How to enable /std:c++17 in VS2017 with CMake

Question

I'm trying to add the /std:c++17 compiler flag to VS2017 with CMake. I'm using the "modern" cross-platform way so far:

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF) # -std=c++11 instead of -std=gnu++11
set(MY_CXX_COMPILE_FEATURES cxx_generic_lambdas cxx_range_for cxx_strong_enums)

add_library(mylib INTERFACE)
target_compile_features(mylib INTERFACE ${MY_CXX_COMPILE_FEATURES})

This adds /std:c++14 in VS2017 (which might be the default anyway?). However I'm having trouble switching this to C++17 (i.e. having it add /std:c++17). If I just add it manually, I get the not-so-nice warning because both flags are present:

1>cl : Command line warning D9025: overriding '/std:c++14' with '/std:c++17'

I've tried set(CMAKE_CXX_STANDARD 17) but it has no effect, in fact the CMake documentation mentions that CMAKE_CXX_STANDARD has no effect on VS anyway.

As for adding a C++17 feature to target_compile_features, it doesn't seem like there are any yet (even in CMake-3.9.0-rc5), and even if there were, I'm specifically only using std::optional from C++17, and there's no target_compile_features flags for library features like std::optional.

So my question is, what's the best (or least ugly) way to do this with CMake? And in a way so it'll also work for gcc and clang? I'm happy to use a very recent CMake version (3.8 or 3.9). I prefer it to be "nice" and not manually looping through CXX_COMPILE_FLAGS and removing the string "/std:c++14" or some hack like that.

(Edit: It can also be the VS/std:c++latest switch - whichever is possible. Both work for the purpose.)

Solution 1

Turning my comment into an answer

1. The CMake team is working on it for VS2017 (as for July 2017, for upcoming CMake version 3.10):

   CMake: MSVC standard version switches

   Those flags seem to be rather new switches (as related to the date of this question):

   VS 2017 15.3 preview now supports /std:c++17

   So for Visual Studio you have to "manually" replace or append the compiler switches until CMake officially does support it.

   Here is a code snippet that I've tested for std:c++latest (which is already supported e.g. in my CMake 3.8.0 version):

   if (MSVC_VERSION GREATER_EQUAL "1900")
       include(CheckCXXCompilerFlag)
       CHECK_CXX_COMPILER_FLAG("/std:c++latest" _cpp_latest_flag_supported)
       if (_cpp_latest_flag_supported)
           add_compile_options("/std:c++latest")
       endif()
   endif()
   

2. For CLang and GNU the support was merged into the main source code branch begin of 2017 and is part of CMake version 3.8 and above:

   CMake: Features: Add support for C++ 17 language standard

Solution 2

CMake versions higher than 3.10 support MSVC C++ standard switches for MSVC versions newer than 19.0.24215. If either of the version requirements are not met, then they have no effect.

The only portable approach, to ensuring your program is compiled with the correct C++ standard mode on Visual Studio, is to require at least CMake 3.10, set the target property CXX_STANDARD to your desired value and CXX_STANDARD_REQUIRED to ON.

Example usage:

set_property(TARGET my_target PROPERTY CXX_STANDARD 17)
set_property(TARGET my_target PROPERTY CXX_STANDARD_REQUIRED ON)

Comments

[Ela782]

Hmm! Too bad that didn't land in CMake yet, the issue is 7 months old :-( (The flags are not really too new, VS2015 already had them). Anyway. Given I'm using above "modern" and target-based approach, what would be the best way to manually replace/add the flag for MSVC?

[Ela782]

I think this is going to be a headache. I can't get_target_property(my_compile_flags mylib COMPILE_FLAGS) because it's an INTERFACE target (header-only library)...

[Ela782]

I've asked this in a new question: stackoverflow.com/questions/44977868/…

[Florian]

@Ela782 Added a code snippet that should work as a global setting. Just don't put any CMAKE_CXX_STANDARD definition parallel to it (to avoid conflicts in the generated project file).

[Mikhail]

Is there a way to set /std:c++latest with it?

[Mikhail]

With add_compile_options we can pass whatever we want, so this is not actually interesting. Is there a way to set /std:c++latest in a portable way with e.g. CMAKE_CXX_STANDARD?

[tambre]

@Mikhail No. Consensus here seems to be to not add a LATEST value for CXX_STANDARD, but rather support each standard version separately. I've created an issue for adding C++2a support.

[tambre]

@Mikhail Support for C++20 is now in CMake nightlies. Mind trying out?

[yano]

bit of an update, using CMake 3.14.3 and VS2019, all I had to do was set(CMAKE_CXX_STANDARD 17) in my CMakeLists.txt files