cmake：不需要时添加C++标准

Question

在Linux下使用cmake 3.7构建C++可执行文件时，我看到要添加一个-std=gnu++11标志来编译标志。问题是，我已经手动添加一个-std=c++1z标志，并且这个新标志覆盖了我的标志。这只发生在可执行文件中，但我无法在文档中找到它。 CMAKE_CXX_STANDARD为空，并且在目标上设置CXX_STANDARD属性不起作用。有没有办法去除这个标志？

这似乎不仅限于可执行文件。

这是我（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

由于@florian怀疑，这是QT5多数民众赞成污染你的编译命令。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。你可以清楚地看到-std=c++1z和编译行-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 

如果您对我们CMakeLists.txt注释掉QT5使用和重新配置并建立，你会看到-std=gnu++11消失（与-fPIC沿Qt也在增加）。

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 
) 

make输出（有删节）：

[...] 
[ 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，一些简单的挖我看不出哪里后的Qt在设置-std=gnu++11其*Config.cmake文件。它必须达到CMake以上的几个grep的能找到的。也许通过cmake --trace阅读将提供一些见解。

不过，奇怪的是，它在做什么都遵守CXX_STANDARD。如果我们调整我们原来的CMakeLists.txt和重新配置并建立：

的CMakeLists.txt（有删节）：

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 
[...] 

make输出（有删节）：

[...] 
[ 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 
[...] 

你可以看到（Qt新增）-fPIC -std=gnu++11现在是-fPIC -std=gnu++14。不幸的是，直到CMake 3.8.0发布和它的CXX_STANDARD/CMAKE_CXX_STANDARDwill understand "C++17"才会对你有所帮助。