diff options
| -rw-r--r-- | CMake/AbseilHelpers.cmake | 49 | ||||
| -rw-r--r-- | CMake/README.md | 45 | ||||
| -rw-r--r-- | CMakeLists.txt | 7 | ||||
| -rw-r--r-- | FAQ.md | 5 |
4 files changed, 97 insertions, 9 deletions
diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake index 6a64a2c7..17c4f449 100644 --- a/CMake/AbseilHelpers.cmake +++ b/CMake/AbseilHelpers.cmake @@ -254,9 +254,23 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n") set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal) endif() - # INTERFACE libraries can't have the CXX_STANDARD property set - set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD}) - set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON) + if(ABSL_PROPAGATE_CXX_STD) + # Abseil libraries require C++11 as the current minimum standard. + # Top-level application CMake projects should ensure a consistent C++ + # standard for all compiled sources by setting CMAKE_CXX_STANDARD. + target_compile_features(${_NAME} PUBLIC cxx_std_11) + else() + # Note: This is legacy (before CMake 3.8) behavior. Setting the + # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is + # initialized by CMAKE_CXX_STANDARD) should have no real effect, since + # that is the default value anyway. + # + # CXX_STANDARD_REQUIRED does guard against the top-level CMake project + # not having enabled CMAKE_CXX_STANDARD_REQUIRED (which prevents + # "decaying" to an older standard if the requested one isn't available). + set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD}) + set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON) + endif() # When being installed, we lose the absl_ prefix. We want to put it back # to have properly named lib files. This is a no-op when we are not being @@ -287,6 +301,16 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n") ${ABSL_DEFAULT_LINKOPTS} ) target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES}) + + if(ABSL_PROPAGATE_CXX_STD) + # Abseil libraries require C++11 as the current minimum standard. + # Top-level application CMake projects should ensure a consistent C++ + # standard for all compiled sources by setting CMAKE_CXX_STANDARD. + target_compile_features(${_NAME} INTERFACE cxx_std_11) + + # (INTERFACE libraries can't have the CXX_STANDARD property set, so there + # is no legacy behavior else case). + endif() endif() # TODO currently we don't install googletest alongside abseil sources, so @@ -390,8 +414,23 @@ function(absl_cc_test) # Add all Abseil targets to a folder in the IDE for organization. set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test) - set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD}) - set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON) + if(ABSL_PROPAGATE_CXX_STD) + # Abseil libraries require C++11 as the current minimum standard. + # Top-level application CMake projects should ensure a consistent C++ + # standard for all compiled sources by setting CMAKE_CXX_STANDARD. + target_compile_features(${_NAME} PUBLIC cxx_std_11) + else() + # Note: This is legacy (before CMake 3.8) behavior. Setting the + # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is + # initialized by CMAKE_CXX_STANDARD) should have no real effect, since + # that is the default value anyway. + # + # CXX_STANDARD_REQUIRED does guard against the top-level CMake project + # not having enabled CMAKE_CXX_STANDARD_REQUIRED (which prevents + # "decaying" to an older standard if the requested one isn't available). + set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD}) + set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON) + endif() add_test(NAME ${_NAME} COMMAND ${_NAME}) endfunction() diff --git a/CMake/README.md b/CMake/README.md index fa8e8d30..f8b27e63 100644 --- a/CMake/README.md +++ b/CMake/README.md @@ -34,15 +34,16 @@ 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.<br> -Here is a short CMakeLists.txt example of a project file using Abseil. +Here is a short CMakeLists.txt example of an application project using Abseil. ```cmake -cmake_minimum_required(VERSION 3.5) -project(my_project) +cmake_minimum_required(VERSION 3.8.2) +project(my_app_project) # Pick the C++ standard to compile with. # Abseil currently supports C++11, C++14, and C++17. set(CMAKE_CXX_STANDARD 11) +set(CMAKE_CXX_STANDARD_REQUIRED ON) add_subdirectory(abseil-cpp) @@ -50,6 +51,44 @@ 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 3.8.2) +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: ${CMAKE_CXX_STANDARD})") +endif() +``` + +Then the top-level application project that uses your library is responsible for +setting a consistent `CMAKE_CXX_STANDARD` that is sufficiently high. + ### Running Abseil Tests with CMake Use the `-DBUILD_TESTING=ON` flag to run Abseil tests. diff --git a/CMakeLists.txt b/CMakeLists.txt index 42bcbe10..7c8bfff4 100644 --- a/CMakeLists.txt +++ b/CMakeLists.txt @@ -67,6 +67,13 @@ else() option(ABSL_ENABLE_INSTALL "Enable install rule" ON) endif() +option(ABSL_PROPAGATE_CXX_STD + "Use CMake C++ standard meta features (e.g. cxx_std_11) that propagate to targets that link to Abseil" + OFF) # TODO: Default to ON for CMake 3.8 and greater. +if((${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.8) AND (NOT ABSL_PROPAGATE_CXX_STD)) + message(WARNING "A future Abseil release will default ABSL_PROPAGATE_CXX_STD to ON for CMake 3.8 and up. We recommend enabling this option to ensure your project still builds correctly.") +endif() + list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/CMake ${CMAKE_CURRENT_LIST_DIR}/absl/copts @@ -27,7 +27,10 @@ compiler, there several ways to do this: file](https://docs.bazel.build/versions/master/guide.html#bazelrc) If you are using CMake as the build system, you'll need to add a line like -`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. See the +`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. If you +are developing a library designed to be used by other clients, you should +instead leave `CMAKE_CXX_STANDARD` unset and configure the minimum C++ standard +required by each of your library targets via `target_compile_features`. See the [CMake build instructions](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md) for more information. |