How to declare constexpr extern?

Question

Is it possible to declare a variable extern constexpr and define it in another file?

I tried it but the compiler gives error:

De

Answer 1

C++17 inline variables

This awesome C++17 feature allow us to:

• conveniently use just a single memory address for each constant
• store it as a constexpr
• do it in a single line from one header

main.cpp

#include 

#include "notmain.hpp"

int main() {
    // Both files see the same memory address.
    assert(¬main_i == notmain_func());
    assert(notmain_i == 42);
}

notmain.hpp

#ifndef NOTMAIN_HPP
#define NOTMAIN_HPP

inline constexpr int notmain_i = 42;

const int* notmain_func();

#endif

notmain.cpp

#include "notmain.hpp"

const int* notmain_func() {
    return ¬main_i;
}

Compile and run:

g++ -c -o notmain.o -std=c++17 -Wall -Wextra -pedantic notmain.cpp
g++ -c -o main.o -std=c++17 -Wall -Wextra -pedantic main.cpp
g++ -o main -std=c++17 -Wall -Wextra -pedantic main.o notmain.o
./main

GitHub upstream.

The C++ standard guarantees that the addresses will be the same. C++17 N4659 standard draft 10.1.6 "The inline specifier":

6 An inline function or variable with external linkage shall have the same address in all translation units.

cppreference https://en.cppreference.com/w/cpp/language/inline explains that if static is not given, then it has external linkage.

See also: How do inline variables work?

Tested in GCC 7.4.0, Ubuntu 18.04.