How to enable C++17 in CMake

Question

I'm using VS 15.3, which supports integrated CMake 3.8. How can I target C++17 without writing flags for each specific compilers? My current global settings don't work:

# https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

# expected behaviour
#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++lastest")

I expected CMake to add "/std:c++lastest" or equivalents when generating VS solution files, but no c++17 flags was found, resulted in compiler error:

C1189 #error: class template optional is only available with C++17.

Answer 1

From the CMake 3.9 documentation:

For compilers that have no notion of a standard level, such as MSVC, this has no effect.

In short, CMake haven't been updated to accommodate for the standard flags added to VC++ 2017.

You have to detect if VC++ 2017 (or later) is used and add the corresponding flags yourself for now.

---

In CMake 3.10 (and later) this have been fixed for newer version of VC++. See the 3.10 documentation.

Answer 2

In modern CMake, I've found it best to assign CXX standards at the target level instead of global variable level and use the built in properties (seen here: https://cmake.org/cmake/help/latest/manual/cmake-properties.7.html) to keep it compiler agnostic.

For Example:

set_target_properties(FooTarget PROPERTIES
            CXX_STANDARD 17
            CXX_EXTENSIONS OFF
            etc..
            )

Answer 3

You can keep that set(CMAKE_CXX_STANDARD 17) for other compilers, like Clang and GCC. But for Visual Studio, it's useless.

If CMake still doesn't support this, you can do the following:

if(MSVC)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++17")
endif(MSVC)

Answer 4

Modern CMake propose an interface for this purpose target_compile_features. Documentation is here: Requiring Language Standards

Use it like this:

target_compile_features(${TARGET_NAME} PRIVATE cxx_std_17)