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c++cmakeif-constexpr

Configure CMake to use newest available C++ standard

I'm building a C++ library with CMake. Is it possible to set the C++ standard to the newest released standard supported by the user's compiler? I checked the docs for the variable CXX_STANDARD, but it only shows how to use one specific standard.

Motivation:

I want my library to be compatible with C++11, but I also want to use the features of newer C++ standards if they're available. For example, I have defined the following macro.

#if (__cplusplus > 201700L || _MSVC_LANG > 201700L)
#define IF_CONSTEXPR_MACRO if constexpr
#else
#define IF_CONSTEXPR_MACRO if
#endif

This is great for header-only libraries because

  1. The code is compatible with old C++ standards, and
  2. If the user compiles with C++17 or newer, they get all the advantages of constexpr if.

Unfortunately, my library is not header only. I want the resulting .so file to use the if constexpr version if it is available.

Solution

  • For MSVC, you can wrap a compiler ID generator expression around /std:c++latest. For the other compilers you might need to do multiple checks with CheckCXXCompilerFlag. Honestly, if the compiler version used to compile your target really doesn't matter, it would be a lot simpler to just tell your user to use whatever they want / have.

    I'm getting some XY problem vibes from the scenario you've described. If the code using that macro is equally valid with if or if constexpr, then you don't really need if constexpr there, and can probably just trust your compiler to do optimizations (which I assume is what you're trying to achieve with that macro).