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  • What does the C++ macro __cplusplus contain and expand to?

  • Does the macro __cplusplus always, even in oldest C++ implementation, contain and expand to a numeric value?

  • Is it safe to use #if __cplusplus or should we use instead of that #ifdef __cplusplus?

Conclusion (added later)

From comments and accepted answer:

  • __cplusplus expands to a number representing the standard's version, except pre-standard C++ in the early 90s (which simply expanded to 1).

  • Yes, even in oldest C++ implementation (expands to a numeric value).

  • No, #ifdef should be used when header is shared with C-language (because some C-compilers will warn when #if checks undefined macro).

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9
  • For C, it should be undefined (unless you're dealing with a malicious compiler writer); for C++, it should be defined, possibly to a useful value, but at least defined. If you want to work with 'even the oldest C++ implementations', do not assume anything more than "it is defined". Commented Apr 19, 2018 at 7:45
  • What do you consider to be "the oldest C++ implementation"? C++98? Older? How much older? Commented Apr 19, 2018 at 7:52
  • The question was originally tagged with C, and the title included 'C' rather than C++. The tags were predominantly C++ tags, though. However, C11 §6.10.8 Predefined macro names ¶3 says: The implementation shall not predefine the macro __cplusplus, nor shall it define it in any standard header. The C99 standard says essentially the same, in a slightly different section. The C90 standard was silent on the issue. Commented Apr 19, 2018 at 7:56
  • since an undefined macro will expand to false/zero we just need __cplusplus evaluate to "true" when defined so we can short up code and conditions Commented Apr 19, 2018 at 8:04
  • 2
    For a long time g++ defined it as 1. #ifdef is the traditional way to test, some compilers will warn if you use #if instead. Commented Apr 19, 2018 at 8:12

3 Answers 3

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19

Yes, it always does expand to numeric value and its meaning is the version of C++ standard that is being used. According to cppreference page, __cplusplus macro should expand to:

  • 199711L (until C++11),
  • 201103L (C++11),
  • 201402L (C++14),
  • 201703L (C++17),
  • 202002L (C++20)
  • 202302L (C++23)

The difference between #if and #ifdef directives is that #ifdef should be used to check whether given macro has been defined to allow a section of code to be compiled.

On the other hand #if (#else, #elif) directives can be used to check whether specified condition is met (just like typical if-condition).

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4 Comments

are you even sure for oldest C++ version ?
Yes, the C++98 standard explicitly says so — I checked in my copy.
Note that for a long time, __cplusplus in some compilers simply expanded to 1.
Of course, if you were experimenting with pre-standard c++ in the early 90s, that standard didn't exist, just as now you can experiment with c++20. If you are still using such a compiler all bets are off, but then your sanity must be questioned. :-)
3

__cplusplus is required to be defined by the C++ standard. For C++11 or higher __cplusplus > 199711L should hold true.

Does the macro __cplusplus always, even in oldest C++ implementation, contain and expand to a numeric value?

Yes it does.

19.8 C++11

__cplusplus

The integer literal 201703L. [ Note: It is intended that future versions of this International Standard will replace the value of this macro with a greater value. — end note  ]

C++98

__cplusplus The name _ _cplusplus is defined to the value 199711L when compiling a C ++ translation unit. 143)

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Comments

2

Unfortunately, the __cplusplus macro has the value 199711 in MS Visual Studio 2022, regardless of the specified C++ standard. Use _MSVC_LANG instead. See VS comment.

This seems to work:

#if defined(_MSVC_LANG)  // MS compiler has different __cplusplus value.
#   if _MSVC_LANG < 201703
#       error Please compile for C++17 or higher
#   endif
#else  // All other compilers.
#   if __cplusplus < 201703
#       error Please compile for C++17 or higher
#   endif
#endif
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4 Comments

You can just use #if __cplusplus < 201703 nowadays, as your link states! The issue was resolved back in 2018. VS2019 and VS2022 have full C++17 support, but VS2017 was still a bit iffy.
No, __cplusplus has the value 199711 in MS Visual Studio 2022 with C++17.
There's also the compiler flag /Zc:__cplusplus learn.microsoft.com/en-us/cpp/build/reference/…
Expanding on user202729's comment to provide the context from that link that addresses the confusion: "The /Zc:__cplusplus compiler option enables the __cplusplus preprocessor macro to report an updated value for recent C++ language standards support. By default, Visual Studio always returns the value 199711L for the __cplusplus preprocessor macro." So, the __cplusplus macro is only reliable if the flag has been passed. Otherwise, the _MSVC_LANG macro is always reliable in MSVC.

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