## File: README.md

# Abseil - C++ Common Libraries

The repository contains the Abseil C++ library code. Abseil is an open-source
collection of C++ code (compliant to C++17) designed to augment the C++
standard library.

## Table of Contents

- [About Abseil](#about)
- [Quickstart](#quickstart)
- [Building Abseil](#build)
- [Support](#support)
- [Codemap](#codemap)
- [Releases](#releases)
- [License](#license)
- [Links](#links)

<a name="about"></a>
## About Abseil

Abseil is an open-source collection of C++ library code designed to augment
the C++ standard library. The Abseil library code is collected from Google's
own C++ code base, has been extensively tested and used in production, and
is the same code we depend on in our daily coding lives.

In some cases, Abseil provides pieces missing from the C++ standard; in
others, Abseil provides alternatives to the standard for special needs
we've found through usage in the Google code base. We denote those cases
clearly within the library code we provide you.

Abseil is not meant to be a competitor to the standard library; we've
just found that many of these utilities serve a purpose within our code
base, and we now want to provide those resources to the C++ community as
a whole.

<a name="quickstart"></a>
## Quickstart

If you want to just get started, make sure you at least run through the
[Abseil Quickstart](https://abseil.io/docs/cpp/quickstart). The Quickstart
contains information about setting up your development environment, downloading
the Abseil code, running tests, and getting a simple binary working.

<a name="build"></a>
## Building Abseil

[Bazel](https://bazel.build) and [CMake](https://cmake.org/) are the official
build systems for Abseil.
See the [quickstart](https://abseil.io/docs/cpp/quickstart) for more information
on building Abseil using the Bazel build system.
If you require CMake support, please check the [CMake build
instructions](CMake/README.md) and [CMake
Quickstart](https://abseil.io/docs/cpp/quickstart-cmake).

<a name="support"></a>
## Support

Abseil follows Google's [Foundational C++ Support
Policy](https://opensource.google/documentation/policies/cplusplus-support). See
[this
table](https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md)
for a list of currently supported versions compilers, platforms, and build
tools.

<a name="codemap"></a>
## Codemap

Abseil contains the following C++ library components:

* [`base`](absl/base/)
   The `base` library contains initialization code and other code which
  all other Abseil code depends on. Code within `base` may not depend on any
  other code (other than the C++ standard library).
* [`algorithm`](absl/algorithm/)
   The `algorithm` library contains additions to the C++ `<algorithm>`
  library and container-based versions of such algorithms.
* [`cleanup`](absl/cleanup/)
   The `cleanup` library contains the control-flow-construct-like type
  `absl::Cleanup` which is used for executing a callback on scope exit.
* [`container`](absl/container/)
   The `container` library contains additional STL-style containers,
  including Abseil's unordered "Swiss table" containers.
* [`crc`](absl/crc/) The `crc` library contains code for
  computing error-detecting cyclic redundancy checks on data.
* [`debugging`](absl/debugging/)
   The `debugging` library contains code useful for enabling leak
  checks, and stacktrace and symbolization utilities.
* [`flags`](absl/flags/)
   The `flags` library contains code for handling command line flags for
  libraries and binaries built with Abseil.
* [`hash`](absl/hash/)
   The `hash` library contains the hashing framework and default hash
  functor implementations for hashable types in Abseil.
* [`log`](absl/log/)
   The `log` library contains `LOG` and `CHECK` macros and facilities
  for writing logged messages out to disk, `stderr`, or user-extensible
  destinations.
* [`memory`](absl/memory/)
   The `memory` library contains memory management facilities that augment
  C++'s `<memory>` library.
* [`meta`](absl/meta/)
   The `meta` library contains type checks
  similar to those available in the C++ `<type_traits>` library.
* [`numeric`](absl/numeric/)
   The `numeric` library contains 128-bit integer types as well as
  implementations of C++20's bitwise math functions.
* [`profiling`](absl/profiling/)
   The `profiling` library contains utility code for profiling C++
  entities.  It is currently a private dependency of other Abseil libraries.
* [`random`](absl/random/)
   The `random` library contains functions for generating pseudorandom
  values.
* [`status`](absl/status/)
   The `status` library contains abstractions for error handling,
  specifically `absl::Status` and `absl::StatusOr<T>`.
* [`strings`](absl/strings/)
   The `strings` library contains a variety of strings routines and
  utilities.
* [`synchronization`](absl/synchronization/)
   The `synchronization` library contains concurrency primitives (Abseil's
  `absl::Mutex` class, an alternative to `std::mutex`) and a variety of
  synchronization abstractions.
* [`time`](absl/time/)
   The `time` library contains abstractions for computing with absolute
  points in time, durations of time, and formatting and parsing time within
  time zones.
* [`types`](absl/types/)
   The `types` library contains non-container utility types.
* [`utility`](absl/utility/)
   The `utility` library contains utility and helper code.

<a name="releases"></a>
## Releases

Abseil recommends users "live-at-head" (update to the latest commit from the
master branch as often as possible). However, we realize this philosophy doesn't
work for every project, so we also provide [Long Term Support
Releases](https://github.com/abseil/abseil-cpp/releases) to which we backport
fixes for severe bugs. See our [release
management](https://abseil.io/about/releases) document for more details.

<a name="license"></a>
## License

The Abseil C++ library is licensed under the terms of the Apache
license. See [LICENSE](LICENSE) for more information.

<a name="links"></a>
## Links

For more information about Abseil:

* Consult our [Abseil Introduction](https://abseil.io/about/intro)
* Read [Why Adopt Abseil](https://abseil.io/about/philosophy) to understand our
  design philosophy.
* Peruse our
  [Abseil Compatibility Guarantees](https://abseil.io/about/compatibility) to
  understand both what we promise to you, and what we expect of you in return.

---

## File: CMake/README.md

# Abseil CMake Build Instructions

Abseil comes with a CMake build script ([CMakeLists.txt](../CMakeLists.txt))
that can be used on a wide range of platforms ("C" stands for cross-platform.).
If you don't have CMake installed already, you can download it for free from
<https://www.cmake.org/>.

CMake works by generating native makefiles or build projects that can
be used in the compiler environment of your choice.

For API/ABI compatibility reasons, we strongly recommend building Abseil in a
subdirectory of your project or as an embedded dependency.

## Incorporating Abseil Into a CMake Project

The recommendations below are similar to those for using CMake within the
googletest framework
(<https://github.com/google/googletest/blob/master/googletest/README.md#incorporating-into-an-existing-cmake-project>)

### Step-by-Step Instructions

1. If you want to build the Abseil tests, integrate the Abseil dependency
[Google Test](https://github.com/google/googletest) into your CMake
project. To disable Abseil tests, you have to pass either
`-DBUILD_TESTING=OFF` or `-DABSL_BUILD_TESTING=OFF` when configuring your
project with CMake.

2. Download Abseil and copy it into a subdirectory in your CMake project or add
Abseil as a [git submodule](https://git-scm.com/docs/git-submodule) in your
CMake project.

3. You can then use the CMake command
[`add_subdirectory()`](https://cmake.org/cmake/help/latest/command/add_subdirectory.html)
to include Abseil directly in your CMake project.

4. Add the **absl::** target you wish to use to the
[`target_link_libraries()`](https://cmake.org/cmake/help/latest/command/target_link_libraries.html)
section of your executable or of your library.
Here is a short CMakeLists.txt example of an application project using Abseil.

```cmake
cmake_minimum_required(VERSION 3.16)
project(my_app_project)

# Pick the C++ standard to compile with.
# Abseil currently supports C++17 and C++20.
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_subdirectory(abseil-cpp)

add_executable(my_exe source.cpp)
target_link_libraries(my_exe absl::base absl::synchronization absl::strings)
```

Note that if you are developing a library designed for use by other clients, you
should instead leave `CMAKE_CXX_STANDARD` unset (or only set if being built as
the current top-level CMake project) and configure the minimum required C++
standard at the target level. If you require a later minimum C++ standard than
Abseil does, it's a good idea to also enforce that `CMAKE_CXX_STANDARD` (which
will control Abseil library targets) is set to at least that minimum. For
example:

```cmake
cmake_minimum_required(VERSION 3.16)
project(my_lib_project)

# Leave C++ standard up to the root application, so set it only if this is the
# current top-level CMake project.
if(CMAKE_SOURCE_DIR STREQUAL my_lib_project_SOURCE_DIR)
  set(CMAKE_CXX_STANDARD 17)
  set(CMAKE_CXX_STANDARD_REQUIRED ON)
endif()

add_subdirectory(abseil-cpp)

add_library(my_lib source.cpp)
target_link_libraries(my_lib absl::base absl::synchronization absl::strings)

# Enforce that my_lib requires C++17. Important to document for clients that they
# must set CMAKE_CXX_STANDARD to 17 or higher for proper Abseil ABI compatibility
# (since otherwise, Abseil library targets could be compiled with a lower C++
# standard than my_lib).
target_compile_features(my_lib PUBLIC cxx_std_17)
if(CMAKE_CXX_STANDARD LESS 17)
  message(FATAL_ERROR
      "my_lib_project requires CMAKE_CXX_STANDARD >= 17 (got: ${CMAKE_CXX_STANDARD})")
endif()
```

Then the top-level application project that uses your library is responsible for
setting a consistent `CMAKE_CXX_STANDARD` that is sufficiently high.

### Running Abseil Tests with CMake

Use the `-DABSL_BUILD_TESTING=ON` flag to run Abseil tests.  Note that
BUILD_TESTING must also be on (the default).

You will need to provide Abseil with a Googletest dependency.  There are two
options for how to do this:

* Use `-DABSL_USE_GOOGLETEST_HEAD`.  This will automatically download the latest
Googletest source into the build directory at configure time.  Googletest will
then be compiled directly alongside Abseil's tests.
* Manually integrate Googletest with your build.  See
https://github.com/google/googletest/blob/master/googletest/README.md#using-cmake
for more information on using Googletest in a CMake project.

For example, to run just the Abseil tests, you could use this script:

```
cd path/to/abseil-cpp
mkdir build
cd build
cmake -DABSL_BUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON ..
make -j
ctest
```

Currently, we only run our tests with CMake in a Linux environment, but we are
working on the rest of our supported platforms. See
https://github.com/abseil/abseil-cpp/projects/1 and
https://github.com/abseil/abseil-cpp/issues/109 for more information.

### Available Abseil CMake Public Targets

Here's a non-exhaustive list of Abseil CMake public targets:

```cmake
absl::algorithm
absl::base
absl::debugging
absl::flat_hash_map
absl::flags
absl::memory
absl::meta
absl::numeric
absl::random_random
absl::strings
absl::synchronization
absl::time
absl::utility
```

## Traditional CMake Set-Up

For larger projects, it may make sense to use the traditional CMake set-up where you build and install projects separately.

First, you'd need to build and install Google Test:
```
cmake -S /source/googletest -B /build/googletest -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/installation/dir -DBUILD_GMOCK=ON
cmake --build /build/googletest --target install
```

Then you need to configure and build Abseil. Make sure you enable `ABSL_USE_EXTERNAL_GOOGLETEST` and `ABSL_FIND_GOOGLETEST`. You also need to enable `ABSL_ENABLE_INSTALL` so that you can install Abseil itself.
```
cmake -S /source/abseil-cpp -B /build/abseil-cpp -DCMAKE_PREFIX_PATH=/installation/dir -DCMAKE_INSTALL_PREFIX=/installation/dir -DABSL_ENABLE_INSTALL=ON -DABSL_USE_EXTERNAL_GOOGLETEST=ON -DABSL_FIND_GOOGLETEST=ON
cmake --build /temporary/build/abseil-cpp
```

(`CMAKE_PREFIX_PATH` is where you already have Google Test installed; `CMAKE_INSTALL_PREFIX` is where you want to have Abseil installed; they can be different.)

Run the tests:
```
ctest --test-dir /temporary/build/abseil-cpp
```

And finally install:
```
cmake --build /temporary/build/abseil-cpp --target install
```

# CMake Option Synopsis

## Enable Standard CMake Installation

`-DABSL_ENABLE_INSTALL=ON`

## Google Test Options

`-DABSL_BUILD_TESTING=ON` must be set to enable testing

- Have Abseil download and build Google Test for you: `-DABSL_USE_EXTERNAL_GOOGLETEST=OFF` (default)
  - Download and build latest Google Test: `-DABSL_USE_GOOGLETEST_HEAD=ON`
  - Download specific Google Test version (ZIP archive): `-DABSL_GOOGLETEST_DOWNLOAD_URL=https://.../version.zip`
  - Use Google Test from specific local directory: `-DABSL_LOCAL_GOOGLETEST_DIR=/path/to/googletest`
- Use Google Test included elsewhere in your project: `-DABSL_USE_EXTERNAL_GOOGLETEST=ON`
- Use standard CMake `find_package(CTest)` to find installed Google Test: `-DABSL_USE_EXTERNAL_GOOGLETEST=ON -DABSL_FIND_GOOGLETEST=ON`