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($<$:-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} ${CXX1XCXXFLAGS}")
add_compile_options(
  "$<$:${c_flags}>"
  "$<$:${cxx_flags}>"
)
# Two alternative variants for single targets that take strings:
target_compile_options(some-target PRIVATE "${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(
  "$<$:${c_flags}>"
  "$<$:${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.