```bash
$ ./build/benchmarks/benchmark
# parsing random integers in the range [0,1)
volume = 2.09808 MB
netlib : 271.18 MB/s (+/- 1.2 %) 12.93 Mfloat/s
doubleconversion : 225.35 MB/s (+/- 1.2 %) 10.74 Mfloat/s
strtod : 190.94 MB/s (+/- 1.6 %) 9.10 Mfloat/s
abseil : 430.45 MB/s (+/- 2.2 %) 20.52 Mfloat/s
fastfloat : 1042.38 MB/s (+/- 9.9 %) 49.68 Mfloat/s
```
See the [Benchmarking](#benchmarking) Section for instructions on how to run our benchmarks.
## Video
[](https://www.youtube.com/watch?v=AVXgvlMeIm4)
## Using as a CMake dependency
This library is header-only by design. The CMake file provides the `fast_float`
target which is merely a pointer to the `include` directory.
If you drop the `fast_float` repository in your CMake project, you should be
able to use it in this manner:
```cmake
add_subdirectory(fast_float)
target_link_libraries(myprogram PUBLIC fast_float)
```
Or you may want to retrieve the dependency automatically if you have a
sufficiently recent version of CMake (3.11 or better at least):
```cmake
FetchContent_Declare(
fast_float
GIT_REPOSITORY https://github.com/fastfloat/fast_float.git
GIT_TAG tags/v8.0.0
GIT_SHALLOW TRUE)
FetchContent_MakeAvailable(fast_float)
target_link_libraries(myprogram PUBLIC fast_float)
```
You should change the `GIT_TAG` line so that you recover the version you wish to
use.
You may also use [CPM](https://github.com/cpm-cmake/CPM.cmake), like so:
```cmake
CPMAddPackage(
NAME fast_float
GITHUB_REPOSITORY "fastfloat/fast_float"
GIT_TAG v8.0.0)
```
## Using as single header
The script `script/amalgamate.py` may be used to generate a single header
version of the library if so desired. Just run the script from the root
directory of this repository. You can customize the license type and output file
if desired as described in the command line help.
You may directly download automatically generated single-header files:
[](https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux)
[](https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos)
[](https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows)
[](https://bugs.chromium.org/p/oss-fuzz/issues/list?\%0Acolspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\%0ASummary&q=proj%3Dfmt&can=1)
[](https://stackoverflow.com/questions/tagged/fmt)
[](https://securityscorecards.dev/viewer/?uri=github.com/fmtlib/fmt)
**{fmt}** is an open-source formatting library providing a fast and safe
alternative to C stdio and C++ iostreams.
If you like this project, please consider donating to one of the funds
that help victims of the war in Ukraine: 
# What is `folly`?
Folly (acronymed loosely after Facebook Open Source Library) is a
library of C++17 components designed with practicality and efficiency
in mind. **Folly contains a variety of core library components used extensively
at Facebook**. In particular, it's often a dependency of Facebook's other
open source C++ efforts and place where those projects can share code.
It complements (as opposed to competing against) offerings
such as Boost and of course `std`. In fact, we embark on defining our
own component only when something we need is either not available, or
does not meet the needed performance profile. We endeavor to remove
things from folly if or when `std` or Boost obsoletes them.
Performance concerns permeate much of Folly, sometimes leading to
designs that are more idiosyncratic than they would otherwise be (see
e.g. `PackedSyncPtr.h`, `SmallLocks.h`). Good performance at large
scale is a unifying theme in all of Folly.
## Check it out in the intro video
[](https://www.youtube.com/watch?v=Wr_IfOICYSs)
# Logical Design
Folly is a collection of relatively independent components, some as
simple as a few symbols. There is no restriction on internal
dependencies, meaning that a given folly module may use any other
folly components.
All symbols are defined in the top-level namespace `folly`, except of
course macros. Macro names are ALL_UPPERCASE and should be prefixed
with `FOLLY_`. Namespace `folly` defines other internal namespaces
such as `internal` or `detail`. User code should not depend on symbols
in those namespaces.
Folly has an `experimental` directory as well. This designation connotes
primarily that we feel the API may change heavily over time. This code,
typically, is still in heavy use and is well tested.
# Physical Design
At the top level Folly uses the classic "stuttering" scheme
`folly/folly` used by Boost and others. The first directory serves as
an installation root of the library (with possible versioning a la
`folly-1.0/`), and the second is to distinguish the library when
including files, e.g. `#include
Inspired by Python's formatting facility, {fmt} provides a safe replacement
for the printf family of functions. Errors in format strings,
which are a common source of vulnerabilities in C, are reported at
compile time. For example:
fmt::format("{:d}", "I am not a number");d is not a valid
format specifier for strings. APIs like
fmt::format prevent buffer overflow errors via
automatic memory management.
→ Learn more
Formatting of most standard types, including all containers, dates, and times is supported out-of-the-box. For example:
fmt::print("{}", std::vector{1, 2, 3});[1, 2, 3]
{fmt} can be anywhere from tens of percent to 20-30 times faster than
iostreams and sprintf, especially for numeric formatting.
The library minimizes dynamic memory allocations and can optionally
compile format strings to optimal code.
{fmt} provides portable Unicode support on major operating systems
with UTF-8 and char strings. For example:
fmt::print("Слава Україні!");The default is locale-independent, but you can opt into localized formatting and {fmt} makes it work with Unicode, addressing issues in the standard libary.
The library makes extensive use of type erasure to achieve fast
compilation. fmt/base.h provides a subset of the API with
minimal include dependencies and enough functionality to replace
all uses of *printf.
Code using {fmt} is usually several times faster to compile than the
equivalent iostreams code, and while printf compiles faster
still, the gap is narrowing.
Type erasure is also used to prevent template bloat, resulting in compact
per-call binary code. For example, a call to fmt::print with
a single argument is just a few
instructions, comparable to printf despite adding
runtime safety, and much smaller than the equivalent iostreams code.
The library itself has small binary footprint and some components such as floating-point formatting can be disabled to make it even smaller for resource-constrained devices.
{fmt} has a small self-contained codebase with the core consisting of just three headers and no external dependencies.
The library is highly portable and requires only a minimal subset of C++11 features which are available in GCC 4.8, Clang 3.4, MSVC 19.0 (2015) and later. Newer compiler and standard library features are used if available, and enable additional functionality.
Where possible, the output of formatting functions is consistent across platforms.
{fmt} is in the top hundred open-source C++ libraries on GitHub and has hundreds of all-time contributors.
The library is distributed under a permissive MIT license and is relied upon by many open-source projects, including Blender, PyTorch, Apple's FoundationDB, Windows Terminal, MongoDB, and others.
replacement_field ::= "{" [arg_id] [":" (format_spec | chrono_format_spec)] "}"
arg_id ::= integer | identifier
integer ::= digit+
digit ::= "0"..."9"
identifier ::= id_start id_continue*
id_start ::= "a"..."z" | "A"..."Z" | "_"
id_continue ::= id_start | digit
format_spec ::= [[fill]align][sign]["#"]["0"][width]["." precision]["L"][type]
fill ::= <a character other than '{' or '}'>
align ::= "<" | ">" | "^"
sign ::= "+" | "-" | " "
width ::= integer | "{" [arg_id] "}"
precision ::= integer | "{" [arg_id] "}"
type ::= "a" | "A" | "b" | "B" | "c" | "d" | "e" | "E" | "f" | "F" |
"g" | "G" | "o" | "p" | "s" | "x" | "X" | "?"
| Option | Meaning |
|---|---|
'<' |
Forces the field to be left-aligned within the available space (this is the default for most objects). |
'>' |
Forces the field to be right-aligned within the available space (this is the default for numbers). |
'^' |
Forces the field to be centered within the available space. |
| Option | Meaning |
|---|---|
'+' |
Indicates that a sign should be used for both nonnegative as well as negative numbers. |
'-' |
Indicates that a sign should be used only for negative numbers (this is the default behavior). |
| space | Indicates that a leading space should be used on nonnegative numbers, and a minus sign on negative numbers. |
| Type | Meaning |
|---|---|
's' |
String format. This is the default type for strings and may be omitted. |
'?' |
Debug format. The string is quoted and special characters escaped. |
| none | The same as 's'. |
| Type | Meaning |
|---|---|
'c' |
Character format. This is the default type for characters and may be omitted. |
'?' |
Debug format. The character is quoted and special characters escaped. |
| none | The same as 'c'. |
| Type | Meaning |
|---|---|
'b' |
Binary format. Outputs the number in base 2. Using the '#'
option with this type adds the prefix "0b" to the output value.
|
'B' |
Binary format. Outputs the number in base 2. Using the '#'
option with this type adds the prefix "0B" to the output value.
|
'c' |
Character format. Outputs the number as a character. |
'd' |
Decimal integer. Outputs the number in base 10. |
'o' |
Octal format. Outputs the number in base 8. |
'x' |
Hex format. Outputs the number in base 16, using lower-case letters for the
digits above 9. Using the '#' option with this type adds the
prefix "0x" to the output value.
|
'X' |
Hex format. Outputs the number in base 16, using upper-case letters for the
digits above 9. Using the '#' option with this type adds the
prefix "0X" to the output value.
|
| none | The same as 'd'. |
| Type | Meaning |
|---|---|
'a' |
Hexadecimal floating point format. Prints the number in base 16 with
prefix "0x" and lower-case letters for digits above 9.
Uses 'p' to indicate the exponent.
|
'A' |
Same as 'a' except it uses upper-case letters for the
prefix, digits above 9 and to indicate the exponent.
|
'e' |
Exponent notation. Prints the number in scientific notation using the letter 'e' to indicate the exponent. |
'E' |
Exponent notation. Same as 'e' except it uses an
upper-case 'E' as the separator character.
|
'f' |
Fixed point. Displays the number as a fixed-point number. |
'F' |
Fixed point. Same as 'f', but converts nan
to NAN and inf to INF.
|
'g' |
General format. For a given precision A precision of |
'G' |
General format. Same as 'g' except switches to
'E' if the number gets too large. The representations of
infinity and NaN are uppercased, too.
|
| none |
Similar to 'g', except that the default precision is as
high as needed to represent the particular value.
|
| Type | Meaning |
|---|---|
'p' |
Pointer format. This is the default type for pointers and may be omitted. |
| none | The same as 'p'. |
chrono_format_spec ::= [[fill]align][width]["." precision][chrono_specs]
chrono_specs ::= conversion_spec |
chrono_specs (conversion_spec | literal_char)
conversion_spec ::= "%" [padding_modifier] [locale_modifier] chrono_type
literal_char ::= <a character other than '{', '}' or '%'>
padding_modifier ::= "-" | "_" | "0"
locale_modifier ::= "E" | "O"
chrono_type ::= "a" | "A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" |
"F" | "g" | "G" | "h" | "H" | "I" | "j" | "m" | "M" |
"n" | "p" | "q" | "Q" | "r" | "R" | "S" | "t" | "T" |
"u" | "U" | "V" | "w" | "W" | "x" | "X" | "y" | "Y" |
"z" | "Z" | "%"
| Type | Meaning |
|---|---|
'a' |
The abbreviated weekday name, e.g. "Sat". If the value does not contain a
valid weekday, an exception of type format_error is thrown.
|
'A' |
The full weekday name, e.g. "Saturday". If the value does not contain a
valid weekday, an exception of type format_error is thrown.
|
'b' |
The abbreviated month name, e.g. "Nov". If the value does not contain a
valid month, an exception of type format_error is thrown.
|
'B' |
The full month name, e.g. "November". If the value does not contain a valid
month, an exception of type format_error is thrown.
|
'c' |
The date and time representation, e.g. "Sat Nov 12 22:04:00 1955". The
modified command %Ec produces the locale's alternate date and
time representation.
|
'C' |
The year divided by 100 using floored division, e.g. "19". If the result
is a single decimal digit, it is prefixed with 0. The modified command
%EC produces the locale's alternative representation of the
century.
|
'd' |
The day of month as a decimal number. If the result is a single decimal
digit, it is prefixed with 0. The modified command %Od
produces the locale's alternative representation.
|
'D' |
Equivalent to %m/%d/%y, e.g. "11/12/55". |
'e' |
The day of month as a decimal number. If the result is a single decimal
digit, it is prefixed with a space. The modified command %Oe
produces the locale's alternative representation.
|
'F' |
Equivalent to %Y-%m-%d, e.g. "1955-11-12". |
'g' |
The last two decimal digits of the ISO week-based year. If the result is a single digit it is prefixed by 0. |
'G' |
The ISO week-based year as a decimal number. If the result is less than four digits it is left-padded with 0 to four digits. |
'h' |
Equivalent to %b, e.g. "Nov". |
'H' |
The hour (24-hour clock) as a decimal number. If the result is a single
digit, it is prefixed with 0. The modified command %OH
produces the locale's alternative representation.
|
'I' |
The hour (12-hour clock) as a decimal number. If the result is a single
digit, it is prefixed with 0. The modified command %OI
produces the locale's alternative representation.
|
'j' |
If the type being formatted is a specialization of duration, the decimal number of days without padding. Otherwise, the day of the year as a decimal number. Jan 1 is 001. If the result is less than three digits, it is left-padded with 0 to three digits. |
'm' |
The month as a decimal number. Jan is 01. If the result is a single digit,
it is prefixed with 0. The modified command %Om produces the
locale's alternative representation.
|
'M' |
The minute as a decimal number. If the result is a single digit, it
is prefixed with 0. The modified command %OM produces the
locale's alternative representation.
|
'n' |
A new-line character. |
'p' |
The AM/PM designations associated with a 12-hour clock. |
'q' |
The duration's unit suffix. |
'Q' |
The duration's numeric value (as if extracted via .count()).
|
'r' |
The 12-hour clock time, e.g. "10:04:00 PM". |
'R' |
Equivalent to %H:%M, e.g. "22:04". |
'S' |
Seconds as a decimal number. If the number of seconds is less than 10, the
result is prefixed with 0. If the precision of the input cannot be exactly
represented with seconds, then the format is a decimal floating-point number
with a fixed format and a precision matching that of the precision of the
input (or to a microseconds precision if the conversion to floating-point
decimal seconds cannot be made within 18 fractional digits). The character
for the decimal point is localized according to the locale. The modified
command %OS produces the locale's alternative representation.
|
't' |
A horizontal-tab character. |
'T' |
Equivalent to %H:%M:%S. |
'u' |
The ISO weekday as a decimal number (1-7), where Monday is 1. The modified
command %Ou produces the locale's alternative representation.
|
'U' |
The week number of the year as a decimal number. The first Sunday of the
year is the first day of week 01. Days of the same year prior to that are
in week 00. If the result is a single digit, it is prefixed with 0.
The modified command %OU produces the locale's alternative
representation.
|
'V' |
The ISO week-based week number as a decimal number. If the result is a
single digit, it is prefixed with 0. The modified command %OV
produces the locale's alternative representation.
|
'w' |
The weekday as a decimal number (0-6), where Sunday is 0. The modified
command %Ow produces the locale's alternative representation.
|
'W' |
The week number of the year as a decimal number. The first Monday of the
year is the first day of week 01. Days of the same year prior to that are
in week 00. If the result is a single digit, it is prefixed with 0.
The modified command %OW produces the locale's alternative
representation.
|
'x' |
The date representation, e.g. "11/12/55". The modified command
%Ex produces the locale's alternate date representation.
|
'X' |
The time representation, e.g. "10:04:00". The modified command
%EX produces the locale's alternate time representation.
|
'y' |
The last two decimal digits of the year. If the result is a single digit
it is prefixed by 0. The modified command %Oy produces the
locale's alternative representation. The modified command %Ey
produces the locale's alternative representation of offset from
%EC (year only).
|
'Y' |
The year as a decimal number. If the result is less than four digits it is
left-padded with 0 to four digits. The modified command %EY
produces the locale's alternative full year representation.
|
'z' |
The offset from UTC in the ISO 8601:2004 format. For example -0430 refers
to 4 hours 30 minutes behind UTC. If the offset is zero, +0000 is used.
The modified commands %Ez and %Oz insert a
: between the hours and minutes: -04:30. If the offset
information is not available, an exception of type
format_error is thrown.
|
'Z' |
The time zone abbreviation. If the time zone abbreviation is not available,
an exception of type format_error is thrown.
|
'%' |
A % character. |
range_format_spec ::= ["n"][range_type][range_underlying_spec]