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# 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
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
### Step-by-Step Instructions
1. If you want to build the Abseil tests, integrate the Abseil dependency
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
CMake project.
3. You can then use the CMake command
to include Abseil directly in your CMake project.
4. Add the **absl::** target you wish to use to the
section of your executable or of your library.<br>
Here is a short CMakeLists.txt example of an application project using Abseil.
```cmake
cmake_minimum_required(VERSION 3.10)
project(my_app_project)
# Pick the C++ standard to compile with.
# Abseil currently supports C++14, C++17, and C++20.
set(CMAKE_CXX_STANDARD 14)
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 3.10)
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
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
### 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`