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(¬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.
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