json Documentation

File: README.md

Design goals
Sponsors
Support (documentation, FAQ, discussions, API, bug issues)
Quick reference
Examples
Customers
Ecosystem
Supported compilers
Integration
License
Contact
Thanks
Used third-party tools
Notes
Execute unit tests

Design goals

There are myriads of JSON libraries out there, and each may even have its reason to exist. Our class had these design goals:

Intuitive syntax. In languages such as Python, JSON feels like a first-class data type. We used all the operator magic of modern C++ to achieve the same feeling in your code. Check out the examples below and you'll know what I mean.
Trivial integration. Our whole code consists of a single header file json.hpp. That's it. No library, no subproject, no dependencies, no complex build system. The class is written in vanilla C++11. All in all, everything should require no adjustment of your compiler flags or project settings. The library is also included in all popular package managers.
Serious testing. Our code is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind and the Clang Sanitizers that there are no memory leaks. Google OSS-Fuzz additionally runs fuzz tests against all parsers 24/7, effectively executing billions of tests so far. To maintain high quality, the project is following the Core Infrastructure Initiative (CII) best practices. See the quality assurance overview documentation.

Other aspects were not so important to us:

Memory efficiency. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). The default generalization uses the following C++ data types: std::string for strings, int64_t, uint64_t or double for numbers, std::map for objects, std::vector for arrays, and bool for Booleans. However, you can template the generalized class basic_json to your needs.
Speed. There are certainly faster JSON libraries out there. However, if your goal is to speed up your development by adding JSON support with a single header, then this library is the way to go. If you know how to use a std::vector or std::map, you are already set.

See the contribution guidelines for more information.

Support

:question: If you have a question, please check if it is already answered in the FAQ or the Q&A section. If not, please ask a new question there.

:books: If you want to learn more about how to use the library, check out the rest of the README, have a look at code examples, or browse through the help pages.

:construction: If you want to understand the API better, check out the API Reference or have a look at the quick reference below.

:bug: If you found a bug, please check the FAQ if it is a known issue or the result of a design decision. Please also have a look at the issue list before you create a new issue. Please provide as much information as possible to help us understand and reproduce your issue.

There is also a docset for the documentation browsers Dash, Velocity, and Zeal that contains the full documentation as an offline resource.

Quick reference

Constructors basic_json, array, binary, object
Object inspection: type, operator value_t, type_name, is_primitive, is_structured, is_null, is_boolean, is_number, is_number_integer, is_number_unsigned, is_number_float, is_object, is_array, is_string, is_binary, is_discarded
Value access; get, get_to, get_ptr, get_ref, operator ValueType, get_binary
Element access: at, operator[], value, front, back
Lookup: find, count, contains
Iterators: begin, cbegin, end, cend, rbegin, rend, crbegin, crend, items
Capacity: empty, size, max_size
Modifiers: clear, push_back, operator+=, emplace_back, emplace, erase, insert, update, swap
Lexicographical comparison operators: operator==, operator!=, operator<, operator>, operator<=, operator>=, operator<=>
Serialization / Dumping: dump
Deserialization / Parsing: parse, accept, sax_parse
JSON Pointer functions: flatten, unflatten
JSON Patch functions: patch, patch_inplace, diff, merge_patch
Static functions: meta, get_allocator
Binary formats: from_bjdata, from_bson, from_cbor, from_msgpack, from_ubjson, to_bjdata, to_bson, to_cbor, to_msgpack, to_ubjson
Non-member functions: operator<<, operator>>, to_string
Literals: operator""_json
Helper classes: std::hash<basic_json>, std::swap<basic_json>

Full API documentation

Examples

Here are some examples to give you an idea how to use the class.

Besides the examples below, you may want to:

→ Check the documentation
→ Browse the standalone example files
→ Read the full API Documentation with self-contained examples for every function

Read JSON from a file

The json class provides an API for manipulating a JSON value. To create a json object by reading a JSON file:

cpp

#include <fstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;

// ...

std::ifstream f("example.json");
json data = json::parse(f);

If using modules (enabled with NLOHMANN_JSON_BUILD_MODULES), this example becomes:

cpp

import std;
import nlohmann.json;

using json = nlohmann::json;

// ...

std::ifstream f("example.json");
json data = json::parse(f);

Creating `json` objects from JSON literals

Assume you want to hard-code this literal JSON value as a json object:

json

{
  "pi": 3.141,
  "happy": true
}

There are various options:

cpp

// Using (raw) string literals and json::parse
json ex1 = json::parse(R"(
  {
    "pi": 3.141,
    "happy": true
  }
)");

// Using user-defined (raw) string literals
using namespace nlohmann::literals;
json ex2 = R"(
  {
    "pi": 3.141,
    "happy": true
  }
)"_json;

// Using initializer lists
json ex3 = {
  {"happy", true},
  {"pi", 3.141},
};

JSON as a first-class data type

Here are some examples to give you an idea how to use the class.

Assume you want to create the JSON object

json

{
  "pi": 3.141,
  "happy": true,
  "name": "Niels",
  "nothing": null,
  "answer": {
    "everything": 42
  },
  "list": [1, 0, 2],
  "object": {
    "currency": "USD",
    "value": 42.99
  }
}

With this library, you could write:

cpp

// create an empty structure (null)
json j;

// add a number stored as double (note the implicit conversion of j to an object)
j["pi"] = 3.141;

// add a Boolean stored as bool
j["happy"] = true;

// add a string stored as std::string
j["name"] = "Niels";

// add another null object by passing nullptr
j["nothing"] = nullptr;

// add an object inside the object
j["answer"]["everything"] = 42;

// add an array stored as std::vector (using an initializer list)
j["list"] = { 1, 0, 2 };

// add another object (using an initializer list of pairs)
j["object"] = { {"currency", "USD"}, {"value", 42.99} };

// instead, you could also write (which looks very similar to the JSON above)
json j2 = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
};

Note that in all these cases, you never need to "tell" the compiler which JSON value type you want to use. If you want to be explicit or express some edge cases, the functions json::array() and json::object() will help:

cpp

// a way to express the empty array []
json empty_array_explicit = json::array();

// ways to express the empty object {}
json empty_object_implicit = json({});
json empty_object_explicit = json::object();

// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });

Serialization / Deserialization

To/from strings

You can create a JSON value (deserialization) by appending _json to a string literal:

cpp

// create object from string literal
json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;

// or even nicer with a raw string literal
auto j2 = R"(
  {
    "happy": true,
    "pi": 3.141
  }
)"_json;

Note that without appending the _json suffix, the passed string literal is not parsed, but just used as JSON string
value. That is, json j = "{ \"happy\": true, \"pi\": 3.141 }" would just store the string
"{ "happy": true, "pi": 3.141 }" rather than parsing the actual object.

The string literal should be brought into scope with using namespace nlohmann::literals;
(see json::parse()).

The above example can also be expressed explicitly using json::parse():

cpp

// parse explicitly
auto j3 = json::parse(R"({"happy": true, "pi": 3.141})");

You can also get a string representation of a JSON value (serialize):

cpp

// explicit conversion to string
std::string s = j.dump();    // {"happy":true,"pi":3.141}

// serialization with pretty printing
// pass in the amount of spaces to indent
std::cout << j.dump(4) << std::endl;
// {
//     "happy": true,
//     "pi": 3.141
// }

Note the difference between serialization and assignment:

cpp

// store a string in a JSON value
json j_string = "this is a string";

// retrieve the string value
auto cpp_string = j_string.get<std::string>();
// retrieve the string value (alternative when a variable already exists)
std::string cpp_string2;
j_string.get_to(cpp_string2);

// retrieve the serialized value (explicit JSON serialization)
std::string serialized_string = j_string.dump();

// output of original string
std::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.get<std::string>() << '\n';
// output of serialized value
std::cout << j_string << " == " << serialized_string << std::endl;

.dump() returns the originally stored string value.

Note the library only supports UTF-8. When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace or json::error_handler_t::ignore are used as error handlers.

To/from streams (e.g., files, string streams)

You can also use streams to serialize and deserialize:

cpp

// deserialize from standard input
json j;
std::cin >> j;

// serialize to standard output
std::cout << j;

// the setw manipulator was overloaded to set the indentation for pretty printing
std::cout << std::setw(4) << j << std::endl;

These operators work for any subclasses of std::istream or std::ostream. Here is the same example with files:

cpp

// read a JSON file
std::ifstream i("file.json");
json j;
i >> j;

// write prettified JSON to another file
std::ofstream o("pretty.json");
o << std::setw(4) << j << std::endl;

Please note that setting the exception bit for failbit is inappropriate for this use case. It will result in program termination due to the noexcept specifier in use.

Read from iterator range

You can also parse JSON from an iterator range; that is, from any container accessible by iterators whose value_type is an integral type of 1, 2, or 4 bytes, which will be interpreted as UTF-8, UTF-16, and UTF-32 respectively. For instance, a std::vector<std::uint8_t>, or a std::list<std::uint16_t>:

cpp

std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
json j = json::parse(v.begin(), v.end());

You may leave the iterators for the range [begin, end):

cpp

std::vector<std::uint8_t> v = {'t', 'r', 'u', 'e'};
json j = json::parse(v);

Custom data source

Since the parse function accepts arbitrary iterator ranges, you can provide your own data sources by implementing the LegacyInputIterator concept.

cpp

struct MyContainer {
  void advance();
  const char& get_current();
};

struct MyIterator {
    using difference_type = std::ptrdiff_t;
    using value_type = char;
    using pointer = const char*;
    using reference = const char&;
    using iterator_category = std::input_iterator_tag;

    explicit MyIterator(MyContainer* tgt = nullptr) : target(tgt) {}

    MyIterator& operator++() {
        target->advance();
        return *this;
    }

    bool operator!=(const MyIterator& rhs) const {
        return rhs.target != target;
    }

    reference operator*() const {
        return target->get_current();
    }

    MyContainer* target = nullptr;
};

MyIterator begin(MyContainer& tgt) {
    return MyIterator{&tgt};
}

MyIterator end(const MyContainer&) {
    return MyIterator{};
}

void foo() {
    MyContainer c;
    json j = json::parse(begin(c), end(c));
}

SAX interface

The library uses a SAX-like interface with the following functions:

cpp

// called when null is parsed
bool null();

// called when a boolean is parsed; value is passed
bool boolean(bool val);

// called when a signed or unsigned integer number is parsed; value is passed
bool number_integer(number_integer_t val);
bool number_unsigned(number_unsigned_t val);

// called when a floating-point number is parsed; value and original string is passed
bool number_float(number_float_t val, const string_t& s);

// called when a string is parsed; value is passed and can be safely moved away
bool string(string_t& val);
// called when a binary value is parsed; value is passed and can be safely moved away
bool binary(binary_t& val);

// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
bool start_object(std::size_t elements);
bool end_object();
bool start_array(std::size_t elements);
bool end_array();
// called when an object key is parsed; value is passed and can be safely moved away
bool key(string_t& val);

// called when a parse error occurs; byte position, the last token, and an exception is passed
bool parse_error(std::size_t position, const std::string& last_token, const detail::exception& ex);

The return value of each function determines whether parsing should proceed.

To implement your own SAX handler, proceed as follows:

Implement the SAX interface in a class. You can use class nlohmann::json_sax<json> as base class, but you can also use any class where the functions described above are implemented and public.
Create an object of your SAX interface class, e.g. my_sax.
Call bool json::sax_parse(input, &my_sax); where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.

Note the sax_parse function only returns a bool indicating the result of the last executed SAX event. It does not return a json value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error -- it is up to you what to do with the exception object passed to your parse_error implementation. Internally, the SAX interface is used for the DOM parser (class json_sax_dom_parser) as well as the acceptor (json_sax_acceptor), see file json_sax.hpp.

STL-like access

We designed the JSON class to behave just like an STL container. In fact, it satisfies the ReversibleContainer requirement.

cpp

// create an array using push_back
json j;
j.push_back("foo");
j.push_back(1);
j.push_back(true);

// also use emplace_back
j.emplace_back(1.78);

// iterate the array
for (json::iterator it = j.begin(); it != j.end(); ++it) {
  std::cout << *it << '\n';
}

// range-based for
for (auto& element : j) {
  std::cout << element << '\n';
}

// getter/setter
const auto tmp = j[0].get<std::string>();
j[1] = 42;
bool foo = j.at(2);

// comparison
j == R"(["foo", 1, true, 1.78])"_json;  // true

// other stuff
j.size();     // 4 entries
j.empty();    // false
j.type();     // json::value_t::array
j.clear();    // the array is empty again

// convenience type checkers
j.is_null();
j.is_boolean();
j.is_number();
j.is_object();
j.is_array();
j.is_string();

// create an object
json o;
o["foo"] = 23;
o["bar"] = false;
o["baz"] = 3.141;

// also use emplace
o.emplace("weather", "sunny");

// special iterator member functions for objects
for (json::iterator it = o.begin(); it != o.end(); ++it) {
  std::cout << it.key() << " : " << it.value() << "\n";
}

// the same code as range for
for (auto& el : o.items()) {
  std::cout << el.key() << " : " << el.value() << "\n";
}

// even easier with structured bindings (C++17)
for (auto& [key, value] : o.items()) {
  std::cout << key << " : " << value << "\n";
}

// find an entry
if (o.contains("foo")) {
  // there is an entry with key "foo"
}

// or via find and an iterator
if (o.find("foo") != o.end()) {
  // there is an entry with key "foo"
}

// or simpler using count()
int foo_present = o.count("foo"); // 1
int fob_present = o.count("fob"); // 0

// delete an entry
o.erase("foo");

Conversion from STL containers

Any sequence container (std::array, std::vector, std::deque, std::forward_list, std::list) whose values can be used to construct JSON values (e.g., integers, floating point numbers, Booleans, string types, or again STL containers described in this section) can be used to create a JSON array. The same holds for similar associative containers (std::set, std::multiset, std::unordered_set, std::unordered_multiset), but in these cases the order of the elements of the array depends on how the elements are ordered in the respective STL container.

cpp

std::vector c_vector {1, 2, 3, 4};
json j_vec(c_vector);
// [1, 2, 3, 4]

std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
json j_deque(c_deque);
// [1.2, 2.3, 3.4, 5.6]

std::list c_list {true, true, false, true};
json j_list(c_list);
// [true, true, false, true]

std::forward_list c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};
json j_flist(c_flist);
// [12345678909876, 23456789098765, 34567890987654, 45678909876543]

std::array c_array {{1, 2, 3, 4}};
json j_array(c_array);
// [1, 2, 3, 4]

std::set<std::string> c_set {"one", "two", "three", "four", "one"};
json j_set(c_set); // only one entry for "one" is used
// ["four", "one", "three", "two"]

std::unordered_set<std::string> c_uset {"one", "two", "three", "four", "one"};
json j_uset(c_uset); // only one entry for "one" is used
// maybe ["two", "three", "four", "one"]

std::multiset<std::string> c_mset {"one", "two", "one", "four"};
json j_mset(c_mset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]

std::unordered_multiset<std::string> c_umset {"one", "two", "one", "four"};
json j_umset(c_umset); // both entries for "one" are used
// maybe ["one", "two", "one", "four"]

Likewise, any associative key-value containers (std::map, std::multimap, std::unordered_map, std::unordered_multimap) whose keys can construct an std::string and whose values can be used to construct JSON values (see examples above) can be used to create a JSON object. Note that in case of multimaps, only one key is used in the JSON object and the value depends on the internal order of the STL container.

cpp

std::map<std::string, int> c_map { {"one", 1}, {"two", 2}, {"three", 3} };
json j_map(c_map);
// {"one": 1, "three": 3, "two": 2 }

std::unordered_map<const char*, double> c_umap { {"one", 1.2}, {"two", 2.3}, {"three", 3.4} };
json j_umap(c_umap);
// {"one": 1.2, "two": 2.3, "three": 3.4}

std::multimap<std::string, bool> c_mmap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
json j_mmap(c_mmap); // only one entry for key "three" is used
// maybe {"one": true, "two": true, "three": true}

std::unordered_multimap<std::string, bool> c_ummap { {"one", true}, {"two", true}, {"three", false}, {"three", true} };
json j_ummap(c_ummap); // only one entry for key "three" is used
// maybe {"one": true, "two": true, "three": true}

JSON Pointer and JSON Patch

The library supports JSON Pointer (RFC 6901) as an alternative means to address structured values. On top of this, JSON Patch (RFC 6902) allows describing differences between two JSON values -- effectively allowing patch and diff operations known from Unix.

cpp

// a JSON value
json j_original = R"({
  "baz": ["one", "two", "three"],
  "foo": "bar"
})"_json;

// access members with a JSON pointer (RFC 6901)
j_original["/baz/1"_json_pointer];
// "two"

// a JSON patch (RFC 6902)
json j_patch = R"([
  { "op": "replace", "path": "/baz", "value": "boo" },
  { "op": "add", "path": "/hello", "value": ["world"] },
  { "op": "remove", "path": "/foo"}
])"_json;

// apply the patch
json j_result = j_original.patch(j_patch);
// {
//    "baz": "boo",
//    "hello": ["world"]
// }

// calculate a JSON patch from two JSON values
json::diff(j_result, j_original);
// [
//   { "op":" replace", "path": "/baz", "value": ["one", "two", "three"] },
//   { "op": "remove","path": "/hello" },
//   { "op": "add", "path": "/foo", "value": "bar" }
// ]

JSON Merge Patch

The library supports JSON Merge Patch (RFC 7386) as a patch format. Instead of using JSON Pointer (see above) to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified.

cpp

// a JSON value
json j_document = R"({
  "a": "b",
  "c": {
    "d": "e",
    "f": "g"
  }
})"_json;

// a patch
json j_patch = R"({
  "a":"z",
  "c": {
    "f": null
  }
})"_json;

// apply the patch
j_document.merge_patch(j_patch);
// {
//  "a": "z",
//  "c": {
//    "d": "e"
//  }
// }

Implicit conversions

Supported types can be implicitly converted to JSON values.

It is recommended to NOT USE implicit conversions FROM a JSON value.
You can find more details about this recommendation here.
You can switch off implicit conversions by defining JSON_USE_IMPLICIT_CONVERSIONS to 0 before including the json.hpp header. When using CMake, you can also achieve this by setting the option JSON_ImplicitConversions to OFF.

cpp

// strings
std::string s1 = "Hello, world!";
json js = s1;
auto s2 = js.get<std::string>();
// NOT RECOMMENDED
std::string s3 = js;
std::string s4;
s4 = js;

// Booleans
bool b1 = true;
json jb = b1;
auto b2 = jb.get();
// NOT RECOMMENDED
bool b3 = jb;
bool b4;
b4 = jb;

// numbers
int i = 42;
json jn = i;
auto f = jn.get<double>();
// NOT RECOMMENDED
double f2 = jn;
double f3;
f3 = jn;

// etc.

Note that char types are not automatically converted to JSON strings, but to integer numbers. A conversion to a string must be specified explicitly:

cpp

char ch = 'A';                       // ASCII value 65
json j_default = ch;                 // stores integer number 65
json j_string = std::string(1, ch);  // stores string "A"

Arbitrary types conversions

Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:

cpp

namespace ns {
    // a simple struct to model a person
    struct person {
        std::string name;
        std::string address;
        int age;
    };
}

ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};

// convert to JSON: copy each value into the JSON object
json j;
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;

// ...

// convert from JSON: copy each value from the JSON object
ns::person p {
    j["name"].get<std::string>(),
    j["address"].get<std::string>(),
    j["age"].get()
};

It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:

cpp

// create a person
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};

// conversion: person -> json
json j = p;

std::cout << j << std::endl;
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

// conversion: json -> person
auto p2 = j.get<ns::person>();

// that's it
assert(p == p2);

Basic usage

To make this work with one of your types, you only need to provide two functions:

cpp

using json = nlohmann::json;

namespace ns {
    void to_json(json& j, const person& p) {
        j = json{{"name", p.name}, {"address", p.address}, {"age", p.age}};
    }

    void from_json(const json& j, person& p) {
        j.at("name").get_to(p.name);
        j.at("address").get_to(p.address);
        j.at("age").get_to(p.age);
    }
} // namespace ns

That's all! When calling the json constructor with your type, your custom to_json method will be automatically called.
Likewise, when calling get<your_type>() or get_to(your_type&), the from_json method will be called.

Some important things:

Those methods MUST be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace ns, where person is defined).
Those methods MUST be available (e.g., proper headers must be included) everywhere you use these conversions. Look at issue 1108 for errors that may occur otherwise.
When using get<your_type>(), your_type MUST be DefaultConstructible. (There is a way to bypass this requirement described later.)
In function from_json, use function at() to access the object values rather than operator[]. In case a key does not exist, at throws an exception that you can handle, whereas operator[] exhibits undefined behavior.
You do not need to add serializers or deserializers for STL types like std::vector: the library already implements these.

Simplify your life with macros

If you just want to serialize/deserialize some structs, the to_json/from_json functions can be a lot of boilerplate. There are several macros to make your life easier as long as you want to use a JSON object as serialization.

Which macro to choose depends on whether private member variables need to be accessed, a deserialization is needed, missing values should yield an error or should be replaced by default values, and if derived classes are used. See this overview to choose the right one for your use case.

Example usage of macros

The to_json/from_json functions for the person struct above can be created with NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE. In all macros, the first parameter is the name of the class/struct, and all remaining parameters name the members.

cpp

namespace ns {
    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
}

If you want to inherit the person struct and add a field to it, it can be done with:

cpp

namespace ns {
    struct person_derived : person {
        std::string email;
    };
    
    NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)
}

Here is another example with private members, where NLOHMANN_DEFINE_TYPE_INTRUSIVE is needed:

cpp

namespace ns {
    class address {
      private:
        std::string street;
        int housenumber;
        int postcode;
  
      public:
        NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
    };
}

Or in case if you use some naming convention that you do not want to expose to JSON:

cpp

namespace ns {
    class address {
      private:
        std::string m_street;
        int m_housenumber;
        int m_postcode;

      public:
        NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, "street", m_street,
                                                           "housenumber", m_housenumber,
                                                           "postcode", m_postcode)
    };
}

How do I convert third-party types?

This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:

The library uses JSON Serializers to convert types to JSON.
The default serializer for nlohmann::json is nlohmann::adl_serializer (ADL means Argument-Dependent Lookup).

It is implemented like this (simplified):

cpp

template <typename T>
struct adl_serializer {
    static void to_json(json& j, const T& value) {
        // calls the "to_json" method in T's namespace
    }

    static void from_json(const json& j, T& value) {
        // same thing, but with the "from_json" method
    }
};

This serializer works fine when you have control over the type's namespace. However, what about boost::optional or std::filesystem::path (C++17)? Hijacking the boost namespace is pretty bad, and it's illegal to add something other than template specializations to std...

To solve this, you need to add a specialization of adl_serializer to the nlohmann namespace, here's an example:

cpp

// partial specialization (full specialization works too)
namespace nlohmann {
    template <typename T>
    struct adl_serializer> {
        static void to_json(json& j, const boost::optional<T>& opt) {
            if (opt == boost::none) {
                j = nullptr;
            } else {
              j = *opt; // this will call adl_serializer<T>::to_json which will
                        // find the free function to_json in T's namespace!
            }
        }

        static void from_json(const json& j, boost::optional<T>& opt) {
            if (j.is_null()) {
                opt = boost::none;
            } else {
                opt = j.get<T>(); // same as above, but with
                                  // adl_serializer<T>::from_json
            }
        }
    };
}

How can I use `get()` for non-default constructible/non-copyable types?

There is a way if your type is MoveConstructible. You will need to specialize the adl_serializer as well, but with a special from_json overload:

cpp

struct move_only_type {
    move_only_type() = delete;
    move_only_type(int ii): i(ii) {}
    move_only_type(const move_only_type&) = delete;
    move_only_type(move_only_type&&) = default;

    int i;
};

namespace nlohmann {
    template <>
    struct adl_serializer<move_only_type> {
        // note: the return type is no longer 'void', and the method only takes
        // one argument
        static move_only_type from_json(const json& j) {
            return {j.get()};
        }

        // Here's the catch! You must provide a to_json method! Otherwise, you
        // will not be able to convert move_only_type to json, since you fully
        // specialized adl_serializer on that type
        static void to_json(json& j, move_only_type t) {
            j = t.i;
        }
    };
}

Can I write my own serializer? (Advanced use)

Yes. You might want to take a look at unit-udt.cpp in the test suite, to see a few examples.

If you write your own serializer, you'll need to do a few things:

use a different basic_json alias than nlohmann::json (the last template parameter of basic_json is the JSONSerializer)
use your basic_json alias (or a template parameter) in all your to_json/from_json methods
use nlohmann::to_json and nlohmann::from_json when you need ADL

Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.

cpp

// You should use void as a second template argument
// if you don't need compile-time checks on T
template<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
struct less_than_32_serializer {
    template <typename BasicJsonType>
    static void to_json(BasicJsonType& j, T value) {
        // we want to use ADL, and call the correct to_json overload
        using nlohmann::to_json; // this method is called by adl_serializer,
                                 // this is where the magic happens
        to_json(j, value);
    }

    template <typename BasicJsonType>
    static void from_json(const BasicJsonType& j, T& value) {
        // same thing here
        using nlohmann::from_json;
        from_json(j, value);
    }
};

Be very careful when reimplementing your serializer, you can stack overflow if you don't pay attention:

cpp

template <typename T, void>
struct bad_serializer
{
    template <typename BasicJsonType>
    static void to_json(BasicJsonType& j, const T& value) {
      // this calls BasicJsonType::json_serializer<T>::to_json(j, value)
      // if BasicJsonType::json_serializer == bad_serializer ... oops!
      j = value;
    }

    template <typename BasicJsonType>
    static void to_json(const BasicJsonType& j, T& value) {
      // this calls BasicJsonType::json_serializer<T>::from_json(j, value)
      // if BasicJsonType::json_serializer == bad_serializer ... oops!
      value = j.get<T>(); // oops!
    }
};

Specializing enum conversion

By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.

It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:

cpp

// example enum type declaration
enum TaskState {
    TS_STOPPED,
    TS_RUNNING,
    TS_COMPLETED,
    TS_INVALID=-1,
};

// map TaskState values to JSON as strings
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
    {TS_INVALID, nullptr},
    {TS_STOPPED, "stopped"},
    {TS_RUNNING, "running"},
    {TS_COMPLETED, "completed"},
})

The NLOHMANN_JSON_SERIALIZE_ENUM() macro declares a set of to_json() / from_json() functions for type TaskState while avoiding repetition and boilerplate serialization code.

Usage:

cpp

// enum to JSON as string
json j = TS_STOPPED;
assert(j == "stopped");

// json string to enum
json j3 = "running";
assert(j3.get<TaskState>() == TS_RUNNING);

// undefined json value to enum (where the first map entry above is the default)
json jPi = 3.14;
assert(jPi.get<TaskState>() == TS_INVALID);

Just as in Arbitrary Type Conversions above,

NLOHMANN_JSON_SERIALIZE_ENUM() MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.

Other Important points:

When using get<ENUM_TYPE>(), undefined JSON values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use NLOHMANN_JSON_SERIALIZE_ENUM_STRICT() which behaves identically except for throwing an exception on unrecognized values.
If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.

Binary formats (BSON, CBOR, MessagePack, UBJSON, and BJData)

Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance over a network. Hence, the library supports BSON (Binary JSON), CBOR (Concise Binary Object Representation), MessagePack, UBJSON (Universal Binary JSON Specification) and BJData (Binary JData) to efficiently encode JSON values to byte vectors and to decode such vectors.

cpp

// create a JSON value
json j = R"({"compact": true, "schema": 0})"_json;

// serialize to BSON
std::vector<std::uint8_t> v_bson = json::to_bson(j);

// 0x1B, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

// roundtrip
json j_from_bson = json::from_bson(v_bson);

// serialize to CBOR
std::vector<std::uint8_t> v_cbor = json::to_cbor(j);

// 0xA2, 0x67, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xF5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00

// roundtrip
json j_from_cbor = json::from_cbor(v_cbor);

// serialize to MessagePack
std::vector<std::uint8_t> v_msgpack = json::to_msgpack(j);

// 0x82, 0xA7, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0xC3, 0xA6, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x00

// roundtrip
json j_from_msgpack = json::from_msgpack(v_msgpack);

// serialize to UBJSON
std::vector<std::uint8_t> v_ubjson = json::to_ubjson(j);

// 0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61, 0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68, 0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D

// roundtrip
json j_from_ubjson = json::from_ubjson(v_ubjson);

The library also supports binary types from BSON, CBOR (byte strings), and MessagePack (bin, ext, fixext). They are stored by default as std::vector<std::uint8_t> to be processed outside the library.

cpp

// CBOR byte string with payload 0xCAFE
std::vector<std::uint8_t> v = {0x42, 0xCA, 0xFE};

// read value
json j = json::from_cbor(v);

// the JSON value has type binary
j.is_binary(); // true

// get reference to stored binary value
auto& binary = j.get_binary();

// the binary value has no subtype (CBOR has no binary subtypes)
binary.has_subtype(); // false

// access std::vector<std::uint8_t> member functions
binary.size(); // 2
binary[0]; // 0xCA
binary[1]; // 0xFE

// set subtype to 0x10
binary.set_subtype(0x10);

// serialize to MessagePack
auto cbor = json::to_msgpack(j); // 0xD5 (fixext2), 0x10, 0xCA, 0xFE

Customers

The library is used in multiple projects, applications, operating systems, etc. The list below is not exhaustive, but the result of an internet search. If you know further customers of the library, please let me know, see contact.

Ecosystem

Beyond projects that use the library, there are third-party projects that build on top of it - schema validators,
language bindings, format converters, and the like. See the curated Ecosystem page.

Supported compilers

Though it's 2026 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:

GCC 4.8 - 14.2 (and possibly later)
Clang 3.4 - 21.0 (and possibly later)
Apple Clang 9.1 - 16.0 (and possibly later)
Intel C++ Compiler 17.0.2 (and possibly later)
Nvidia CUDA Compiler 11.0.221 (and possibly later)
Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0 (and possibly later)
Microsoft Visual C++ 2017 / Build Tools 15.5.180.51428 (and possibly later)
Microsoft Visual C++ 2019 / Build Tools 16.3.1+1def00d3d (and possibly later)
Microsoft Visual C++ 2022 / Build Tools 19.30.30709.0 (and possibly later)

I would be happy to learn about other compilers/versions.

Please note:

GCC 4.8 has a bug 57824: multiline raw strings cannot be the arguments to macros. Don't use multiline raw strings directly in macros with this compiler.
Android defaults to using very old compilers and C++ libraries. To fix this, add the following to your Application.mk. This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.
makefile
```
APP_STL := c++_shared
NDK_TOOLCHAIN_VERSION := clang3.6
APP_CPPFLAGS += -frtti -fexceptions
```
The code compiles successfully with Android NDK, Revision 9 - 11 (and possibly later) and CrystaX's Android NDK version 10.
For GCC running on MinGW or Android SDK, the error 'to_string' is not a member of 'std' (or similarly, for strtod or strtof) may occur. Note this is not an issue with the code, but rather with the compiler itself. On Android, see above to build with a newer environment. For MinGW, please refer to this site and this discussion for information on how to fix this bug. For Android NDK using APP_STL := gnustl_static, please refer to this discussion.
Unsupported versions of GCC and Clang are rejected by #error directives. This can be switched off by defining JSON_SKIP_UNSUPPORTED_COMPILER_CHECK. Note that you can expect no support in this case.

See the page quality assurance on the compilers used to check the library in the CI.

Integration

json.hpp is the single required file in single_include/nlohmann or released here. You need to add

cpp

#include <nlohmann/json.hpp>

// for convenience
using json = nlohmann::json;

to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., -std=c++11 for GCC and Clang).

You can further use file include/nlohmann/json_fwd.hpp for forward-declarations. The installation of json_fwd.hpp (as part of cmake's install step) can be achieved by setting -DJSON_MultipleHeaders=ON.

CMake

You can also use the nlohmann_json::nlohmann_json interface target in CMake. This target populates the appropriate usage requirements for INTERFACE_INCLUDE_DIRECTORIES to point to the appropriate include directories and INTERFACE_COMPILE_FEATURES for the necessary C++11 flags.

External

To use this library from a CMake project, you can locate it directly with find_package() and use the namespaced imported target from the generated package configuration:

cmake

# CMakeLists.txt
find_package(nlohmann_json 3.12.0 REQUIRED)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

The package configuration file, nlohmann_jsonConfig.cmake, can be used either from an install tree or directly out of the build tree.

Embedded

To embed the library directly into an existing CMake project, place the entire source tree in a subdirectory and call add_subdirectory() in your CMakeLists.txt file:

cmake

# Typically you don't care so much for a third party library's tests to be
# run from your own project's code.
set(JSON_BuildTests OFF CACHE INTERNAL "")

# If you only include this third party in PRIVATE source files, you do not
# need to install it when your main project gets installed.
# set(JSON_Install OFF CACHE INTERNAL "")

# Don't use include(nlohmann_json/CMakeLists.txt) since that carries with it
# unintended consequences that will break the build.  It's generally
# discouraged (although not necessarily well documented as such) to use
# include(...) for pulling in other CMake projects anyways.
add_subdirectory(nlohmann_json)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

Embedded (FetchContent)

Since CMake v3.11,
FetchContent can
be used to automatically download a release as a dependency at configure time.

Example:

cmake

include(FetchContent)

FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.12.0/json.tar.xz)
FetchContent_MakeAvailable(json)

target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

Note: It is recommended to use the URL approach described above, which is supported as of version 3.10.0. See
https://json.nlohmann.me/integration/cmake/#fetchcontent for more information.

Supporting Both

To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following:

cmake

# Top level CMakeLists.txt
project(FOO)
...
option(FOO_USE_EXTERNAL_JSON "Use an external JSON library" OFF)
...
add_subdirectory(thirdparty)
...
add_library(foo ...)
...
# Note that the namespaced target will always be available regardless of the
# import method
target_link_libraries(foo PRIVATE nlohmann_json::nlohmann_json)

cmake

# thirdparty/CMakeLists.txt
...
if(FOO_USE_EXTERNAL_JSON)
  find_package(nlohmann_json 3.12.0 REQUIRED)
else()
  set(JSON_BuildTests OFF CACHE INTERNAL "")
  add_subdirectory(nlohmann_json)
endif()
...

thirdparty/nlohmann_json is then a complete copy of this source tree.

Package Managers

Use your favorite package manager to use the library.

Homebrew nlohmann-json
Meson nlohmann_json
Bazel nlohmann_json
Conan nlohmann_json
Spack nlohmann-json
Hunter nlohmann_json
vcpkg nlohmann-json
cget nlohmann/json
Swift Package Manager nlohmann/json
Nuget nlohmann.json
Conda nlohmann_json
MacPorts nlohmann-json
cpm.cmake gh:nlohmann/json
xmake nlohmann_json

The library is part of many package managers. See the documentation for detailed descriptions and examples.

Pkg-config

If you are using bare Makefiles, you can use pkg-config to generate the include flags that point to where the library is installed:

pkg-config nlohmann_json --cflags

License

The class is licensed under the MIT License:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the MIT License (see above). Copyright © 2008-2009 Björn Hoehrmann
The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the MIT License (see above). Copyright © 2009 Florian Loitsch
The class contains a copy of Hedley from Evan Nemerson which is licensed as CC0-1.0.
The class contains parts of Google Abseil which is licensed under the Apache 2.0 License.

The library is compliant to version 3.3 of the REUSE specification:

Every source file contains an SPDX copyright header.
The full text of all licenses used in the repository can be found in the LICENSES folder.
File .reuse/dep5 contains an overview of all files' copyrights and licenses.
Run pipx run reuse lint to verify the project's REUSE compliance and pipx run reuse spdx to generate a SPDX SBOM.

Contact

If you have questions regarding the library, I would like to invite you to open an issue at GitHub. Please describe your request, problem, or question as detailed as possible, and also mention the version of the library you are using as well as the version of your compiler and operating system. Opening an issue at GitHub allows other users and contributors to this library to collaborate. For instance, I have little experience with MSVC, and most issues in this regard have been solved by a growing community. If you have a look at the closed issues, you will see that we react quite timely in most cases.

Only if your request would contain confidential information, please send me an email. For encrypted messages, please use this key.

Security

Commits by Niels Lohmann and releases are signed with this PGP Key.

Thanks

I deeply appreciate the help of the following people.

Teemperor implemented CMake support and lcov integration, realized escape and Unicode handling in the string parser, and fixed the JSON serialization.
elliotgoodrich fixed an issue with double deletion in the iterator classes.
kirkshoop made the iterators of the class composable to other libraries.
wancw fixed a bug that hindered the class to compile with Clang.
Tomas Åblad found a bug in the iterator implementation.
Joshua C. Randall fixed a bug in the floating-point serialization.
Aaron Burghardt implemented code to parse streams incrementally. Furthermore, he greatly improved the parser class by allowing the definition of a filter function to discard undesired elements while parsing.
Daniel Kopeček fixed a bug in the compilation with GCC 5.0.
Florian Weber fixed a bug in and improved the performance of the comparison operators.
Eric Cornelius pointed out a bug in the handling with NaN and infinity values. He also improved the performance of the string escaping.
易思龙 implemented a conversion from anonymous enums.
kepkin patiently pushed forward the support for Microsoft Visual Studio.
gregmarr simplified the implementation of reverse iterators and helped with numerous hints and improvements. In particular, he pushed forward the implementation of user-defined types.
Caio Luppi fixed a bug in the Unicode handling.
dariomt fixed some typos in the examples.
Daniel Frey cleaned up some pointers and implemented exception-safe memory allocation.
Colin Hirsch took care of a small namespace issue.
Huu Nguyen corrected a variable name in the documentation.
Silverweed overloaded parse() to accept an rvalue reference.
dariomt fixed a subtlety in MSVC type support and implemented the get_ref() function to get a reference to stored values.
ZahlGraf added a workaround that allows compilation using Android NDK.
whackashoe replaced a function that was marked as unsafe by Visual Studio.
406345 fixed two small warnings.
Glen Fernandes noted a potential portability problem in the has_mapped_type function.
Corbin Hughes fixed some typos in the contribution guidelines.
twelsby fixed the array subscript operator, an issue that failed the MSVC build, and floating-point parsing/dumping. He further added support for unsigned integer numbers and implemented better roundtrip support for parsed numbers.
Volker Diels-Grabsch fixed a link in the README file.
msm- added support for American Fuzzy Lop.
Annihil fixed an example in the README file.
Themercee noted a wrong URL in the README file.
Lv Zheng fixed a namespace issue with int64_t and uint64_t.
abc100m analyzed the issues with GCC 4.8 and proposed a partial solution.
zewt added useful notes to the README file about Android.
Róbert Márki added a fix to use move iterators and improved the integration via CMake.
Chris Kitching cleaned up the CMake files.
Tom Needham fixed a subtle bug with MSVC 2015 which was also proposed by Michael K..
Mário Feroldi fixed a small typo.
duncanwerner found a really embarrassing performance regression in the 2.0.0 release.
Damien fixed one of the last conversion warnings.
Thomas Braun fixed a warning in a test case and adjusted MSVC calls in the CI.
Théo DELRIEU patiently and constructively oversaw the long way toward iterator-range parsing. He also implemented the magic behind the serialization/deserialization of user-defined types and split the single header file into smaller chunks.
Stefan fixed a minor issue in the documentation.
Vasil Dimov fixed the documentation regarding conversions from std::multiset.
ChristophJud overworked the CMake files to ease project inclusion.
Vladimir Petrigo made a SFINAE hack more readable and added Visual Studio 17 to the build matrix.
Denis Andrejew fixed a grammar issue in the README file.
Pierre-Antoine Lacaze found a subtle bug in the dump() function.
TurpentineDistillery pointed to std::locale::classic() to avoid too much locale joggling, found some nice performance improvements in the parser, improved the benchmarking code, and realized locale-independent number parsing and printing.
cgzones had an idea how to fix the Coverity scan.
Jared Grubb silenced a nasty documentation warning.
Yixin Zhang fixed an integer overflow check.
Bosswestfalen merged two iterator classes into a smaller one.
Daniel599 helped to get Travis to execute the tests with Clang's sanitizers.
Jonathan Lee fixed an example in the README file.
gnzlbg supported the implementation of user-defined types.
Alexej Harm helped to get the user-defined types working with Visual Studio.
Jared Grubb supported the implementation of user-defined types.
EnricoBilla noted a typo in an example.
Martin Hořeňovský found a way for a 2x speedup for the compilation time of the test suite.
ukhegg proposed an improvement for the examples section.
rswanson-ihi noted a typo in the README.
Mihai Stan fixed a bug in the comparison with nullptrs.
Tushar Maheshwari added cotire support to speed up the compilation.
TedLyngmo noted a typo in the README, removed unnecessary bit arithmetic, and fixed some -Weffc++ warnings.
Krzysztof Woś made exceptions more visible.
ftillier fixed a compiler warning.
tinloaf made sure all pushed warnings are properly popped.
Fytch found a bug in the documentation.
Jay Sistar implemented a Meson build description.
Henry Lee fixed a warning in ICC and improved the iterator implementation.
Vincent Thiery maintains a package for the Conan package manager.
Steffen fixed a potential issue with MSVC and std::min.
Mike Tzou fixed some typos.
amrcode noted misleading documentation about comparison of floats.
Oleg Endo reduced the memory consumption by replacing with.
dan-42 cleaned up the CMake files to simplify including/reusing of the library.
Nikita Ofitserov allowed for moving values from initializer lists.
Greg Hurrell fixed a typo.
Dmitry Kukovinets fixed a typo.
kbthomp1 fixed an issue related to the Intel OSX compiler.
Markus Werle fixed a typo.
WebProdPP fixed a subtle error in a precondition check.
Alex noted an error in a code sample.
Tom de Geus reported some warnings with ICC and helped to fix them.
Perry Kundert simplified reading from input streams.
Sonu Lohani fixed a small compilation error.
Jamie Seward fixed all MSVC warnings.
Nate Vargas added a Doxygen tag file.
pvleuven helped to fix a warning in ICC.
Pavel helped to fix some warnings in MSVC.
Jamie Seward avoided unnecessary string copies in find() and count().
Mitja fixed some typos.
Jorrit Wronski updated the Hunter package links.
Matthias Möller added a .natvis for the MSVC debug view.
bogemic fixed some C++17 deprecation warnings.
Eren Okka fixed some MSVC warnings.
abolz integrated the Grisu2 algorithm for proper floating-point formatting, allowing more roundtrip checks to succeed.
Vadim Evard fixed a Markdown issue in the README.
zerodefect fixed a compiler warning.
Kert allowed to template the string type in the serialization and added the possibility to override the exceptional behavior.
mark-99 helped fix an ICC error.
Patrik Huber fixed links in the README file.
johnfb found a bug in the implementation of CBOR's indefinite length strings.
Paul Fultz II added a note on the cget package manager.
Wilson Lin made the integration section of the README more concise.
RalfBielig detected and fixed a memory leak in the parser callback.
agrianius allowed dumping JSON to an alternative string type.
Kevin Tonon overworked the C++11 compiler checks in CMake.
Axel Huebl simplified a CMake check and added support for the Spack package manager.
Carlos O'Ryan fixed a typo.
James Upjohn fixed a version number in the compilers section.
Chuck Atkins adjusted the CMake files to the CMake packaging guidelines and provided documentation for the CMake integration.
Jan Schöppach fixed a typo.
martin-mfg fixed a typo.
Matthias Möller removed the dependency from std::stringstream.
agrianius added code to use alternative string implementations.
Daniel599 allowed to use more algorithms with the items() function.
Julius Rakow fixed the Meson include directory and fixed the links to cppreference.com.
Sonu Lohani fixed the compilation with MSVC 2015 in debug mode.
grembo fixed the test suite and re-enabled several test cases.
Hyeon Kim introduced the macro JSON_INTERNAL_CATCH to control the exception handling inside the library.
thyu fixed a compiler warning.
David Guthrie fixed a subtle compilation error with Clang 3.4.2.
Dennis Fischer allowed to call find_package without installing the library.
Hyeon Kim fixed an issue with a double macro definition.
Ben Berman made some error messages more understandable.
zakalibit fixed a compilation problem with the Intel C++ compiler.
mandreyel fixed a compilation problem.
Kostiantyn Ponomarenko added version and license information to the Meson build file.
Henry Schreiner added support for GCC 4.8.
knilch made sure the test suite does not stall when run in the wrong directory.
Antonio Borondo fixed an MSVC 2017 warning.
Dan Gendreau implemented the NLOHMANN_JSON_SERIALIZE_ENUM macro to quickly define an enum/JSON mapping.
efp added line and column information to parse errors.
julian-becker added BSON support.
Pratik Chowdhury added support for structured bindings.
David Avedissian added support for Clang 5.0.1 (PS4 version).
Jonathan Dumaresq implemented an input adapter to read from FILE*.
kjpus fixed a link in the documentation.
Manvendra Singh fixed a typo in the documentation.
ziggurat29 fixed an MSVC warning.
Sylvain Corlay added code to avoid an issue with MSVC.
mefyl fixed a bug when JSON was parsed from an input stream.
Millian Poquet allowed to install the library via Meson.
Michael Behrns-Miller found an issue with a missing namespace.
Nasztanovics Ferenc fixed a compilation issue with libc 2.12.
Andreas Schwab fixed the endian conversion.
Mark-Dunning fixed a warning in MSVC.
Gareth Sylvester-Bradley added operator/ for JSON Pointers.
John-Mark noted a missing header.
Vitaly Zaitsev fixed compilation with GCC 9.0.
Laurent Stacul fixed compilation with GCC 9.0.
Ivor Wanders helped to reduce the CMake requirement to version 3.1.
njlr updated the Buckaroo instructions.
Lion fixed a compilation issue with GCC 7 on CentOS.
Isaac Nickaein improved the integer serialization performance and implemented the contains() function.
past-due suppressed an unfixable warning.
Elvis Oric improved Meson support.
Matěj Plch fixed an example in the README.
Mark Beckwith fixed a typo.
scinart fixed a bug in the serializer.
Patrick Boettcher implemented push_back() and pop_back() for JSON Pointers.
Bruno Oliveira added support for Conda.
Michele Caini fixed links in the README.
Hani documented how to install the library with NuGet.
Mark Beckwith fixed a typo.
yann-morin-1998 helped to reduce the CMake requirement to version 3.1.
Konstantin Podsvirov maintains a package for the MSYS2 software distro.
remyabel added GNUInstallDirs to the CMake files.
Taylor Howard fixed a unit test.
Gabe Ron implemented the to_string method.
Watal M. Iwasaki fixed a Clang warning.
Viktor Kirilov switched the unit tests from Catch to doctest
Juncheng E fixed a typo.
tete17 fixed a bug in the contains function.
Xav83 fixed some cppcheck warnings.
0xflotus fixed some typos.
Christian Deneke added a const version of json_pointer::back.
Julien Hamaide made the items() function work with custom string types.
Evan Nemerson updated fixed a bug in Hedley and updated this library accordingly.
Florian Pigorsch fixed a lot of typos.
Camille Bégué fixed an issue in the conversion from std::pair and std::tuple to json.
Anthony VH fixed a compile error in an enum deserialization.
Yuriy Vountesmery noted a subtle bug in a preprocessor check.
Chen fixed numerous issues in the library.
Antony Kellermann added a CI step for GCC 10.1.
Alex fixed an MSVC warning.
Rainer proposed an improvement in the floating-point serialization in CBOR.
Francois Chabot made performance improvements in the input adapters.
Arthur Sonzogni documented how the library can be included via FetchContent.
Rimas Misevičius fixed an error message.
Alexander Myasnikov fixed some examples and a link in the README.
Hubert Chathi made CMake's version config file architecture-independent.
OmnipotentEntity implemented the binary values for CBOR, MessagePack, BSON, and UBJSON.
ArtemSarmini fixed a compilation issue with GCC 10 and fixed a leak.
Evgenii Sopov integrated the library to the wsjcpp package manager.
Sergey Linev fixed a compiler warning.
Miguel Magalhães fixed the year in the copyright.
Gareth Sylvester-Bradley fixed a compilation issue with MSVC.
Alexander “weej” Jones fixed an example in the README.
Antoine Cœur fixed some typos in the documentation.
jothepro updated links to the Hunter package.
Dave Lee fixed a link in the README.
Joël Lamotte added instruction for using Build2's package manager.
Paul Jurczak fixed an example in the README.
Sonu Lohani fixed a warning.
Carlos Gomes Martinho updated the Conan package source.
Konstantin Podsvirov fixed the MSYS2 package documentation.
Tridacnid improved the CMake tests.
Michael fixed MSVC warnings.
Quentin Barbarat fixed an example in the documentation.
XyFreak fixed a compiler warning.
TotalCaesar659 fixed links in the README.
Tanuj Garg improved the fuzzer coverage for UBSAN input.
AODQ fixed a compiler warning.
jwittbrodt made NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE inline.
pfeatherstone improved the upper bound of arguments of the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE/NLOHMANN_DEFINE_TYPE_INTRUSIVE macros.
Jan Procházka fixed a bug in the CBOR parser for binary and string values.
T0b1-iOS fixed a bug in the new hash implementation.
Matthew Bauer adjusted the CBOR writer to create tags for binary subtypes.
gatopeich implemented an ordered map container for nlohmann::ordered_json.
Érico Nogueira Rolim added support for pkg-config.
KonanM proposed an implementation for the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE/NLOHMANN_DEFINE_TYPE_INTRUSIVE macros.
Guillaume Racicot implemented string_view support and allowed C++20 support.
Alex Reinking improved CMake support for FetchContent.
Hannes Domani provided a GDB pretty printer.
Lars Wirzenius reviewed the README file.
Jun Jie fixed a compiler path in the CMake scripts.
Ronak Buch fixed typos in the documentation.
Alexander Karzhenkov fixed a move constructor and the Travis builds.
Leonardo Lima added CPM.Cmake support.
Joseph Blackman fixed a warning.
Yaroslav updated doctest and implemented unit tests.
Martin Stump fixed a bug in the CMake files.
Jaakko Moisio fixed a bug in the input adapters.
bl-ue fixed some Markdown issues in the README file.
William A. Wieselquist fixed an example from the README.
abbaswasim fixed an example from the README.
Remy Jette fixed a warning.
Fraser fixed the documentation.
Ben Beasley updated doctest.
Doron Behar fixed pkg-config.pc.
raduteo fixed a warning.
David Pfahler added the possibility to compile the library without I/O support.
Morten Fyhn Amundsen fixed a typo.
jpl-mac allowed treating the library as a system header in CMake.
Jason Dsouza fixed the indentation of the CMake file.
offa added a link to Conan Center to the documentation.
TotalCaesar659 updated the links in the documentation to use HTTPS.
Rafail Giavrimis fixed the Google Benchmark default branch.
Louis Dionne fixed a conversion operator.
justanotheranonymoususer made the examples in the README more consistent.
Finkman suppressed some -Wfloat-equal warnings.
Ferry Huberts fixed -Wswitch-enum warnings.
Arseniy Terekhin made the GDB pretty-printer robust against unset variable names.
Amir Masoud Abdol updated the Homebrew command as nlohmann/json is now in homebrew-core.
Hallot fixed some -Wextra-semi-stmt warnings.
Giovanni Cerretani fixed -Wunused warnings on JSON_DIAGNOSTICS.
Bogdan Popescu hosts the docset for offline documentation viewers.
Carl Smedstad fixed an assertion error when using JSON_DIAGNOSTICS.
miikka75 provided an important fix to compile C++17 code with Clang 9.
Maarten Becker fixed a warning for shadowed variables.
Cristi Vîjdea fixed typos in the operator[] documentation.
Alex Beregszaszi fixed spelling mistakes in comments.
Dirk Stolle fixed typos in documentation.
Daniel Albuschat corrected the parameter name in the parse documentation.
Prince Mendiratta fixed a link to the FAQ.
Florian Albrechtskirchinger implemented std::string_view support for object keys and made dozens of other improvements.
Qianqian Fang implemented the Binary JData (BJData) format.
pketelsen added macros NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.
DarkZeros adjusted to code to not clash with Arduino defines.
flagarde fixed the output of meta() for MSVC.
Giovanni Cerretani fixed a check for std::filesystem.
Dimitris Apostolou fixed a typo.
Ferry Huberts fixed a typo.
Michael Nosthoff fixed a typo.
JungHoon Lee fixed a typo.
Faruk D. fixed the CITATION.CFF file.
Andrea Cocito added a clarification on macro usage to the documentation.
Krzysiek Karbowiak refactored the tests to use CHECK_THROWS_WITH_AS.
Chaoqi Zhang fixed a typo.
ivanovmp fixed a whitespace error.
KsaNL fixed a build error when including <windows.h>.
Andrea Pappacoda moved .pc and .cmake files to share directory.
Wolf Vollprecht added the patch_inplace function.
Jake Zimmerman highlighted common usage patterns in the README file.
NN added the Visual Studio output directory to .gitignore.
Romain Reignier improved the performance of the vector output adapter.
Mike fixed the std::iterator_traits.
Richard Hozák added macro JSON_NO_ENUM to disable default enum conversions.
vakokako fixed tests when compiling with C++20.
Alexander “weej” Jones fixed an example in the README.
Eli Schwartz added more files to the include.zip archive.
Kevin Lu fixed a compilation issue when typedefs with certain names were present.
Trevor Hickey improved the description of an example.
Jef LeCompte updated the year in the README file.
Alexandre Hamez fixed a warning.
Maninderpal Badhan fixed a typo.
kevin-- added a note to an example in the README file.
I fixed a typo.
Gregorio Litenstein fixed the Clang detection.
Andreas Smas added a Doozer badge.
WanCW fixed the string conversion with Clang.
zhaohuaxishi fixed a Doxygen error.
emvivre removed an invalid parameter from CMake.
Tobias Hermann fixed a link in the README file.
Michael fixed a warning.
Ryan Mulder added ensure_ascii to the dump function.
Muri Nicanor fixed the sed discovery in the Makefile.
David Avedissian implemented SFINAE-friendly iterator_traits.
AQNOUCH Mohammed fixed a typo in the README.
Gareth Sylvester-Bradley added operator/= and operator/ to construct JSON pointers.
Michael Macnair added support for afl-fuzz testing.
Berkus Decker fixed a typo in the README.
Illia Polishchuk improved the CMake testing.
Ikko Ashimine fixed a typo.
Raphael Grimm added the possibility to define a custom base class.
tocic fixed typos in the documentation.
Vertexwahn added Bazel build support.
Dirk Stolle fixed typos in the documentation.
DavidKorczynski added a CIFuzz CI GitHub action.
Finkman fixed the debug pretty-printer.
Florian Segginger bumped the years in the README.
haadfida cleaned up the badges of used services.
Arsen Arsenović fixed a build error.
theevilone45 fixed a typo in a CMake file.
Sergei Trofimovich fixed the custom allocator support.
Joyce fixed some security issues in the GitHub workflows.
Nicolas Jakob add vcpkg version badge.
Tomerkm added tests.
No. fixed the use of get<> calls.
taro fixed a typo in the CODEOWNERS file.
Ikko Eltociear Ashimine fixed a typo.
Felix Yan fixed a typo in the README.
HO-COOH fixed a parenthesis in the documentation.
Ivor Wanders fixed the examples to catch exception by const&.
miny1233 fixed a parenthesis in the documentation.
tomalakgeretkal fixed a compilation error.
alferov fixed a compilation error.
Craig Scott fixed a deprecation warning in CMake.
Vyacheslav Zhdanovskiy added macros for serialization-only types.
Mathieu Westphal fixed typos.
scribam fixed the MinGW workflow.
Aleksei Sapitskii added support for Apple's Swift Package Manager.
Benjamin Buch fixed the installation path in CMake.
Colby Haskell clarified the parse error message in case a file cannot be opened.
Juan Carlos Arevalo Baeza fixed the enum conversion.
alferov fixed a version in the documentation.
ss fixed the amalgamation call.
AniketDhemare fixed a version in the documentation.
Philip Müller fixed an example.
Leila Shcheglova fixed a warning in a test.
Alex Prabhat Bara fixed a function name in the documentation.
laterlaugh fixed some typos.
Yuanhao Jia fixed the GDB pretty printer.
Fallen_Breath fixed an example for JSON Pointer.
Nikhil Idiculla fixed some typos.
Griffin Myers updated the Natvis file.
thetimr fixed a typo in the documentation.
Balazs Erseki fixed a URL in the contribution guidelines.
Niccolò Iardella added NLOHMANN_DEFINE_DERIVED_TYPE_* macros.
Borislav Stanimirov allowed overriding the CMake target name.
Captain Crutches made iterator_proxy_value a std::forward_iterator.
Fredrik Sandhei added type conversion support for std::optional.
jh96 added exceptions when nullptr is passed to parse.
Stuart Gorman fixed number parsing when EINTR set in errno.
Dylan Baker generated a pkg-config file that follows the pkg-config conventions.
Tianyi Chen optimized the binary get_number implementation.
peng-wang-cn added type conversion support for multidimensional arrays.
Einars Netlis-Galejs added ONLY_SERIALIZE for NLOHMANN_DEFINE_DERIVED_TYPE_* macros.
Marcel removed alwayslink=True Bazel flag.
Harinath Nampally added diagnostic positions to exceptions.
Nissim Armand Ben Danan fixed NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT with an empty JSON instance.
Michael Valladolid added support for BSON uint64 serialization/deserialization.
Nikhil updated the documentation.
Nebojša Cvetković added support for BJDATA optimized binary array type.
Sushrut Shringarputale added support for diagnostic positions.
kimci86 templated to NLOHMANN_DEFINE_TYPE macros to also support ordered_json.
Richard Topchii added support for VisionOS in the Swift Package Manager.
Robert Chisholm fixed a typo.
zjyhjqs added CPack support.
bitFiedler made GDB pretty printer work with Python 3.8.
Gianfranco Costamagna fixed a compiler warning.
risa2000 made std::filesystem::path conversion to/from UTF-8 encoded string explicit.

Thanks a lot for helping out! Please let me know if I forgot someone.

Used third-party tools

The library itself consists of a single header file licensed under the MIT license. However, it is built, tested, documented, and whatnot using a lot of third-party tools and services. Thanks a lot!

amalgamate.py - Amalgamate C source and header files to create a single header file
American fuzzy lop for fuzz testing
AppVeyor for continuous integration on Windows
Artistic Style for automatic source code indentation
Clang for compilation with code sanitizers
CMake for build automation
Codacy for further code analysis
Coveralls to measure code coverage
Coverity Scan for static analysis
cppcheck for static analysis
doctest for the unit tests
GitHub Changelog Generator to generate the ChangeLog
Google Benchmark to implement the benchmarks
Hedley to avoid re-inventing several compiler-agnostic feature macros
lcov to process coverage information and create an HTML view
libFuzzer to implement fuzz testing for OSS-Fuzz
Material for MkDocs for the style of the documentation site
MkDocs for the documentation site
OSS-Fuzz for continuous fuzz testing of the library (project repository)
Probot for automating maintainer tasks such as closing stale issues, requesting missing information, or detecting toxic comments.
Valgrind to check for correct memory management

Notes

Standards compliance

The library targets strict conformance with RFC 8259. Both the original JSONTestSuite and its updated revision are exercised in CI; their test data is downloaded from nlohmann/json_test_data at configure time rather than committed to this repository (see tests/src/unit-testsuites.cpp):

The updated revision runs all mandatory y_ (must-accept) and n_ (must-reject) cases through the strict parse() entry point; the original suite runs its n_ cases through parse() and its y_ cases through operator>>.
The i_ (implementation-defined) cases are, by RFC 8259, free to be accepted or rejected, so "passing all i_ cases" is not a meaningful conformance metric. The library makes deliberate, documented choices there: nesting depth is not artificially limited, a leading UTF-8 byte order mark is silently ignored, Unicode noncharacters are forwarded unchanged, invalid UTF-8 and lone/unpaired UTF-16 surrogates are rejected (stricter than required), and a number that cannot be stored without becoming NaN/INF raises out_of_range.406.

One behavioral nuance is worth calling out, because a superficial test often misreads it as non-compliance: parse() is strict and rejects trailing data after a value, whereas operator>> follows relaxed iostream semantics — it parses a single value and leaves the stream positioned right after it. Feeding "a valid document followed by trailing bytes" through operator>> reports success; the same input through parse() is rejected. This is a documented two-API design, not a conformance gap. See parsing for details.

Character encoding

The library supports Unicode input as follows:

Only UTF-8 encoded input is supported, which is the default encoding for JSON according to RFC 8259.
std::u16string and std::u32string can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
Unicode noncharacters will not be replaced by the library.
Invalid surrogates (e.g., incomplete pairs such as \uDEAD) will yield parse errors.
The strings stored in the library are UTF-8 encoded. When using the default string type (std::string), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace or json::error_handler_t::ignore are used as error handlers.
To store wide strings (e.g., std::wstring), you need to convert them to a UTF-8 encoded std::string before, see an example.

Comments in JSON

This library does not support comments by default. It does so for three reasons:

Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.
This was not an oversight: Douglas Crockford wrote on this in May 2012:

I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
It is dangerous for interoperability if some libraries would add comment support while others don't. Please check The Harmful Consequences of the Robustness Principle on this.

However, you can set parameter ignore_comments to true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace.

Trailing commas

The JSON specification does not allow trailing commas in arrays and objects, and hence this library is treating them as parsing errors by default.

Like comments, you can set parameter ignore_trailing_commas to true in the parse function to ignore trailing commas in arrays and objects. Note that a single comma as the only content of the array or object ([,] or {,}) is not allowed, and multiple trailing commas ([1,,]) are not allowed either.

This library does not add trailing commas when serializing JSON data.

For more information, see JSON With Commas and Comments (JWCC).

Order of object keys

By default, the library does not preserve the insertion order of object elements. This is standards-compliant, as the JSON standard defines objects as "an unordered collection of zero or more name/value pairs".

If you do want to preserve the insertion order, you can try the type nlohmann::ordered_json. Alternatively, you can use a more sophisticated ordered map like tsl::ordered_map (integration) or nlohmann::fifo_map (integration).

See the documentation on object order for more information.

Memory Release

We checked with Valgrind and the Address Sanitizer (ASAN) that there are no memory leaks.

If you find that a parsing program with this library does not release memory, please consider the following case, and it may be unrelated to this library.

Your program is compiled with glibc. There is a tunable threshold that glibc uses to decide whether to actually return memory to the system or whether to cache it for later reuse. If in your program you make lots of small allocations and those small allocations are not a contiguous block and are presumably below the threshold, then they will not get returned to the OS.
Here is a related issue #1924.

Further notes

The code contains numerous debug assertions which can be switched off by defining the preprocessor macro NDEBUG, see the documentation of assert. In particular, note operator[] implements unchecked access for const objects: If the given key is not present, the behavior is undefined (think of a dereferenced null pointer) and yields an assertion failure if assertions are switched on. If you are not sure whether an element in an object exists, use checked access with the at() function. Furthermore, you can define JSON_ASSERT(x) to replace calls to assert(x). See the documentation on runtime assertions for more information.
As the exact number type is not defined in the JSON specification, this library tries to choose the best fitting C++ number type automatically. As a result, the type double may be used to store numbers which may yield floating-point exceptions in certain rare situations if floating-point exceptions have been unmasked in the calling code. These exceptions are not caused by the library and need to be fixed in the calling code, such as by re-masking the exceptions prior to calling library functions.
The code can be compiled without C++ runtime type identification features; that is, you can use the -fno-rtti compiler flag.
Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag -fno-exceptions or by defining the symbol JSON_NOEXCEPTION. In this case, exceptions are replaced by abort() calls. You can further control this behavior by defining JSON_THROW_USER (overriding throw), JSON_TRY_USER (overriding try), and JSON_CATCH_USER (overriding catch). Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior. Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824. See the documentation of exceptions for more information.

Execute unit tests

To compile and run the tests, you need to execute

shell

mkdir build
cd build
cmake .. -DJSON_BuildTests=On
cmake --build .
ctest --output-on-failure

Note that during the ctest stage, several JSON test files are downloaded from an external repository. If policies forbid downloading artifacts during testing, you can download the files yourself and pass the directory with the test files via -DJSON_TestDataDirectory=path to CMake. Then, no Internet connectivity is required. See issue #2189 for more information.

If the testdata is not found, several test suites will fail like this:

text

===============================================================================
json/tests/src/make_test_data_available.hpp:21:
TEST CASE:  check test suite is downloaded

json/tests/src/make_test_data_available.hpp:23: FATAL ERROR: REQUIRE( utils::check_testsuite_downloaded() ) is NOT correct!
  values: REQUIRE( false )
  logged: Test data not found in 'json/cmake-build-debug/json_test_data'.
          Please execute target 'download_test_data' before running this test suite.
          See <https://github.com/nlohmann/json#execute-unit-tests> for more information.

===============================================================================

In case you have downloaded the library rather than checked out the code via Git, test cmake_fetch_content_configure will fail. Please execute ctest -LE git_required to skip these tests. See issue #2189 for more information.

Some tests are requiring network to be properly execute. They are labeled as git_required. Please execute ctest -LE git_required to skip these tests. See issue #4851 for more information.

Some tests change the installed files and hence make the whole process not reproducible. Please execute ctest -LE not_reproducible to skip these tests. See issue #2324 for more information. Furthermore, assertions must be switched off to ensure reproducible builds (see discussion 4494).

Note you need to call cmake -LE "not_reproducible|git_required" to exclude both labels. See issue #2596 for more information.

As Intel compilers use unsafe floating point optimization by default, the unit tests may fail. Use flag /fp:precise then.

File: docs/docset/README.md

docset

The folder contains the required files to create a docset which can be used in
documentation browsers like Dash, Velocity, or
Zeal.

The docset can be created with

make nlohmann_json.docset

The generated folder nlohmann_json.docset can then be opened in the documentation browser.

A recent version is also part of the Dash user contributions.

Licenses

The JSON logo is public domain.

File: docs/mkdocs/includes/glossary.md

*[ADL]: Argument-dependent lookup
*[API]: Application Programming Interfaces
*[ASCII]: American Standard Code for Information Interchange
*[BDFL]: Benevolent Dictator for Life
*[BJData]: Binary JData
*[BSON]: Binary JSON
*[CBOR]: Concise Binary Object Representation
*[CC0]: Creative Commons Zero
*[CI]: Continuous Integration
*[DOM]: Document Object Model
*[EOF]: End of File
*[FAQ]: Frequently Asked Questions
*[GCC]: GNU Compiler Collection
*[HTTP]: Hypertext Transfer Protocol
*[ICC]: Intel C++ Compiler
*[IEEE]: Institute of Electrical and Electronics Engineers
*[ISO]: International Organization for Standardization
*[JSON]: JavaScript Object Notation
*[MIT]: Massachusetts Institute of Technology
*[MSVC]: Microsoft Visual C++
*[MsgPack]: MessagePack
*[NASA]: National Aeronautics and Space Administration
*[NDK]: Native Development Kit
*[NaN]: Not a Number
*[RFC]: Request for Comments
*[RTTI]: Runtime Type Information
*[SAX]: Simple API for XML
*[SDK]: Software Development Kit
*[SFINAE]: Substitution failure is not an error
*[SHA]: Secure Hash Algorithm
*[SPDX]: Software Package Data Exchange
*[STL]: Standard Template Library
*[UBJSON]: Universal Binary JSON
*[UTF]: Unicode Transformation Format

File: docs/mkdocs/docs/home/architecture.md

Architecture

!!! info

text

This page is still under construction. Its goal is to provide a high-level overview of the library's architecture.
This should help new contributors to get an idea of the used concepts and where to make changes.

Overview

The main structure is class nlohmann::basic_json.

public API
container interface
iterators

Template specializations

describe template parameters of basic_json
json
ordered_json via ordered_map

Value storage

Values are stored as a tagged union of value_t and json_value.

cpp

/// the type of the current element
value_t m_type = value_t::null;

/// the value of the current element
json_value m_value = {};

with

cpp

enum class value_t : std::uint8_t
{
    null,             ///< null value
    object,           ///< object (unordered set of name/value pairs)
    array,            ///< array (ordered collection of values)
    string,           ///< string value
    boolean,          ///< boolean value
    number_integer,   ///< number value (signed integer)
    number_unsigned,  ///< number value (unsigned integer)
    number_float,     ///< number value (floating-point)
    binary,           ///< binary array (ordered collection of bytes)
    discarded         ///< discarded by the parser callback function
};

union json_value {
  /// object (stored with pointer to save storage)
  object_t *object;
  /// array (stored with pointer to save storage)
  array_t *array;
  /// string (stored with pointer to save storage)
  string_t *string;
  /// binary (stored with pointer to save storage)
  binary_t *binary;
  /// boolean
  boolean_t boolean;
  /// number (integer)
  number_integer_t number_integer;
  /// number (unsigned integer)
  number_unsigned_t number_unsigned;
  /// number (floating-point)
  number_float_t number_float;
};

Parsing inputs (deserialization)

Input is read via input adapters that abstract a source with a common interface:

cpp

/// read a single character
std::char_traits<char>::int_type get_character() noexcept;

/// read multiple characters to a destination buffer and
/// returns the number of characters successfully read
template<class T>
std::size_t get_elements(T* dest, std::size_t count = 1);

List examples of input adapters.

SAX Interface

TODO

Writing outputs (serialization)

Output is written via output adapters:

cpp

template<typename T>
void write_character(CharType c);

template<typename CharType>
void write_characters(const CharType* s, std::size_t length);

List examples of output adapters.

Value conversion

cpp

template<class T>
void to_json(basic_json& j, const T& t);

template<class T>
void from_json(const basic_json& j, T& t);

Additional features

JSON Pointers
Binary formats
Custom base class
Conversion macros

Details namespace

C++ feature backports

File: docs/mkdocs/docs/home/customers.md

Customers

The library is used in multiple projects, applications, operating systems, etc. The list below is not exhaustive, but
the result of an internet search. If you know further customers of the library, please let me know.

Space Exploration

Peregrine Lunar Lander Flight 01 - The library was used for payload management in the Peregrine Moon Lander, developed by Astrobotic Technology and launched as part of NASA's Commercial Lunar Payload Services (CLPS) program. After six days in orbit, the spacecraft was intentionally redirected into Earth's atmosphere, where it burned up over the Pacific Ocean on January 18, 2024.

Automotive

Alexa Auto SDK, a software development kit enabling the integration of Alexa into automotive systems
Apollo, a framework for building autonomous driving systems
Automotive Grade Linux (AGL), a collaborative open-source platform for automotive software development
Genesis Motor (infotainment), a luxury automotive brand
Hyundai (infotainment), a global automotive brand
Kia (infotainment), a global automotive brand
Mercedes-Benz Operating System (MB.OS), a core component of the vehicle software ecosystem from Mercedes-Benz
Rivian (infotainment), an electric vehicle manufacturer
Suzuki (infotainment), a global automotive and motorcycle manufacturer

Gaming and Entertainment

Assassin's Creed: Mirage, a stealth-action game set in the Middle East, focusing on the journey of a young assassin with classic parkour and stealth mechanics
Chasm: The Rift, a first-person shooter blending horror and adventure, where players navigate dark realms and battle monsters
College Football 25, a college football simulation game featuring gameplay that mimics real-life college teams and competitions
Concepts, a digital sketching app designed for creative professionals, offering flexible drawing tools for illustration, design, and brainstorming
Depthkit, a tool for creating and capturing volumetric video, enabling immersive 3D experiences and interactive content
IMG.LY, a platform offering creative tools and SDKs for integrating advanced image and video editing in applications
LOOT, a tool for optimizing the load order of game plugins, commonly used in The Elder Scrolls and Fallout series
Madden NFL 25, a sports simulation game capturing the excitement of American football with realistic gameplay and team management features
Marne, an unofficial private server platform for hosting custom Battlefield 1 game experiences
Minecraft, a popular sandbox video game
NHL 22, a hockey simulation game offering realistic gameplay, team management, and various modes to enhance the hockey experience
Pixelpart, a 2D animation and video compositing software that allows users to create animated graphics and visual effects with a focus on simplicity and ease of use
Razer Cortex, a gaming performance optimizer and system booster designed to enhance the gaming experience
Red Dead Redemption II, an open-world action-adventure game following an outlaw's story in the late 1800s, emphasizing deep storytelling and immersive gameplay
Snapchat, a multimedia messaging and augmented reality app for communication and entertainment
Tactics Ogre: Reborn, a tactical role-playing game featuring strategic battles and deep storytelling elements
Throne and Liberty, an MMORPG that offers an expansive fantasy world with dynamic gameplay and immersive storytelling
Unity Vivox, a communication service that enables voice and text chat functionality in multiplayer games developed with Unity
Zool: Redimensioned, a modern reimagining of the classic platformer featuring fast-paced gameplay and vibrant environments
immersivetech, a technology company focused on immersive experiences, providing tools and solutions for virtual and augmented reality applications

Consumer Electronics

Audinate, a provider of networked audio solutions specializing in Dante technology, which facilitates high-quality digital audio transport over IP networks
Canon CanoScan LIDE, a series of flatbed scanners offering high-resolution image scanning for home and office use
Canon PIXMA Printers, a line of all-in-one inkjet printers known for high-quality printing and wireless connectivity
Cisco Webex Desk Camera, a video camera designed for professional-quality video conferencing and remote collaboration
Philips Hue Personal Wireless Lighting, a smart lighting system for customizable and wireless home illumination
Ray-Ban Meta Smart glasses, a pair of smart glasses designed for capturing photos and videos with integrated connectivity and social features
Razer Synapse, a unified configuration software enabling hardware customization for Razer devices
Siemens SINEMA Remote Connect, a remote connectivity solution for monitoring and managing industrial networks and devices securely
Sony PlayStation 4, a gaming console developed by Sony that offers a wide range of games and multimedia entertainment features
Sony Virtual Webcam Driver for Remote Camera, a software driver that enables the use of Sony cameras as virtual webcams for video conferencing and streaming

Operating Systems

Apple iOS and macOS, a family of operating systems developed by Apple, including iOS for mobile devices and macOS for desktop computers
Google Fuchsia, an open-source operating system developed by Google, designed to be secure, updatable, and adaptable across various devices
SerenityOS, an open-source operating system that aims to provide a simple and beautiful user experience with a focus on simplicity and elegance
Yocto, a Linux-based build system for creating custom operating systems and software distributions, tailored for embedded devices and IoT applications

Development Tools and IDEs

Accentize SpectralBalance, an adaptive speech analysis tool designed to enhance audio quality by optimizing frequency balance in recordings
Arm Compiler for Linux, a software development toolchain for compiling and optimizing applications on Arm-based Linux systems
BBEdit, a professional text and code editor for macOS
CoderPad, a collaborative coding platform that enables real-time code interviews and assessments for developers; the library is included in every CoderPad instance and can be accessed with a simple #include "json.hpp"
Compiler Explorer, a web-based tool that allows users to write, compile, and visualize the assembly output of code in various programming languages; the library is readily available and accessible with the directive #include <nlohmann/json.hpp>.
GitHub CodeQL, a code analysis tool used for identifying security vulnerabilities and bugs in software through semantic queries
Hex-Rays, a reverse engineering toolset for analyzing and decompiling binaries, primarily used for security research and vulnerability analysis
ImHex, a hex editor designed for reverse engineering, providing advanced features for data analysis and manipulation
Intel GPA Framework, a suite of cross-platform tools for capturing, analyzing, and optimizing graphics applications across different APIs
Intopix, a provider of advanced image processing and compression solutions used in software development and AV workflows
Java SE, the core Java platform that provides the libraries and runtime needed to build and run general-purpose Java applications
MKVToolNix, a set of tools for creating, editing, and inspecting MKV (Matroska) multimedia container files
Meta Yoga, a layout engine that facilitates flexible and efficient user interface design across multiple platforms
NVIDIA Nsight Compute, a performance analysis tool for CUDA applications that provides detailed insights into GPU performance metrics
Notepad++, a free source code editor that supports various programming languages
OpenRGB, an open source RGB lighting control that doesn't depend on manufacturer software
OpenTelemetry C++, a library for collecting and exporting observability data in C++, enabling developers to implement distributed tracing and metrics in their application
Qt Creator, an IDE for developing applications using the Qt application framework
Scanbot SDK, a software development kit (SDK) that provides tools for integrating advanced document scanning and barcode scanning capabilities into applications

Machine Learning and AI

Apple Core ML Tools, a set of tools for converting and configuring machine learning models for deployment in Apple's Core ML framework
Avular Mobile Robotics, a platform for developing and deploying mobile robotics solutions
Google gemma.cpp, a lightweight C++ inference engine designed for running AI models from the Gemma family
llama.cpp, a C++ library designed for efficient inference of large language models (LLMs), enabling streamlined integration into applications
MLX, an array framework for machine learning on Apple Silicon
Mozilla llamafile, a tool designed for distributing and executing large language models (LLMs) efficiently using a single file format
NVIDIA ACE, a suite of real-time AI solutions designed for the development of interactive avatars and digital human applications, enabling scalable and sophisticated user interactions
Peer, a platform offering personalized AI assistants for interactive learning and creative collaboration
stable-diffusion.cpp, a C++ implementation of the Stable Diffusion image generation model
TanvasTouch, a software development kit (SDK) that enables developers to create tactile experiences on touchscreens, allowing users to feel textures and physical sensations in a digital environment
TensorFlow, a machine learning framework that facilitates the development and training of models, supporting data serialization and efficient data exchange between components

Scientific Research and Analysis

BLACK, a bounded linear temporal logic (LTL) satisfiability checker
CERN Atlas Athena, a software framework used in the ATLAS experiment at the Large Hadron Collider (LHC) for performance monitoring
ICU, the International Components for Unicode, a mature library for software globalization and multilingual support
KAMERA, a platform for synchronized data collection and real-time deep learning to map marine species like polar bears and seals, aiding Arctic ecosystem research
KiCad, a free and open-source software suite for electronic design automation
Maple, a symbolic and numeric computing environment for advanced mathematical modeling and analysis
MeVisLab, a software framework for medical image processing and visualization.
OpenPMD API, a versatile programming interface for accessing and managing scientific data, designed to facilitate the efficient storage, retrieval, and sharing of simulation data across various applications and platforms
ParaView, an open-source tool for large-scale data visualization and analysis across various scientific domains
QGIS, a free and open-source geographic information system (GIS) application that allows users to create, edit, visualize, and analyze geospatial data across a variety of formats
VTK, a software library for 3D computer graphics, image processing, and visualization
VolView, a lightweight application for interactive visualization and analysis of 3D medical imaging data.

Business and Productivity Software

ArcGIS PRO, a desktop geographic information system (GIS) application developed by Esri for mapping and spatial analysis
Autodesk Desktop, a software platform developed by Autodesk for creating and managing desktop applications and services
Check Point, a cybersecurity company specializing in threat prevention and network security solutions, offering a range of products designed to protect enterprises from cyber threats and ensure data integrity
Microsoft Office for Mac, a suite of productivity applications developed by Microsoft for macOS, including tools for word processing, spreadsheets, and presentations
Microsoft Teams, a team collaboration application offering workspace chat and video conferencing, file storage, and integration of proprietary and third-party applications and services
Nexthink Infinity, a digital employee experience management platform for monitoring and improving IT performance
Sophos Connect Client, a secure VPN client from Sophos that allows remote users to connect to their corporate network, ensuring secure access to resources and data
Stonebranch, a cloud-based cybersecurity solution that integrates backup, disaster recovery, and cybersecurity features to protect data and ensure business continuity for organizations
Tablecruncher, a data analysis tool that allows users to import, analyze, and visualize spreadsheet data, offering interactive features for better insights and decision-making
magicplan, a mobile application for creating floor plans and interior designs using augmented reality

Databases and Big Data

ADIOS2, a data management framework designed for high-performance input and output operations
Cribl Stream, a real-time data processing platform that enables organizations to collect, route, and transform observability data, enhancing visibility and insights into their systems
DB Browser for SQLite, a visual open-source tool for creating, designing, and editing SQLite database files
MySQL Connector/C++, a C++ library for connecting and interacting with MySQL databases
MySQL NDB Cluster, a distributed database system that provides high availability and scalability for MySQL databases
MySQL Shell, an advanced client and code editor for interacting with MySQL servers, supporting SQL, Python, and JavaScript
PrestoDB, a distributed SQL query engine designed for large-scale data analytics, originally developed by Facebook
ROOT Data Analysis Framework, an open-source data analysis framework widely used in high-energy physics and other fields for data processing and visualization
WiredTiger, a high-performance storage engine for databases, offering support for compression, concurrency, and checkpointing

Simulation and Modeling

Arcturus HoloSuite, a software toolset for capturing, editing, and streaming volumetric video, featuring advanced compression technologies for high-quality 3D content creation
azul, a fast and efficient 3D city model viewer designed for visualizing urban environments and spatial data
Blender, a free and open-source 3D creation suite for modeling, animation, rendering, and more
cpplot, a library for creating interactive graphs and charts in C++, which can be viewed in web browsers
Foundry Nuke, a powerful node-based digital compositing and visual effects application used in film and television post-production
GAMS, a high-performance mathematical modeling system for optimization and decision support
Kitware SMTK, a software toolkit for managing simulation models and workflows in scientific and engineering applications
M-Star, a computational fluid dynamics software for simulating and analyzing fluid flow
MapleSim CAD Toolbox, a software extension for MapleSim that integrates CAD models, allowing users to import, manipulate, and analyze 3D CAD data within the MapleSim environment for enhanced modeling and simulation
NVIDIA Omniverse, a platform for 3D content creation and collaboration that enables real-time simulations and interactive experiences across various industries
Pixar Renderman, a photorealistic 3D rendering software developed by Pixar, widely used in the film industry for creating high-quality visual effects and animations
ROS - Robot Operating System, a set of software libraries and tools that assist in developing robot applications
UBS, a multinational financial services and banking company

Enterprise and Cloud Applications

Acronis Cyber Protect Cloud, an all-in-one data protection solution that combines backup, disaster recovery, and cybersecurity to safeguard business data from threats like ransomware
Baereos, a backup solution that provides data protection and recovery options for various environments, including physical and virtual systems
Bitdefender Home Scanner, a tool from Bitdefender that scans devices for malware and security threats, providing a safeguard against potential online dangers
Citrix Provisioning, a solution that streamlines the delivery of virtual desktops and applications by allowing administrators to manage and provision resources efficiently across multiple environments
Citrix Virtual Apps and Desktops, a solution from Citrix that delivers virtual apps and desktops
Cyberarc, a security solution that specializes in privileged access management, enabling organizations to control and monitor access to critical systems and data, thereby enhancing overall cybersecurity posture
Egnyte Desktop, a secure cloud storage solution designed for businesses, enabling file sharing, collaboration, and data management across teams while ensuring compliance and data protection
Elster, a digital platform developed by German tax authorities for secure and efficient electronic tax filing and management using secunet protect4use
Ethereum Solidity, a high-level, object-oriented programming language designed for implementing smart contracts on the Ethereum platform
Inciga, a monitoring tool for IT infrastructure, designed to provide insights into system performance and availability through customizable dashboards and alerts
Intel Accelerator Management Daemon for VMware ESXi, a management tool designed for monitoring and controlling Intel hardware accelerators within VMware ESXi environments, optimizing performance and resource allocation
Juniper Identity Management Service
Microsoft Azure IoT SDK, a collection of tools and libraries to help developers connect, build, and deploy Internet of Things (IoT) solutions on the Azure cloud platform
Microsoft WinGet, a command-line utility included in the Windows Package Manager
plexusAV, a high-performance AV-over-IP transceiver device capable of video encoding and decoding using the IPMX standard
Pointr, a platform for indoor positioning and navigation solutions, offering tools and SDKs for developers to create location-based applications
secunet protect4use, a secure, passwordless multifactor authentication solution that transforms smartphones into digital keyrings, ensuring high security for online services and digital identities
Sencore MRD 7000, a professional multi-channel receiver and decoder supporting UHD and HD stream decoding

File: docs/mkdocs/docs/home/design_goals.md

Design goals

There are myriads of JSON libraries out there, and each may even have its reason to exist. Our class had these design goals:

Intuitive syntax. In languages such as Python, JSON feels like a first-class data type. We used all the operator magic of modern C++ to achieve the same feeling in your code.
Trivial integration. Our whole code consists of a single header file json.hpp. That's it. No library, no subproject, no dependencies, no complex build system. The class is written in vanilla C++11. All in all, everything should require no adjustment of your compiler flags or project settings.
Serious testing. Our class is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind and the Clang Sanitizers that there are no memory leaks. Google OSS-Fuzz additionally runs fuzz tests against all parsers 24/7, effectively executing billions of tests so far. To maintain high quality, the project is following the Core Infrastructure Initiative (CII) best practices.

Other aspects were not so important to us:

Memory efficiency. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). The default generalization uses the following C++ data types: std::string for strings, int64_t, uint64_t or double for numbers, std::map for objects, std::vector for arrays, and bool for Booleans. However, you can template the generalized class basic_json to your needs.
Speed. There are certainly faster JSON libraries out there. However, if your goal is to speed up your development by adding JSON support with a single header, then this library is the way to go. If you know how to use a std::vector or std::map, you are already set.

See the contribution guidelines for more information.

File: docs/mkdocs/docs/home/exceptions.md

Exceptions

Overview

Base type

All exceptions inherit from class json::exception (which in turn inherits from std::exception). It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as "wildcard" to catch exceptions.

mermaid

classDiagram
  direction LR
    class `std::exception` {
        <>
    }

    class `json::exception` {
        +const int id
        +const char* what() const
    }

    class `json::parse_error` {
        +const std::size_t byte
    }

    class `json::invalid_iterator`
    class `json::type_error`
    class `json::out_of_range`
    class `json::other_error`

    `std::exception` <|-- `json::exception`
    `json::exception` <|-- `json::parse_error`
    `json::exception` <|-- `json::invalid_iterator`
    `json::exception` <|-- `json::type_error`
    `json::exception` <|-- `json::out_of_range`
    `json::exception` <|-- `json::other_error`

Switch off exceptions

Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag -fno-exceptions or by defining the symbol JSON_NOEXCEPTION. In this case, exceptions are replaced by abort() calls. You can further control this behavior by defining JSON_THROW_USER (overriding #!cpp throw), JSON_TRY_USER (overriding #!cpp try), and JSON_CATCH_USER (overriding #!cpp catch).

Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.

??? example

text

The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

```cpp
#include 

#define JSON_TRY_USER if(true)
#define JSON_CATCH_USER(exception) if(false)
#define JSON_THROW_USER(exception)                           \
    {std::clog << "Error in " << __FILE__ << ":" << __LINE__ \
               << " (function " << __FUNCTION__ << ") - "    \
               << (exception).what() << std::endl;           \
     std::abort();}

#include <nlohmann/json.hpp>
```

Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

See documentation of JSON_TRY_USER, JSON_CATCH_USER and JSON_THROW_USER for more information.

Extended diagnostic messages

Exceptions in the library are thrown in the local context of the JSON value they are detected. This makes detailed diagnostics messages, and hence debugging, difficult.

??? example

text

```cpp
--8<-- "examples/diagnostics_standard.cpp"
```

Output:

```
--8<-- "examples/diagnostics_standard.output"
```

This exception can be hard to debug if storing the value `#!c "12"` and accessing it is further apart.

To create better diagnostics messages, each JSON value needs a pointer to its parent value such that a global context (i.e., a path from the root value to the value that led to the exception) can be created. That global context is provided as JSON Pointer.

As this global context comes at the price of storing one additional pointer per JSON value and runtime overhead to maintain the parent relation, extended diagnostics are disabled by default. They can, however, be enabled by defining the preprocessor symbol JSON_DIAGNOSTICS to 1 before including json.hpp.

??? example

text

```cpp
--8<-- "examples/diagnostics_extended.cpp"
```

Output:

```
--8<-- "examples/diagnostics_extended.output"
```

Now the exception message contains a JSON Pointer `/address/housenumber` that indicates which value has the wrong type.

See documentation of JSON_DIAGNOSTICS for more information.

Parse errors

The library throws this exception when a parse error occurs. Parse errors
can occur during the deserialization of JSON text, CBOR, MessagePack, as well
as when using JSON Patch.

Exceptions have ids 1xx.

!!! info "Byte index"

text

Member `byte` holds the byte index of the last read character in the input
file.

For an input with n bytes, 1 is the index of the first character and n+1
is the index of the terminating null byte or the end of file. This also
holds true when reading a byte vector (CBOR or MessagePack).

??? example

text

The following code shows how a `parse_error` exception can be caught.

```cpp
--8<-- "examples/parse_error.cpp"
```

Output:

```
--8<-- "examples/parse_error.output"
```

json.exception.parse_error.101

This error indicates a syntax error while deserializing a JSON text. The error message describes that an unexpected token (character) was encountered, and the member byte indicates the error position.

!!! failure "Example message"

text

Input ended prematurely:

```
[json.exception.parse_error.101] parse error at 2: unexpected end of input; expected string literal
```

No input:

```
[json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON
```

Control character was not escaped:

```
[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \u0009 or \\; last read: '"'"
```

String was not closed:

```
[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '"'
```

Invalid number format:

```
[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E'
```

`\u` was not be followed by four hex digits:

```
[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\u' must be followed by 4 hex digits; last read: '"\u01"'
```

Invalid UTF-8 surrogate pair:

```
[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '"\uD7FF\uDC00'"
```

Invalid UTF-8 byte:

```
[json.exception.parse_error.101] parse error at line 3, column 24: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"vous \352t'
```

!!! tip

text

- Make sure the input is correctly read. Try to write the input to standard output to check if, for instance, the input file was successfully opened.
- Paste the input to a JSON validator like <http://jsonlint.com> or a tool like [jq](https://stedolan.github.io/jq/).

json.exception.parse_error.102

JSON uses the \uxxxx format to describe Unicode characters. Code points above 0xFFFF are split into two \uxxxx entries ("surrogate pairs"). This error indicates that the surrogate pair is incomplete or contains an invalid code point.

!!! failure "Example message"

text

```
parse error at 14: missing or wrong low surrogate
```

!!! note

text

This exception is not used any more. Instead [json.exception.parse_error.101](#jsonexceptionparse_error101) with a more detailed description is used.

json.exception.parse_error.103

Unicode supports code points up to 0x10FFFF. Code points above 0x10FFFF are invalid.

!!! failure "Example message"

text

```
parse error: code points above 0x10FFFF are invalid
```

!!! note

text

This exception is not used any more. Instead [json.exception.parse_error.101](#jsonexceptionparse_error101) with a more detailed description is used.

json.exception.parse_error.104

RFC 6902 requires a JSON Patch document to be a JSON document that represents an array of objects.

!!! failure "Example message"

text

```
[json.exception.parse_error.104] parse error: JSON patch must be an array of objects
```

json.exception.parse_error.105

An operation of a JSON Patch document must contain exactly one "op" member, whose value indicates the operation to perform. Its value must be one of "add", "remove", "replace", "move", "copy", or "test"; other values are errors.

!!! failure "Example message"

text

```
[json.exception.parse_error.105] parse error: operation 'add' must have member 'value'
```
```
[json.exception.parse_error.105] parse error: operation 'copy' must have string member 'from'
```
```
[json.exception.parse_error.105] parse error: operation value 'foo' is invalid
```

json.exception.parse_error.106

An array index in a JSON Pointer (RFC 6901) may be 0 or any number without a leading 0.

!!! failure "Example message"

text

```
[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'
```

json.exception.parse_error.107

A JSON Pointer must be a Unicode string containing a sequence of zero or more reference tokens, each prefixed by a / character.

!!! failure "Example message"

text

```
[json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'
```

json.exception.parse_error.108

In a JSON Pointer, only ~0 and ~1 are valid escape sequences.

!!! failure "Example message"

text

```
[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'
```

json.exception.parse_error.109

A JSON Pointer array index must be a number.

!!! failure "Example messages"

text

```
[json.exception.parse_error.109] parse error: array index 'one' is not a number
```
```
[json.exception.parse_error.109] parse error: array index '+1' is not a number
```

json.exception.parse_error.110

When parsing a binary format, the byte vector ends before the complete value has
been read.

!!! failure "Example messages"

text

```
[json.exception.parse_error.110] parse error at byte 5: syntax error while parsing CBOR string: unexpected end of input
```
```
[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing UBJSON value: expected end of input; last byte: 0x5A
```
```
[json.exception.parse_error.110] parse error at byte 8: syntax error while parsing BSON number: unexpected end of input
```

json.exception.parse_error.112

An unexpected byte was read in a binary format or length information is invalid (BSON).

!!! failure "Example messages"

text

```
[json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0x1C
```
```
[json.exception.parse_error.112] parse error at byte 1: syntax error while parsing MessagePack value: invalid byte: 0xC1
```
```
[json.exception.parse_error.112] parse error at byte 4: syntax error while parsing BJData size: expected '#' after type information; last byte: 0x02
```
```
[json.exception.parse_error.112] parse error at byte 4: syntax error while parsing UBJSON size: expected '#' after type information; last byte: 0x02
```
```
[json.exception.parse_error.112] parse error at byte 10: syntax error while parsing BSON string: string length must be at least 1, is -2147483648
```
```
[json.exception.parse_error.112] parse error at byte 15: syntax error while parsing BSON binary: byte array length cannot be negative, is -1
```
```
[json.exception.parse_error.112] parse error at byte 9: syntax error while parsing CBOR value: negative integer overflow
```
```
[json.exception.parse_error.112] parse error at byte 5: syntax error while parsing BSON document: document size 6 does not match the number of bytes read (5)
```

json.exception.parse_error.113

A string could not be read from a binary format: either a value that is not a
string was read where one was required (for instance as a map key), or the string's length specification is invalid.

!!! failure "Example messages"

text

```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF
```
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xFF
```
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing UBJSON char: byte after 'C' must be in range 0x00..0x7F; last byte: 0x82
```
```
[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing BJData string: string length must not be negative
```

json.exception.parse_error.114

The parsing of the corresponding BSON record type is not implemented (yet).

!!! failure "Example message"

text

```
[json.exception.parse_error.114] parse error at byte 5: Unsupported BSON record type 0xFF
```

json.exception.parse_error.115

A UBJSON high-precision number could not be parsed.

!!! failure "Example message"

text

```
[json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A
```

Iterator errors

This exception is thrown if iterators passed to a library function do not match
the expected semantics.

Exceptions have ids 2xx.

??? example

text

The following code shows how an `invalid_iterator` exception can be caught.

```cpp
--8<-- "examples/invalid_iterator.cpp"
```

Output:

```
--8<-- "examples/invalid_iterator.output"
```

json.exception.invalid_iterator.201

The iterators passed to constructor basic_json(InputIT first, InputIT last) are not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.201] iterators are not compatible
```

json.exception.invalid_iterator.202

In the erase or insert function, the passed iterator pos does not belong to the JSON value for which the function was called. It hence does not define a valid position for the deletion/insertion.

!!! failure "Example messages"

text

```
[json.exception.invalid_iterator.202] iterator does not fit current value
```
```
[json.exception.invalid_iterator.202] iterators first and last must point to objects
```

json.exception.invalid_iterator.203

Either iterator passed to function erase(IteratorType first, IteratorType last) does not belong to the JSON value from which values shall be erased. It hence does not define a valid range to delete values from.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.203] iterators do not fit current value
```

json.exception.invalid_iterator.204

When an iterator range for a primitive type (number, boolean, or string) is passed to a constructor or an erase function, this range has to be exactly (begin(), end()), because this is the only way the single stored value is expressed. All other ranges are invalid.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.204] iterators out of range
```

json.exception.invalid_iterator.205

When an iterator for a primitive type (number, boolean, or string) is passed to an erase function, the iterator has to be the begin() iterator, because it is the only way to address the stored value. All other iterators are invalid.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.205] iterator out of range
```

json.exception.invalid_iterator.206

The iterators passed to constructor basic_json(InputIT first, InputIT last) belong to a JSON null value and hence to not define a valid range.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.206] cannot construct with iterators from null
```

json.exception.invalid_iterator.207

The key() member function can only be used on iterators belonging to a JSON object, because other types do not have a concept of a key.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.207] cannot use key() for non-object iterators
```

json.exception.invalid_iterator.208

The operator[] to specify a concrete offset cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.208] cannot use operator[] for object iterators
```

json.exception.invalid_iterator.209

The offset operators (+, -, +=, -=) cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.209] cannot use offsets with object iterators
```

json.exception.invalid_iterator.210

The iterator range passed to the insert function is not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.210] iterators do not fit
```

json.exception.invalid_iterator.211

The iterator range passed to the insert function must not be a subrange of the container to insert to.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.211] passed iterators may not belong to container
```

json.exception.invalid_iterator.212

When two iterators are compared, they must belong to the same container.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.212] cannot compare iterators of different containers
```

json.exception.invalid_iterator.213

The order of object iterators cannot be compared, because JSON objects are unordered.

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.213] cannot compare order of object iterators
```

json.exception.invalid_iterator.214

Cannot retrieve value from iterator: The iterator either refers to a null value, or it refers to a primitive type (number, boolean, or string), but does not match the iterator returned by begin().

!!! failure "Example message"

text

```
[json.exception.invalid_iterator.214] cannot get value
```

Type errors

This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.

Exceptions have ids 3xx.

??? example

text

The following code shows how a `type_error` exception can be caught.

```cpp
--8<-- "examples/type_error.cpp"
```

Output:

```
--8<-- "examples/type_error.output"
```

json.exception.type_error.301

To create an object from an initializer list, the initializer list must consist only of a list of pairs whose first element is a string. When this constraint is violated, an array is created instead.

!!! failure "Example message"

text

```
[json.exception.type_error.301] cannot create object from initializer list
```

json.exception.type_error.302

During implicit or explicit value conversion, the JSON type must be compatible with the target type. For instance, a JSON string can only be converted into string types, but not into numbers or boolean types.

!!! failure "Example messages"

text

```
[json.exception.type_error.302] type must be object, but is null
```
```
[json.exception.type_error.302] type must be string, but is object
```

json.exception.type_error.303

To retrieve a reference to a value stored in a basic_json object with get_ref, the type of the reference must match the value type. For instance, for a JSON array, the ReferenceType must be array_t &.

!!! failure "Example messages"

text

```
[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is object
```
```
[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number"
```

json.exception.type_error.304

The at() member functions can only be executed for certain JSON types.

!!! failure "Example messages"

text

```
[json.exception.type_error.304] cannot use at() with string
```
```
[json.exception.type_error.304] cannot use at() with number
```

json.exception.type_error.305

The operator[] member functions can only be executed for certain JSON types.

!!! failure "Example messages"

text

```
[json.exception.type_error.305] cannot use operator[] with a string argument with array
```
```
[json.exception.type_error.305] cannot use operator[] with a numeric argument with object
```

json.exception.type_error.306

The value() member functions can only be executed for certain JSON types.

!!! failure "Example message"

text

```
[json.exception.type_error.306] cannot use value() with number
```

json.exception.type_error.307

The erase() member functions can only be executed for certain JSON types.

!!! failure "Example message"

text

```
[json.exception.type_error.307] cannot use erase() with string
```

json.exception.type_error.308

The push_back() and operator+= member functions can only be executed for certain JSON types.

!!! failure "Example message"

text

```
[json.exception.type_error.308] cannot use push_back() with string
```

json.exception.type_error.309

The insert() member functions can only be executed for certain JSON types.

!!! failure "Example messages"

text

```
[json.exception.type_error.309] cannot use insert() with array
```
```
[json.exception.type_error.309] cannot use insert() with number
```

json.exception.type_error.310

The swap() member functions can only be executed for certain JSON types.

!!! failure "Example message"

text

```
[json.exception.type_error.310] cannot use swap() with number
```

json.exception.type_error.311

The emplace() and emplace_back() member functions can only be executed for certain JSON types.

!!! failure "Example messages"

text

```
[json.exception.type_error.311] cannot use emplace() with number
```
```
[json.exception.type_error.311] cannot use emplace_back() with number
```

json.exception.type_error.312

The update() member functions can only be executed for certain JSON types.

!!! failure "Example message"

text

```
[json.exception.type_error.312] cannot use update() with array
```

json.exception.type_error.313

The unflatten function converts an object whose keys are JSON Pointers back into an arbitrary nested JSON value. The JSON Pointers must not overlap, because then the resulting value would not be well-defined.

!!! failure "Example message"

text

```
[json.exception.type_error.313] invalid value to unflatten
```

json.exception.type_error.314

The unflatten function only works for an object whose keys are JSON Pointers.

!!! failure "Example message"

text

Calling `unflatten()` on an array `#!json [1,2,3]`:

```
[json.exception.type_error.314] only objects can be unflattened
```

json.exception.type_error.315

The unflatten() function only works for an object whose keys are JSON Pointers and whose values are primitive.

!!! failure "Example message"

text

Calling `unflatten()` on an object `#!json {"/1", [1,2,3]}`:

```
[json.exception.type_error.315] values in object must be primitive
```

json.exception.type_error.316

The dump() function only works with UTF-8 encoded strings; that is, if you assign a std::string to a JSON value, make sure it is UTF-8 encoded.

!!! failure "Example message"

text

Calling `dump()` on a JSON value containing an ISO 8859-1 encoded string:
```
[json.exception.type_error.316] invalid UTF-8 byte at index 15: 0x6F
```

!!! tip

text

- Store the source file with UTF-8 encoding.
- Pass an error handler as last parameter to the `dump()` function to avoid this exception:
    - `json::error_handler_t::replace` will replace invalid bytes sequences with `U+FFFD` 
    - `json::error_handler_t::ignore` will silently ignore invalid byte sequences

json.exception.type_error.317

The dynamic type of the object cannot be represented in the requested serialization format (e.g., a raw true or null JSON object cannot be serialized to BSON)

!!! failure "Example messages"

text

Serializing `#!json null` to BSON:
```
[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is null
```
Serializing `#!json [1,2,3]` to BSON:
```
[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array
```

!!! tip

text

Encapsulate the JSON value in an object. That is, instead of serializing `#!json true`, serialize `#!json {"value": true}`

Out of range

This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

Exceptions have ids 4xx.

??? example

text

The following code shows how an `out_of_range` exception can be caught.

```cpp
--8<-- "examples/out_of_range.cpp"
```

Output:

```
--8<-- "examples/out_of_range.output"
```

json.exception.out_of_range.401

The provided array index i is larger than size-1.

!!! failure "Example message"

text

```
array index 3 is out of range
```

json.exception.out_of_range.402

The special array index - in a JSON Pointer never describes a valid element of the array, but the index past the end. That is, it can only be used to add elements at this position, but not to read it.

!!! failure "Example message"

text

```
array index '-' (3) is out of range
```

json.exception.out_of_range.403

The provided key was not found in the JSON object.

!!! failure "Example message"

text

```
key 'foo' not found
```

json.exception.out_of_range.404

A reference token in a JSON Pointer could not be resolved.

!!! failure "Example message"

text

```
unresolved reference token 'foo'
```

json.exception.out_of_range.405

The JSON Patch operations 'remove' and 'add' cannot be applied to the root element of the JSON value.

!!! failure "Example message"

text

```
JSON pointer has no parent
```

json.exception.out_of_range.406

A parsed number could not be stored as without changing it to NaN or INF.

!!! failure "Example message"

text

```
number overflow parsing '10E1000'
```

json.exception.out_of_range.407

This exception previously indicated that the UBJSON and BSON binary formats did not support integer numbers greater than
9223372036854775807 due to limitations in the implemented mapping. However, these limitations have since been resolved,
and this exception no longer occurs.

!!! success "Exception cannot occur any more"

text

- Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
- Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.

json.exception.out_of_range.408

The size of an array or object in a binary format exceeds the maximal capacity:
the size following # for UBJSON/BJData,
or the encoded length for CBOR.

!!! failure "Example messages"

text

```
excessive array size: 8658170730974374167
```
```
[json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive array size
```
```
[json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive map size
```

json.exception.out_of_range.409

Key identifiers to be serialized to BSON cannot contain code point U+0000, since the key is stored as zero-terminated c-string.

!!! failure "Example message"

text

```
BSON key cannot contain code point U+0000 (at byte 2)
```

json.exception.out_of_range.410

This exception is thrown when an undefined value is used with
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT, or when an array index in
a JSON pointer exceeds the range of size_type (e.g., on 32-bit platforms).

!!! failure "Example message"

text

```
enum value out of range
array index 18446744073709551616 exceeds size_type
```

json.exception.out_of_range.411

A JSON Patch add operation cannot be applied because the target location's parent is neither an object nor an array. Per RFC 6902, an add target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) cannot receive a new member or element.

!!! failure "Example message"

text

```
cannot add value: the JSON Patch 'add' target's parent is of type string, but must be an object or array
```

!!! note

text

This exception was added in version 3.13.0. Before that, this situation hit an internal assertion (aborting the program in debug builds) or was silently ignored when assertions were disabled.

json.exception.out_of_range.412

BSON stores the length of documents, arrays, strings, and binary values in a signed 32-bit integer. This exception is thrown when a value is too large to be described by such a length field.

!!! failure "Example message"

text

```
BSON length 2147483661 exceeds maximum of 2147483647
```

!!! note

text

This exception was added in version 3.13.0. Before that, the length was silently truncated, and
[`to_bson`](../api/basic_json/to_bson.md) produced documents with negative length prefixes that
[`from_bson`](../api/basic_json/from_bson.md) rejected.

Further exceptions

This exception is thrown in case of errors that cannot be classified with the
other exception types.

Exceptions have ids 5xx.

??? example

text

The following code shows how an `other_error` exception can be caught.

```cpp
--8<-- "examples/other_error.cpp"
```

Output:

```
--8<-- "examples/other_error.output"
```

json.exception.other_error.501

A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.

!!! failure "Example message"

text

Executing `#!json {"op":"test", "path":"/baz", "value":"bar"}` on `#!json {"baz": "qux"}`:

```
[json.exception.other_error.501] unsuccessful: {"op":"test","path":"/baz","value":"bar"}
```

File: docs/mkdocs/docs/home/faq.md

Frequently Asked Questions (FAQ)

Known bugs

Brace initialization yields arrays

!!! question

text

Why does

```cpp
json j{true};
```

and

```cpp
json j(true);
```

yield different results (`#!json [true]` vs. `#!json true`)?

This is a known issue, and -- even worse -- the behavior differs between GCC and Clang. The "culprit" for this is the library's constructor overloads for initializer lists to allow syntax like

cpp

json array = {1, 2, 3, 4};

for arrays and

cpp

json object = {{"one", 1}, {"two", 2}};

for objects.

!!! tip

text

To avoid any confusion and ensure portable code, **do not** use brace initialization with the types `basic_json`, `json`, or `ordered_json` unless you want to create an object or array as shown in the examples above.

To explicitly create a single-element array, use `json::array({value})`:

```cpp
json j = json::array({true});  // [true]
```

Opt-in copy semantics (since version 3.12.0)

If you define JSON_BRACE_INIT_COPY_SEMANTICS to 1 before including the library, single-element brace initialization is treated as copy/move instead of creating a single-element array:

cpp

#define JSON_BRACE_INIT_COPY_SEMANTICS 1
#include <nlohmann/json.hpp>

json obj = {{"key", "value"}};
json j{obj};   // -> {"key":"value"}  (copy, not array)

Without the macro (default behavior), json j{obj} creates [{"key":"value"}]. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.

Limitations

Relaxed parsing

!!! question

text

Can you add an option to ignore trailing commas?

This library does not support any feature that would jeopardize interoperability.

Parse errors reading non-ASCII characters

!!! question "Questions"

text

- Why is the parser complaining about a Chinese character?
- Does the library support Unicode?
- I get an exception `[json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"Testé$')"`

The library supports Unicode input as follows:

Only UTF-8 encoded input is supported, which is the default encoding for JSON, according to RFC 8259.
std::u16string and std::u32string can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
The library will not replace Unicode noncharacters.
Invalid surrogates (e.g., incomplete pairs such as \uDEAD) will yield parse errors.
The strings stored in the library are UTF-8 encoded. When using the default string type (std::string), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace or json::error_handler_t::ignore are used as error handlers.

In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or ISO 8859-1 is often the standard encoding.

Wide string handling

!!! question

text

Why are wide strings (e.g., `std::wstring`) dumped as arrays of numbers?

As described above, the library assumes UTF-8 as encoding. To store a wide string, you need to change the encoding.

!!! example

text

```cpp
#include <codecvt> // codecvt_utf8
#include <locale>  // wstring_convert

// encoding function
std::string to_utf8(std::wstring& wide_string)
{
    static std::wstring_convert<std::codecvt_utf8<wchar_t>> utf8_conv;
    return utf8_conv.to_bytes(wide_string);
}

json j;
std::wstring ws = L"車B1234 こんにちは";

j["original"] = ws;
j["encoded"] = to_utf8(ws);

std::cout << j << std::endl;
```

The result is:

```json
{
  "encoded": "車B1234 こんにちは",
  "original": [36554, 66, 49, 50, 51, 52, 32, 12371, 12435, 12395, 12385, 12399]
}
```

Usage

Thread safety

!!! question

text

Is `basic_json` thread-safe?

No. basic_json provides no built-in synchronization, the same as std::map or std::vector. Concurrent reads of
the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent json
objects. However, any concurrent write to a json object -- or a concurrent read while another thread writes to the
same object -- is a data race and requires external synchronization (e.g., a std::mutex) by the caller.

Schema validation

!!! question

text

Does this library support JSON Schema validation?

Not directly, but the companion project json-schema-validator
builds JSON Schema (draft 4, 6, 7, and 2019-09) validation on top of this library and is a common recommendation
for this use case.

Exceptions

Parsing without exceptions

!!! question

text

Is it possible to indicate a parse error without throwing an exception?

Yes, see Parsing and exceptions.

Key name in exceptions

!!! question

text

Can I get the key of the object item that caused an exception?

Yes, you can. Please define the symbol JSON_DIAGNOSTICS to get extended diagnostics messages.

Serialization issues

Number precision

!!! question

text

- It seems that precision is lost when serializing a double.
- Can I change the precision for floating-point serialization?

The library uses std::numeric_limits<number_float_t>::digits10 (15 for IEEE doubles) digits for serialization. This value is sufficient to guarantee roundtripping. If one uses more than this number of digits of precision, then string -> value -> string is not guaranteed to round-trip.

!!! quote "cppreference.com"

text

The value of `std::numeric_limits<T>::digits10` is the number of base-10 digits that can be represented by the type T without change, that is, any number with this many significant decimal digits can be converted to a value of type T and back to decimal form, without change due to rounding or overflow.

!!! tip

text

The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.

See this section on the library's number handling for more information.

Serializing untrusted or invalid UTF-8

!!! question "Questions"

text

- Why does `dump()` throw when I serialize data that came from the network?
- Is CVE-2024-34363 a vulnerability in this library?

Crashes reported against this library that stem from an uncaught
type_error.316 while serializing unvalidated input (e.g.,
CVE-2024-34363) are a usage issue, not a library vulnerability:
dump() throws in its default strict mode because
RFC 8259 requires JSON text to be valid UTF-8.

The recommended pattern is to pass a non-strict error_handler or to handle the
exception:

cpp

// replace invalid sequences with U+FFFD instead of throwing
const auto s = j.dump(-1, ' ', false, json::error_handler_t::replace);

Using JSON values with `std::format` or `fmt`

!!! question

text

- Can I use `std::format("{}", j)` on a JSON value?
- Can I use `fmt::format("{}", j)` or `fmt::print("{}", j)` (the [{fmt}](https://github.com/fmtlib/fmt) library) on a JSON value?

std::format works out of the box since version 3.13.0, as long as the standard library provides
<format> (see JSON_HAS_STD_FORMAT); see
std::formatter for details, including the #!cpp "{:#}"
pretty-print spec, indent widths (#!cpp "{:2}"), and custom indent characters (#!cpp "{:.>#}").

For fmt, the library ships format_as, a small customization point
fmt looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from
fmt 11.1.0 onwards, fmt no longer picks up a format_as overload that returns a std::string. On such
versions (or any version, if you also want the same #!cpp "{:#}"/width/fill-and-align spec support that
std::formatter has), define your own fmt::formatter specialization; see
format_as for a recipe that mirrors it.

If you get ambiguous-overload errors when passing a JSON value to fmt::format/fmt::print without any
fmt::formatter<json> specialization in scope, that's fmt picking up basic_json's implicit
operator ValueType() conversion operator (see #964 and
#958); disabling it via
JSON_USE_IMPLICIT_CONVERSIONS 0 avoids the ambiguity.

Compilation issues

Android SDK

!!! question

text

Why does the code not compile with Android SDK?

Android defaults to using very old compilers and C++ libraries. To fix this, add the following to your Application.mk. This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.

ini

APP_STL := c++_shared
NDK_TOOLCHAIN_VERSION := clang3.6
APP_CPPFLAGS += -frtti -fexceptions

The code compiles successfully with Android NDK, Revision 9 - 11 (and possibly later) and CrystaX's Android NDK version 10.

Missing STL function

!!! question "Questions"

text

- Why do I get a compilation error `'to_string' is not a member of 'std'` (or similarly, for `strtod` or `strtof`)?
- Why does the code not compile with MinGW or Android SDK?

This is not an issue with the code, but rather with the compiler itself. On Android, see above to build with a newer environment. For MinGW, please refer to this site and this discussion for information on how to fix this bug. For Android NDK using APP_STL := gnustl_static, please refer to this discussion.

File: docs/mkdocs/docs/home/releases.md

Releases

This page summarizes the notable changes of every release and links to the relevant documentation.
The complete release notes — including all changes, the download files, and their checksums — are
published on the GitHub releases page.

v3.12.0 (2025-04-11)

Fixes bugs found in 3.11.3 and adds several features. All changes are backward-compatible.

Adds diagnostic byte positions via JSON_DIAGNOSTIC_POSITIONS,
exposed through the new start_pos and
end_pos member functions.
Makes the conversion macros
templated (so they also work with ordered_json) and adds
NLOHMANN_DEFINE_DERIVED_TYPE for derived classes.
Adds std::optional support (C++17) and lets patch,
diff, and flatten work with
arbitrary string types.
Extends the binary formats:
BJData draft 3 and unsigned 64-bit integers for
BSON.
Adds multidimensional C-array conversion and UTF-8 encoded std::filesystem::path conversions, and
lowers the minimum CMake version to allow CMake 4.0.

Full release notes.

v3.11.3 (2023-11-28)

Adds features and fixes bugs found in 3.11.2. All changes are backward-compatible.

Adds a custom base class as a node customization point.
Adds serialization-only conversion macros
(NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE)
and a clearer parse error for empty input.
Adds Bazel and
Swift Package Manager build support.
Fixes custom allocators, a memory leak in adl_serializer's
to_json, initializer-list construction when size_type is not int, and many compiler warnings.

Full release notes.

v3.11.2 (2022-08-12)

Fixes bugs found in 3.11.1 and restructures the namespace. All changes are backward-compatible.

Fixes the value function (broken for strings, size types, and
nullptr in 3.11.0) and makes json_fwd.hpp self-contained.
Restores using json_pointer as a key in associative containers and
comparing it with strings.
Restructures the inline namespace and allows disabling the version
component, and avoids heap allocations in the BJData parser.

Full release notes.

v3.11.1 (2022-08-01)

Fixes a regression from 3.11.0. All changes are backward-compatible.

Restores the global user-defined string literals
operator""_json and
operator""_json_pointer, which 3.11.0 had moved into a
namespace by default.

Full release notes.

v3.11.0 (2022-08-01)

One of the largest releases ever. All changes are backward-compatible.

Allows std::string_view as object keys in at,
operator[], value,
erase, find,
contains, and count.
Adds the BJData binary format (the fifth supported format).
Improves C++20 support, including operator<=> and
<ranges>-compatible iterators.
Adds a versioned, ABI-tagged inline namespace
(NLOHMANN_JSON_NAMESPACE) and the option to move the
UDLs out of the global namespace (JSON_USE_GLOBAL_UDLS).
Adds patch_inplace, default values for the
conversion macros, and an option to
disable enum serialization (JSON_DISABLE_ENUM_SERIALIZATION).

This release introduced a UDL regression that was fixed in
3.11.1.
Full release notes.

v3.10.5 (2022-01-03)

Bug-fix release. All changes are backward-compatible.

Guards the std::filesystem conversions behind compiler-support checks
(JSON_HAS_FILESYSTEM), which can be set to 0 to disable
them altogether.

Full release notes.

v3.10.4 (2021-10-16)

Fixes regressions introduced in 3.10.0. All changes are backward-compatible.

Fixes the std::filesystem::path conversion (which could trigger a stack overflow and broke
compilation on Windows).
Fixes compilation for types with an explicit defaulted constructor and for code relying on the
return values of std::find and std::remove.

Full release notes.

v3.10.3 (2021-10-08)

Fixes more regressions from 3.10.0. All changes are backward-compatible.

Fixes extended-diagnostics assertions triggered by
update and by inserting into arrays.
Supports custom allocators when writing binary formats into a std::vector, and allows conversion
from types that only provide begin()/end().

Full release notes.

v3.10.2 (2021-08-26)

Re-release of 3.10.1, whose Git tag pointed at the wrong commit due to a bug in the release script.
All changes are backward-compatible.
Full release notes.

v3.10.1 (2021-08-24)

Fixes a regression from 3.10.0. All changes are backward-compatible.

Fixes an extended-diagnostics assertion triggered when used
with ordered_json, and hardens the GDB pretty-printer.

Full release notes.

v3.10.0 (2021-08-17)

Feature release. All changes are backward-compatible.

Adds extended diagnostic messages
(JSON_DIAGNOSTICS) that prepend a JSON pointer to exception
messages to pinpoint the offending value.
Adds a GDB pretty-printer and a cbor_tag_handler_t
store option to keep CBOR tags as binary subtypes.
Supports containers with non-default-constructible types and parsing from std::byte.
Adds JSON_NO_IO to exclude the I/O headers and the
JSON_HAS_CPP_* macros to override the detected C++ standard.

Full release notes.

v3.9.1 (2020-08-06)

Fixes two regressions from 3.9.0. All changes are backward-compatible.

Accepts consecutive comments and completes the
ordered_json interface (e.g. ordered_json::parse).

Full release notes.

v3.9.0 (2020-07-27)

Feature release adding four long-requested features. All changes are backward-compatible.

Optional comment parsing in parse via the
ignore_comments parameter.
ordered_json to preserve the insertion order
of object keys.
An option to switch off implicit conversions.
The NLOHMANN_DEFINE_TYPE_*
convenience macros, plus high-precision-number support for
UBJSON and CBOR tag handling.

Full release notes.

v3.8.0 (2020-06-14)

Feature release. All changes are backward-compatible.

Introduces a binary value type that is read from and written to
CBOR, BSON, and
MessagePack, and can be shared between formats.
Generalizes the input adapters to read from any LegacyInputIterator container (3–10 % faster
parsing).
Fixes contains for JSON pointers and makes the binary
from_cbor/from_msgpack/etc.
functions respect allow_exceptions.

Full release notes.

v3.7.3 (2019-11-17)

Fixes a regression from 3.7.2 that could yield quadratic complexity in destructor calls. All changes
are backward-compatible. Full release notes.

v3.7.2 (2019-11-10)

Fixes a stack overflow for deeply nested input by making the destructor iterative; parsing is now
bounded only by available memory. All changes are backward-compatible.
Full release notes.

v3.7.1 (2019-11-06)

Bug-fix release. All changes are backward-compatible.

Fixes a segmentation fault when serializing the std::int64_t minimum value and fixes
contains for JSON pointers.
Allows items with a custom string type and makes json_pointer::back
const.

Full release notes.

v3.7.0 (2019-07-28)

Convenience features and house-keeping. All changes are backward-compatible.

Adds a contains overload that checks a JSON pointer without
throwing, a generic to_string, and a return value for
emplace_back.

Full release notes.

v3.6.1 (2019-03-20)

Fixes a regression (GCC 7/8 compilation) and a <Windows.h> build error introduced in 3.6.0. All
changes are backward-compatible. Full release notes.

v3.6.0 (2019-03-20)

Feature release. All changes are backward-compatible.

Reworks the JSON pointer interface (operator/, push_back,
parent_pointer, …).
Adds a contains function to test for an object key and greatly
improves the performance of integer serialization.

Full release notes.

v3.5.0 (2018-12-22)

Feature release. All changes are backward-compatible.

Adds structured-binding support via the items function and reading
from FILE* in the parse function.
Fixes the eofbit handling on input streams and a bug in the BSON SAX parser.

Full release notes.

v3.4.0 (2018-10-30)

Feature release. All changes are backward-compatible.

Adds BSON read/write support.
Adds configurable Unicode error handlers to dump (throw, replace with
U+FFFD, or ignore) and the
NLOHMANN_JSON_SERIALIZE_ENUM macro for
enum conversion.
Improves parse-error messages with line/column positions and context.

Full release notes.

v3.3.0 (2018-10-05)

Feature release. All changes are backward-compatible.

Adds GCC 4.8 support, the get_to function, and an overhauled and
documented CMake integration.

Full release notes.

v3.2.0 (2018-08-20)

Feature release. All changes are backward-compatible.

Adds a SAX interface and a non-recursive parser.
Adds parsing from wide-string types (std::wstring, std::u16string, std::u32string) and
std::string_view (C++17), and round-tripping of std::map/std::unordered_map with non-string
keys.

Full release notes.

v3.1.2 (2018-03-14)

Bug-fix release. All changes are backward-compatible.

Fixes a memory leak in the parser callback and adds user-defined string-type support to the parser
and serializer.

Full release notes.

v3.1.1 (2018-02-13)

Bug-fix release. All changes are backward-compatible.

Fixes parsing of indefinite-length CBOR strings, a user-defined conversion to vector types, and
overflow detection for UBJSON containers.

Full release notes.

v3.1.0 (2018-02-01)

Feature release. All changes are backward-compatible.

Adds UBJSON read/write support and
JSON Merge Patch via merge_patch.
Switches to the Grisu2 algorithm for short, round-trippable floating-point output, and splits the
header into multiple files with a forward-declaration header.

Full release notes.

v3.0.1 (2017-12-29)

Fixes small issues in the JSON Pointer and
JSON Patch implementations (invalid "copy" targets and non-integer array
indices). All changes are backward-compatible.
Full release notes.

v3.0.0 (2017-12-17)

First 3.x release — a major release with breaking changes (see the
migration guide).

Introduces user-defined exceptions (json::exception and subtypes, each with an
identifier).
Adds a non-throwing accept function and an allow_exceptions flag
for parse, and an update function to
merge objects.
Adds streaming for CBOR and MessagePack and allows storing NaN/infinity.
Non-UTF-8 strings now throw on serialization, and the iterator category changed to bidirectional.

Full release notes.

v2.1.1 (2017-02-25)

Bug-fix release. All changes are backward-compatible.

Makes number parsing and serialization locale-independent with correct floating-point
round-tripping; released files are now GPG-signed.

Full release notes.

v2.1.0 (2017-01-28)

Feature release. All changes are backward-compatible.

Adds conversions from and to arbitrary user-defined types via
to_json/from_json, the meta function, and the option to switch off
exceptions (JSON_NOEXCEPTION).

Full release notes.

v2.0.10 (2017-01-02)

Fixes several security-relevant bugs in the CBOR and MessagePack parsers found by OSS-Fuzz. All
changes are backward-compatible. Full release notes.

v2.0.9 (2016-12-16)

Adds the CBOR and
MessagePack binary formats. All changes are
backward-compatible. Full release notes.

v2.0.8 (2016-12-02)

Adds the emplace and
emplace_back functions and improves parsing and serialization
performance. All changes are backward-compatible.
Full release notes.

v2.0.7 (2016-11-02)

Fixes several parser bugs found through the "Parsing JSON is a Minefield" study (short files, encoding
detection, surrogate pairs). All changes are backward-compatible.
Full release notes.

v2.0.6 (2016-10-15)

Fixes operator[] for JSON pointers
so that it creates missing values like the other overloads. All changes are backward-compatible.
Full release notes.

v2.0.5 (2016-09-14)

Fixes a remaining stream end-of-file detection bug in the parser. All changes are backward-compatible.
Full release notes.

v2.0.4 (2016-09-11)

Fixes stream end-of-file detection in the parser. All changes are backward-compatible.
Full release notes.

v2.0.3 (2016-08-31)

Generalizes the parser to accept any contiguous sequence of one-byte elements and deprecates the
input-stream constructor in favor of the parse function. All changes
are backward-compatible. Full release notes.

v2.0.2 (2016-07-31)

Overhauls the parser (now rejecting unescaped control characters), tightens the class invariants, and
cleans up the code. All changes are backward-compatible.
Full release notes.

v2.0.1 (2016-06-28)

Fixes a performance regression in the dump function by adjusting the
stream locale once per serialization. All changes are backward-compatible.
Full release notes.

v2.0.0 (2016-06-24)

Feature release with a minor (potentially non-backward-compatible) API change from added noexcept
and constexpr specifiers.

Adds JSON Pointer support in at and
operator[], plus flatten
and unflatten.
Adds JSON Patch via diff and
patch, unsigned 64-bit integer support, and locale-independent
serialization.

Full release notes.

v1.1.0 (2016-01-24)

Bug-fix and feature release. All changes are backward-compatible.

Improves floating-point round-tripping, adds a get_ref accessor for stored values, and introduces
runtime assertions.

Full release notes.

v1.0.0 (2015-12-28)

First official release. Full release notes.

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Sponsors

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Quick reference

Examples

Read JSON from a file

Creating `json` objects from JSON literals

JSON as a first-class data type

Serialization / Deserialization

STL-like access

Conversion from STL containers

JSON Pointer and JSON Patch

JSON Merge Patch

Implicit conversions

Arbitrary types conversions

Specializing enum conversion

Binary formats (BSON, CBOR, MessagePack, UBJSON, and BJData)

Customers

Ecosystem

Supported compilers

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File: docs/docset/README.md

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Value storage

Parsing inputs (deserialization)

SAX Interface

Writing outputs (serialization)

Value conversion

Additional features

Details namespace

File: docs/mkdocs/docs/home/customers.md

Space Exploration

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Consumer Electronics

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Scientific Research and Analysis

Business and Productivity Software

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Simulation and Modeling

Enterprise and Cloud Applications

File: docs/mkdocs/docs/home/design_goals.md

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json.exception.parse_error.105