cmake: adding C++ standard when not required - c++

Question

When building a C++ executable under Linux using cmake 3.7, I see a -std=gnu++11 flag being added to compile flags. The problem is, I'm already manually adding a -std=c++1z flag, and this new one overwrites mine. This happens only for executables, but I cannot find this being mentioned in the docs. The CMAKE_CXX_STANDARD is empty, and setting the CXX_STANDARD property on the target has no effect. Is there a way to remove this flag?
This seems to be not only limited to executables.
Here's my (simplified) 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

As #florian suspected, it's Qt5 that's polluting your compile commands. Using a similar 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
)
I configured and built a dummy executable. You can plainly see the -std=c++1z and the -std=gnu++11 on the compile line:
❯ 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
If you comment out the Qt5 usage in our CMakeLists.txt and configure and build again, you'll see the -std=gnu++11 disappear (along with the -fPIC which Qt is also adding).
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 ${Boost_LIBRARIES}
# PRIVATE Qt5::Widgets
)
make output (abridged):
[...]
[ 50%] Building CXX object CMakeFiles/foo.dir/main.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -I/usr/local/opt/boost#1.55/include -std=c++1z -o CMakeFiles/foo.dir/main.cpp.o -c /Users/nega/foo/main.cpp
[100%] Linking CXX executable foo
/usr/local/Cellar/cmake/3.7.2/bin/cmake -E cmake_link_script CMakeFiles/foo.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -std=c++1z -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/foo.dir/main.cpp.o -o foo -L/usr/local/opt/boost#1.55/lib -Wl,-rpath,/usr/local/opt/boost#1.55/lib /usr/local/opt/boost#1.55/lib/libboost_serialization-mt.dylib /usr/local/opt/boost#1.55/lib/libboost_system-mt.dylib
[100%] Built target foo
[...]
Unfortunately, after some brief digging I couldn't see where Qt was setting -std=gnu++11 in its *Config.cmake files. It must be reaching into CMake more than just a few grep's could find. Maybe reading through cmake --trace will provide some insight.
Curiously though, what ever it's doing respects CXX_STANDARD. If we tweak our original CMakeLists.txt and configure and build again:
CMakeLists.txt (abridged):
cmake_minimum_required(VERSION 3.7.2 FATAL_ERROR)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z")
set(BOOST_ROOT "/usr/local/opt/boost#1.55")
execute_process(COMMAND brew --prefix qt5
[...]
make output (abridged):
[...]
[ 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
[...]
You can see that the (Qt added) -fPIC -std=gnu++11 is now -fPIC -std=gnu++14. Unfortunately this won't help you until CMake 3.8.0 is released and its CXX_STANDARD/CMAKE_CXX_STANDARD will understand "C++17".