How to detect which (C++) language standard was selected in the Project->General Properties->C++ Language Standard

Question

I'm developing with Visual Studio 2019, and would like to be able to compile my C++ program conditionally based on the language standard chosen (C++20, C++17, etc.) from Project Properties -> General Properties -> C++ Language Standard. What gets defined when I set it C++20, for example, so that I can use it as:

#ifdef WHAT_DO_I_PUT_HERE_FOR_C++_20 
#else
#ifdef WHAT_DO_I_PUT_HERE_FOR_C++_17
...

Why? See my answer, it will definitely work for conditional compilation. — jpo38, Dec 10 '21 at 21:30
@jpo378 _HAS_CXX17 or _HAS_CXX20 seems to work, but are they standard? — Vectorizer, Dec 10 '21 at 21:32
Dunno, this may be only set by Visual Studio. `__cplusplus` proposed in my answer should work for all compilers. — jpo38, Dec 10 '21 at 21:35

viraltaco_ · Answer 1 · 2021-12-11T16:23:11.647

You could use the MSVC predefined macro _MSVC_LANG:

#ifndef VT_CPP_VERSION
# if defined (_MSVC_LANG)
#  define VT_CPP_VERSION _MSVC_LANG
# else
#  define VT_CPP_VERSION __cplusplus
# endif
#endif

#if defined (VT_CPP_VERSION)
# if   (VT_CPP_VERSION >= 202002L)
  // 20 or above
# elif (VT_CPP_VERSION >= 201703L)
  // 17
# elif (VT_CPP_VERSION >= 201402L)
  // 14
# elif (VT_CPP_VERSION >= 201103L)
  // 11
# else
  // cry.
# endif
#endif

If you need to get this value after the code has already been compiled (for example: a shared library), then you can use a function.

[[maybe_unused]] long version_used() noexcept {
#if defined (_MSVC_LANG)
  return _MSVC_LANG;
#else
  return __cplusplus;
#endif
}

score 1 · Accepted Answer · answered Dec 10 '21 at 21:29

1

This should work:

#if (__cplusplus >= 202002L)
    // C++20 (or later) code
#elif (__cplusplus == 201703L) 
    // C++17 code
#else
    // C++14 or older code
#endif

answered Dec 10 '21 at 21:29

jpo38

20,821
10
70
151

Unfortunately it does not work with Visual Studio 2019 Version 16.10.4 Thanks for your input – Vectorizer Dec 10 '21 at 22:27
@Vectorizer That would then be a compliancy defect of Visual Studio, and a whooping big one at that. – DevSolar Dec 11 '21 at 16:28
@DevSolar: Maybe I am missing something. All I am doing is setting the C++ language standard to C++17 and expecting the relevant portion of the code to be active. – Vectorizer Dec 11 '21 at 16:53
@Vectorizer: Understood. Testing `__cplusplus` as shown here is a mechanism that is defined by the language standard, so every standard-compliant compiler should allow it. Unfortunately Microsoft thinks standards are for other people only. If it does not work as shown, that would be a fault in Microsoft's compiler, making it non-compliant in a pretty important way. Working around the issue by using some of Microsoft's non-standard extensions (as shown by ViralTaco_'s answer) makes your code less portable / readable, the *real* solution would be MS getting their act together. – DevSolar Dec 11 '21 at 17:23
@DevSolar Understood. I am afraid I don't have the means to influence Microsoft to get its act together. By the way, the problem is still here in the latest version: 16.11.7 – Vectorizer Dec 11 '21 at 17:29
@Vectorizer: What value does `__cplusplus` evaluate to in that version? – DevSolar Dec 11 '21 at 17:38
@DevSolar: Always to 199711 (for C++14, 17, 20) – Vectorizer Dec 11 '21 at 17:59
2

@Vectorizer: [Set /Zc to enable updated __cplusplus macro.](https://learn.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170). Sorry, didn't know about that myself. – DevSolar Dec 11 '21 at 18:04
1

@DevSolar: Thank you for your help on this while I was offline ;-)! – jpo38 Dec 11 '21 at 20:23

How to detect which (C++) language standard was selected in the Project->General Properties->C++ Language Standard

2 Answers2