c++ - cmake:在不需要时添加 C++ 标准

Question

当使用 cmake 3.7 在 Linux 下构建 C++ 可执行文件时，我看到一个 -std=gnu++11 标志被添加到编译标志中。问题是，我已经手动添加了一个 -std=c++1z 标志，这个新标志覆盖了我的。这只发生在可执行文件上，但我找不到文档中提到的这一点。 CMAKE_CXX_STANDARD 为空，在目标上设置 CXX_STANDARD 属性无效。有没有办法删除这个标志？

这似乎不仅限于可执行文件。

这是我的(简化的)cmake:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z")    
find_boost(serialization system)    
find_package(Qt5Widgets REQUIRED)    
link_directories(${Boost_LIBRARY_DIRS})
include_directories(
    ${Boost_INCLUDE_DIRS}
    ${ZMQ_INCLUDE_DIR}
    ${CPPZMQ_INCLUDE_DIR}
)

if(WIN32)
    add_definitions(-DNOMINMAX)
endif()

add_executable(
    ${PROFILER_CLIENT_NAME}
    main.cpp
    MainWindow.cpp
    MainWindow.h
    ProfilerWidget.cpp
    ProfilerWidget.h
    TimelineWidget.cpp
    TimelineWidget.h
    ZmqReceiver.cpp
    ZmqReceiver.h
)

add_dependencies(${PROFILER_CLIENT_NAME} boost zeromq)

target_link_libraries(
    ${PROFILER_CLIENT_NAME}
    PRIVATE ${PROFILER_NAME}
    PRIVATE ${Boost_LIBRARIES}
    PRIVATE Qt5::Widgets
)

Answer 1

正如@florian 所怀疑的那样，是 Qt5 污染了您的编译命令。使用类似的 CMakeLists.txt:

cmake_minimum_required(VERSION 3.7.2 FATAL_ERROR)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z")
set(BOOST_ROOT "/usr/local/opt/boost@1.55")
execute_process(COMMAND brew --prefix qt5
  COMMAND tr -d \\n
  OUTPUT_VARIABLE QT5_BREW_PATH)

find_package(Boost COMPONENTS serialization system)
find_package(Qt5 COMPONENTS Widgets HINTS ${QT5_BREW_PATH})

link_directories(${Boost_LIBRARY_DIRS})
include_directories(${Boost_INCLUDE_DIRS})

add_executable(foo main.cpp)
target_link_libraries(foo
  PRIVATE ${Boost_LIBRARIES}
  PRIVATE Qt5::Widgets
  )

我配置并构建了一个虚拟可执行文件。您可以在编译行上清楚地看到 -std=c++1z 和 -std=gnu++11:

❯ make VERBOSE=1
/usr/local/Cellar/cmake/3.7.2/bin/cmake -H/Users/nega/foo -B/Users/nega/foo --check-build-system CMakeFiles/Makefile.cmake 0
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_progress_start /Users/nega/foo/CMakeFiles /Users/nega/foo/CMakeFiles/progress.marks
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/Makefile2 all
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/foo.dir/build.make CMakeFiles/foo.dir/depend
cd /Users/nega/foo && /usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_depends "Unix Makefiles" /Users/nega/foo /Users/nega/foo /Users/nega/foo /Users/nega/foo /Users/nega/foo/CMakeFiles/foo.dir/DependInfo.cmake --color=
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/foo.dir/build.make CMakeFiles/foo.dir/build
[ 50%] Building CXX object CMakeFiles/foo.dir/main.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++   -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NO_DEBUG -DQT_WIDGETS_LIB -I/usr/local/opt/boost@1.55/include -iframework /usr/local/opt/qt5/lib -isystem /usr/local/opt/qt5/lib/QtWidgets.framework/Headers -isystem /usr/local/opt/qt5/lib/QtGui.framework/Headers -isystem /System/Library/Frameworks/OpenGL.framework/Headers -isystem /usr/local/opt/qt5/lib/QtCore.framework/Headers -isystem /usr/local/opt/qt5/./mkspecs/macx-clang  -std=c++1z   -fPIC -std=gnu++11 -o CMakeFiles/foo.dir/main.cpp.o -c /Users/nega/foo/main.cpp
[100%] Linking CXX executable foo
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_link_script CMakeFiles/foo.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++    -std=c++1z -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/foo.dir/main.cpp.o  -o foo  -L/usr/local/opt/boost@1.55/lib -Wl,-rpath,/usr/local/opt/boost@1.55/lib /usr/local/opt/boost@1.55/lib/libboost_serialization-mt.dylib /usr/local/opt/boost@1.55/lib/libboost_system-mt.dylib /usr/local/opt/qt5/lib/QtWidgets.framework/QtWidgets /usr/local/opt/qt5/lib/QtGui.framework/QtGui /usr/local/opt/qt5/lib/QtCore.framework/QtCore 
[100%] Built target foo
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_progress_start /Users/nega/foo/CMakeFiles 0

如果您在我们的 CMakeLists.txt 中注释掉 Qt5 用法并重新配置和构建，您将看到 -std=gnu++11 消失(以及使用 Qt 也添加的 -fPIC。

CMakeLists.txt:

cmake_minimum_required(VERSION 3.7.2 FATAL_ERROR)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z")
set(BOOST_ROOT "/usr/local/opt/boost@1.55")
execute_process(COMMAND brew --prefix qt5
  COMMAND tr -d \\n
  OUTPUT_VARIABLE QT5_BREW_PATH)

find_package(Boost COMPONENTS serialization system)
#find_package(Qt5 COMPONENTS Widgets HINTS ${QT5_BREW_PATH})

link_directories(${Boost_LIBRARY_DIRS})
include_directories(${Boost_INCLUDE_DIRS})

add_executable(foo main.cpp)
target_link_libraries(foo
  PRIVATE ${Boost_LIBRARIES}
# PRIVATE Qt5::Widgets
  )

制作输出(删节):

[...]
[ 50%] Building CXX object CMakeFiles/foo.dir/main.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++    -I/usr/local/opt/boost@1.55/include  -std=c++1z   -o CMakeFiles/foo.dir/main.cpp.o -c /Users/nega/foo/main.cpp
[100%] Linking CXX executable foo
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_link_script CMakeFiles/foo.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++    -std=c++1z -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/foo.dir/main.cpp.o  -o foo  -L/usr/local/opt/boost@1.55/lib -Wl,-rpath,/usr/local/opt/boost@1.55/lib /usr/local/opt/boost@1.55/lib/libboost_serialization-mt.dylib /usr/local/opt/boost@1.55/lib/libboost_system-mt.dylib 
[100%] Built target foo
[...]

不幸的是，经过一些简短的挖掘后，我看不到 Qt 在其 *Config.cmake 文件中设置 -std=gnu++11 的位置。它必须深入到 CMake 中，而不仅仅是几个 grep 可以找到的。也许通读 cmake --trace 会提供一些见解。

奇怪的是，它所做的一切都符合 CXX_STANDARD。如果我们调整原始 CMakeLists.txt 并重新配置和构建:

CMakeLists.txt(删节版):

cmake_minimum_required(VERSION 3.7.2 FATAL_ERROR)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z")
set(BOOST_ROOT "/usr/local/opt/boost@1.55")
execute_process(COMMAND brew --prefix qt5
[...]

制作输出(删节):

[...]
[ 50%] Building CXX object CMakeFiles/foo.dir/main.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++   -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NO_DEBUG -DQT_WIDGETS_LIB -I/usr/local/opt/boost@1.55/include -iframework /usr/local/opt/qt5/lib -isystem /usr/local/opt/qt5/lib/QtWidgets.framework/Headers -isystem /usr/local/opt/qt5/lib/QtGui.framework/Headers -isystem /System/Library/Frameworks/OpenGL.framework/Headers -isystem /usr/local/opt/qt5/lib/QtCore.framework/Headers -isystem /usr/local/opt/qt5/./mkspecs/macx-clang  -std=c++1z   -fPIC -std=gnu++14 -o CMakeFiles/foo.dir/main.cpp.o -c /Users/nega/foo/main.cpp
[100%] Linking CXX executable foo
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_link_script CMakeFiles/foo.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++    -std=c++1z -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/foo.dir/main.cpp.o  -o foo  -L/usr/local/opt/boost@1.55/lib -Wl,-rpath,/usr/local/opt/boost@1.55/lib /usr/local/opt/boost@1.55/lib/libboost_serialization-mt.dylib /usr/local/opt/boost@1.55/lib/libboost_system-mt.dylib /usr/local/opt/qt5/lib/QtWidgets.framework/QtWidgets /usr/local/opt/qt5/lib/QtGui.framework/QtGui /usr/local/opt/qt5/lib/QtCore.framework/QtCore 
[100%] Built target foo
[...]

您可以看到(Qt 添加的)-fPIC -std=gnu++11 现在是 -fPIC -std=gnu++14。不幸的是，在发布 CMake 3.8.0 及其 CXX_STANDARD/CMAKE_CXX_STANDARD will understand "C++17" 之前，这对您没有帮助。 .