How to enable C++17 in CMake

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.

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)

Your approach is the correct one, but it will not work for MSVC on versions of CMake prior to 3.10.

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.

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..
            )

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 for Visual Studio:

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

EDIT: As the question title doesn't mention the compiler, let me add that for gcc, clang and similar compilers, this is the command to enable C++17:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17")

when using vs2019

set(CMAKE_CXX_STANDARD 17)

Based on my test, the following code will enable VS2019 to select /std:c++latest, otherwise for other platforms it will set the C++ version to C++17. I have tested this with emscripten, raspbian, and windows.

if(MSVC)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++latest")
else(MSVC)
target_compile_features(${PROJECT_NAME} PRIVATE cxx_std_17)
endif(MSVC)

Bash command line in CMake flags:

-DCMAKE_CXX_STANDARD=17 \
-DCMAKE_CXX_STANDARD_REQUIRED=ON \
-DCMAKE_CXX_EXTENSIONS=OFF \

You can also use target_compile_options to set /std:c++latest flag for Visual Studio 2019

if (MSVC_VERSION GREATER_EQUAL "1900")
    include(CheckCXXCompilerFlag)
    CHECK_CXX_COMPILER_FLAG("/std:c++latest" _cpp_latest_flag_supported)
    if (_cpp_latest_flag_supported)
        target_compile_options(${TARGET_NAME} PRIVATE "/std:c++latest")
    endif()
endif()

Replace ${TARGET_NAME} with the actual target name.