How to declare constexpr extern?

no you can't do it, here's what the standard says (section 7.1.5):

1 The constexpr specifier shall be applied only to the definition of a variable or variable template, the declaration of a function or function template, or the declaration of a static data member of a literal type (3.9). If any declaration of a function, function template, or variable template has a constexpr specifier, then all its declarations shall contain the constexpr specifier. [Note: An explicit specialization can differ from the template declaration with respect to the constexpr specifier. Function parameters cannot be declared constexpr. — end note ]

some examples given by the standard:

  constexpr void square(int &x);  // OK: declaration
  constexpr int bufsz = 1024;  // OK: definition
  constexpr struct pixel {  // error: pixel is a type
    int x;
    int y;
    constexpr pixel(int);  // OK: declaration
  };

  extern constexpr int memsz; // error: not a definition

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 <cassert>

#include "notmain.hpp"

int main() {
    // Both files see the same memory address.
    assert(&notmain_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 &notmain_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.

What you probably want is extern and constexpr initialization, e.g.:

// in header
extern const int g_n;

// in cpp
constexpr int g_n = 2;

This is support though in Visual Studio 2017 only through conformance mode:

• /Zc:externConstexpr (Enable extern constexpr variables)
• constexpr definition of extern const variable