CMake generator expression, differentiate C / C++ code

Question

I would like to add -std=c++11 to my

add_compile_options(\"-std=c++11\")

However, this also adds them to compilation of C

Answer 1

When you have mixed C and C++ sources, the LINKER_LANGUAGE property might apply the wrong flags for compilation of individual sources. The solution is to use the COMPILE_LANGUAGE generator expression (introduced with CMake 3.3). The simplest example for your original C++1x flag is:

add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-std=c++11>)

When you have a string of compile options (for example, for usage with the COMPILE_FLAGS target property), you have to split the flags

set(WARNCFLAGS "-Wall -Wextra -Wfuzzle -Wbar")
# ...
string(REPLACE " " ";" c_flags "${WARNCFLAGS}")
string(REPLACE " " ";" cxx_flags "${WARNCXXFLAGS} ${CXX1XCXXFLAGS}")
add_compile_options(
  "$<$<COMPILE_LANGUAGE:C>:${c_flags}>"
  "$<$<COMPILE_LANGUAGE:CXX>:${cxx_flags}>"
)
# Two alternative variants for single targets that take strings:
target_compile_options(some-target PRIVATE "${WARNCFLAGS}")
set_target_properties(some-target PROPERTIES
  COMPILE_FLAGS "${WARNCFLAGS}")

Use of strings is however deprecated in favor of lists. When lists are in use, you can use:

set(c_flags -Wall -Wextra -Wfuzzle -Wbar)
# ...
add_compile_options(
  "$<$<COMPILE_LANGUAGE:C>:${c_flags}>"
  "$<$<COMPILE_LANGUAGE:CXX>:${cxx_flags}>"
)
# Two alternative variants for single targets given a list:
target_compile_options(some-target PRIVATE ${f_flags})
set_target_properties(some-target PROPERTIES
  COMPILE_OPTIONS "${c_flags}")

Pay attention to the quoting. If a list is not quotes, it is expanded to its items (and is no longer a list). To pass a list between commands, quote it.

Answer 2

You can use LINKER_LANGUAGE target property to add flag only to C++ targets*:

add_compile_options(
    "$<$<STREQUAL:$<TARGET_PROPERTY:LINKER_LANGUAGE>,CXX>:-std=c++11>"
)

_{*Note that this will not work for targets with mixed C/C++ sources}

CMAKE_CXX_FLAGS should work fine too:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

Probably you need to add them to cache if it set before project command (e.g. in toolchain):

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11" CACHE STRING "" FORCE)

Answer 3

I'd rather do it like this:

set_source_files_properties(
    ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp
    PROPERTIES COMPILE_FLAGS "-std=c++11")

where the documentation for set_source_files_properties is at http://www.cmake.org/cmake/help/v3.0/command/set_source_files_properties.html