CMake disable -std=c++11 flag for C files

Question

I'm trying to build bkchaind. One build option is to use cmake, so I installed it with Homebrew (OSX 10.9.1). When I do cmake, though, I get:

[  2%] Building C object json-rpc-cpp/src/jsonrpc/CMakeFiles/jsonrpcStatic.dir/connectors/mongoose.c.o
error: invalid argument '-std=c++11' not allowed with 'C/ObjC'

I am none too sure why cmake would try to pass a C++-specific compiler option to a C/ObjC file. If I comment out this line in the main CMakeLists.txt file:

ADD_DEFINITIONS(-std=c++11)

then it no longer passes the flag to any file. However, the C++ files do need it. How do I get cmake to include the flag for C++ files, but not for C files?

Answer 1

This type of thing can be done explicitly without any dependence on the specific format of a compiler's flags:

target_compile_features(someTargetName PUBLIC cxx_std_14)

target_compile_features can also do this more implicitly I think, by requesting particular language features that are only found in the desired version of the language.

Another way is to set a target property:

set_property(TARGET server PROPERTY CXX_STANDARD 14)

Or one may set CMAKE_CXX_STANDARD, whose value is used as the default value for CXX_STANDARD on targets, and then allow targets to inherit that value implicitly.

(Here I'm using the 2014 standard, but of course one may specify 11 or some other value.)

Answer 2

Use CMAKE_CXX_FLAGS to set c++ specific flags:

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