How to find default supported C++ standard by g++ compiler in cmake

Question

I saw many threads regarding how to enable certain C++ standard e.g.

add_definitions("-std=c++14")

and

set (CMAKE_CXX_STANDARD 11)

But how to find which standard by default g++ compiler following while compiling code ?

Have cmake use the compiler to try and compile a small program that uses C++-some version only features. — Hatted Rooster, Feb 22 '19 at 12:24
Or check the version of the compiler and match it to known versions of that compiler that support some version of the C++ standard — Hatted Rooster, Feb 22 '19 at 12:25

score 0 · Answer 1 · answered Feb 22 '19 at 12:25

0

add_definitions("-std=c++14") is not the correct way to enable C++14. The right way either the second one you mentioned: set(CMAKE_CXX_STANDARD 14), or set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14").

add_definitions is for macro definitions, equivalent to #define in your source code.

About your question, it's bad practice to depend on the default version of C++ in your compiler. Even Visual Studio now defines the standard you want to use, which is a new thing in VS. Standards are made to solve this problem by having them specified. So, in your project, you define the target standard, and you define it in your make file. You should not expect your code to work on all C++ standards.

answered Feb 22 '19 at 12:25

The Quantum Physicist

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Teach me about particle physics please – Hatted Rooster Feb 22 '19 at 12:37
@The Quantum , thanks for very useful answer, my question was, If I dnt set compiler standard flag (bad practice accepted), by default which standard g++ following ? How to find this. One is too use documentation but I want to do runtime. – TonyParker Feb 22 '19 at 12:42
@SombreroChicken Check out my Stack Exchange Physics account. – The Quantum Physicist Feb 22 '19 at 12:47
@TonyParker I don't know of a way to do this. I don't expect it to be possible. There's a way around this, but it'll ruin the possibility to cross-compile. You create a small program that uses the macros that tell you the C++ version, then you print them, and parse them in cmake while compiling the target program. – The Quantum Physicist Feb 22 '19 at 12:49
... and the macro to use would be `__cplusplus`. For the possible values (so far!), see https://en.cppreference.com/w/cpp/preprocessor/replace#Predefined_macros – aschepler Feb 23 '19 at 02:09

score 0 · Answer 2 · answered Feb 22 '19 at 13:20

If you're developing an application, then you should not adjust your code based on available compiler version, instead you should specify which compiler standard your code requires:

set (CMAKE_CXX_STANDARD 11)

If you're building a code library that is meant to be used in different contexts, use compiler feature detection and conditional compilation. For example:

write_compiler_detection_header(
  FILE "${CMAKE_CURRENT_BINARY_DIR}/compiler_features.h"
  PREFIX Foo
  COMPILERS GNU
  FEATURES
    cxx_variadic_templates
)

And then:

#include "compiler_features.h"
#if Foo_COMPILER_CXX_VARIADIC_TEMPLATES
#  include "with_variadics/interface.h"
#else
#  include "no_variadics/interface.h"
#endif

This is especially useful with compilers like MSVC which have many features from C++17 while also lacking some basic features from C++11 (e.g. expression SFINAE).

CMake has built-in detectors for many popular C++ features.

PS. If you're really curious, G++ up to 5.0 uses by default gnu++98 standard, and from 6.0 gnu++14 (GNU dialect of -std=c++14).

How to find default supported C++ standard by g++ compiler in cmake

2 Answers2