Enable the latest C++ standard with Visual Studio

[naszta]

If the following flag would be added to MSVC part, the MSVC C++14 and C++17 features could be used in the code (including std::string_view and variadic template std::less).

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++latest")

I'm happy to write the code (opt-out as the latest VS runs properly with all tests passed).

[nlohmann]

Is it good style to set these flags from the project? Wouldn't this be something the user sets?

[naszta]

In visual studio project I normally specify explicitly /std:c++14 or /std:c++17 for an application. But for tests or libs (when both VS2015 and VS2017 should be supported) I definitely would use the latest flag.

Reference

[nlohmann]

I'm not using MSVC myself, that's why I would like to discuss whether it is best practice to set such switches.

[gregmarr]

What features are you looking to use, and how would you make sure that the library is still usable with only C++11?

[naszta]

There are standard switch macros in the code (JSON_HAS_CPP_17 and JSON_HAS_CPP_14). They are set accordingly on used standard by the compiler (in VS case _HAS_CXX17 and _HAS_CXX14). With this flag always the latest standard could be used what supported by VS. (In current code variadic std::less and std::string_ref are protected by these macros.)

[nlohmann]

I still have not understood why this library should set the C++ version when this is usually done by the code that actually uses the library. Do I miss something between CMake and MSVC?

[naszta]

No: my idea here is using the latest supported C++ in MSVC even in tests: MSVC is not a major platform for you and in this way you will have less macros for VS (as it uses the latest standard what it could handle).

[nlohmann]

I am already testing with these flags on AppVeyor. And if I would remove macros, wouldn't this break support for older MSVC versions?

[naszta]

The flag is supported since Visual C++ 2015 Update 3. If older VS versions are supported in tests, it shouldn't be introduced. Otherwise it's safe to go.

[nlohmann]

I still have not understood the benefit of enforcing this flag to projects using the library. FWIW, we use this AppVeyor configuration, where we additionally check /permissive- /std:c++latest /utf-8 for MSVC 2017.

[naszta]

Ok, nevermind.