Ensure that g++ does not compile a program using features added in newer versions of C++

Question

Compiling with the flag -std=c++14 compiles programs which use features implemented in newer versions of C++ as well, issuing a warning like the following:

warning: inline variables are only available with -std=c++17 or -std=gnu++17

I don't want g++ to compile the program in this situation and do not know why it does in the first place.

I found out that adding the flag -Werror converts the above warning to an error, ensuring that the program does not compile but I am not sure whether that is the recommended way to do so.

Answer 1

To provoke compiler errors specifically for the use of language features that are legal only in C++ standards later than your chosen one, the best-targeted diagnostic option is probably -pedantic-errors , which is documented

-pedantic-errors

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic , in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard...

[my emphasis]

The "base standard" here is the C++ standard named by the specified or default value of -std=... (or if that one is a GNU dialect like gnu++14 , then it is the C++ standard on which that dialect is based).

If you compile source code with, say, std=c++14 that uses a construct first legalised in C++17, then that code is ill-formed per the C++14 standard and a diagnostic is required. The addition of -pedantic-errors to -std=c++14 will therefore oblige the compiler to diagnose the C++17 innovations as errors.

Eg, without -pedantic-errors

$ cat foo.cpp
struct foo
{
    inline static const int value = 42;
};

$ g++ -std=c++14 -Wall -Wextra -pedantic -c foo.cpp
foo.cpp:3:5: warning: inline variables are only available with ‘-std=c++17’ or ‘-std=gnu++17’
    3 |     inline static const int value = 42;
      |     ^~~~~~

And with -pedantic-errors

$ g++ -std=c++14 -Wall -Wextra -pedantic-errors -c foo.cpp
foo.cpp:3:5: error: inline variables are only available with ‘-std=c++17’ or ‘-std=gnu++17’
    3 |     inline static const int value = 42;
      |     ^~~~~~

-pedantic-errors will make the compiler more fussy about C++14 conformity than std-c++14 by itself, or with -Werror . But I guess you won't object to that. And you can make an unconstrained choice as to whether you also practice the blanket discipline of compiling with zero warnings ( -Werror )