Does set_target_properties in CMake override CMAKE_CXX_FLAGS? - c++

Question

At the beginning of my CMake project, I'm setting general compilation flags in the variable CMAKE_CXX_FLAGS, like
set(CMAKE_CXX_FLAGS "-W -Wall ${CMAKE_CXX_FLAGS}")
Later on, I need to append additional configuration-specific compilation flags (stored in BUILD_FLAGS). Can I use the following command for this:
set_target_properties(${TARGET} PROPERTIES COMPILE_FLAGS ${BUILD_FLAGS})
or do I have to add the CMAKE_CXX_FLAGS manually:
set_target_properties(${TARGET} PROPERTIES COMPILE_FLAGS "${CMAKE_CXX_FLAGS} ${BUILD_FLAGS}")
to prevent CMAKE_CXX_FLAGS being overriden by BUILD_FLAGS?

Answer 1

The accepted answer is still working but outdated since 2013.
This answer is based and new functions from CMake v2.8.12, v3.3 and v3.13.
Since CMake-2.8.12 (2013)
Two new commands to set CMAKE_CXX_FLAGS:
target_compile_options() (for one single target)
add_compile_options() (for all targets)
The documentation of last version has not changed a lot since cmake-2.8.12:
target_compile_options()
add_compile_options()
In you case you can use:
target_compile_options(${TARGET} PRIVATE ${BUILD_FLAGS})
Or simply if you have a single target:
add_compile_options(${BUILD_FLAGS})
More examples
target_compile_options(mylib PRIVATE -O2) # only internal
target_compile_options(mylib INTERFACE -gl) # only external
target_compile_options(mylib PUBLIC -g) # same as PRIVATE + INTERFACE
# multiple targets and flags
target_compile_options(mylib1 mylib2 PRIVATE -Wall -Wextra)
target_compile_options( mylib PUBLIC -DUSEXX) # Bad
target_compile_definitions(mylib PUBLIC -DUSEXX) # OK
add_compile_options(-Wall -Wextra) # for all targets in current directory
add_compile_options(-DUSEXX) # Bad
add_definitions(-DUSEXX) # OK
Deprecated COMPILE_FLAGS
cmake-3.0 documentation flags COMPILE_FLAGS as deprecated:
COMPILE_FLAGS
Additional flags to use when compiling this target’s sources.
The COMPILE_FLAGS property sets additional compiler flags used to
build sources within the target. Use COMPILE_DEFINITIONS to pass
additional preprocessor definitions.
This property is deprecated. Use the COMPILE_OPTIONS property or the
target_compile_options command instead.
If you still want to use set_target_properties() you may use COMPILE_OPTIONS instead of COMPILE_FLAGS:
set_target_properties(${TARGET} PROPERTIES COMPILE_OPTIONS ${BUILD_FLAGS})
Since CMake-3.3 (2015)
Anton Petrov suggests to use generator expressions as presented in an answer of ar31.
The CMake generator expressions applies your ${BUILD_FLAGS} to:
C++ language using $<COMPILE_LANGUAGE:CXX> (can also be C, CUDA...)
Clang compiler using $<CXX_COMPILER_ID:Clang>
(can also be GNU for gcc, or MSVCfor Visual C++... see full list)
(use $<C_COMPILER_ID:Clang> instead if language is C)
and more as supported C++ feature or compiler version... (see documentation)
In you case you can use:
target_compile_options(${TARGET} PRIVATE
$<$<COMPILE_LANGUAGE:CXX>:${BUILD_FLAGS_FOR_CXX}>
$<$<COMPILE_LANGUAGE:C>:${BUILD_FLAGS_FOR_C}>)
or about compilers:
target_compile_options(${TARGET} PRIVATE
$<$<CXX_COMPILER_ID:Clang>:${BUILD_FLAGS_FOR_CLANG}>
$<$<CXX_COMPILER_ID:GNU>:${BUILD_FLAGS_FOR_GCC}>
$<$<CXX_COMPILER_ID:MSVC>:${BUILD_FLAGS_FOR_VISUAL}>)
Since CMake-3.13 (2018)
A new function target_link_options() allow to pass options to the linker, as mentioned by Craig Scott.
Different options for C and C++ files
The best way is to distinguish C files and C++ files using two different targets.

Answer 2

Use the first one:
set_target_properties(${TARGET} PROPERTIES COMPILE_FLAGS ${BUILD_FLAGS})
The flags stored in BUILD_FLAGS are appended after CMAKE_CXX_FLAGS when compiling the sources of TARGET. The documentation hints at this, but I've just tried it to make sure.
COMPILE_FLAGS
Additional flags to use when compiling this target's sources.
The COMPILE_FLAGS property sets additional compiler flags used to
build sources within the target. Use COMPILE_DEFINITIONS to
pass additional preprocessor definitions.
The full command line will be the equivalent of:
${CMAKE_CXX_COMPILER} ${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS} -o foo.o -c foo.cc
And as Ramon said, you can always check with make VERBOSE=1.