Difference between constexpr and static constexpr

Question

In the C++11 standard, what is the difference between constexpr and static constexpr global variables when defined in a header? More specifically, when multiple translation units include the same header, which declaration (if any) is guaranteed to define the same variable across the translation units?

e.g.,

cexpr.h:

#ifndef CEXPR_H
#define CEXPR_H

constexpr int cint = 1;
static constexpr int scint = 1;

#endif

a.cpp:

#include "cexpr.h"

b.cpp:

#include "cexpr.h"

Answer 1

In your current example there is no difference: On variable declarations, constexpr implies const, and a const variable at namespace scope has internal linkage by default (so adding static does not change anything).

In C++14, you cannot declare a variable as constexpr and have it have external linkage unless you only ever do this in one single translation unit. The reason is that constexpr variables require an initializer, and a declaration with initializer is a definition, and you must only have a single definition.

However, what you can do is use a normal integral constant, which you can declare (not define) as extern, and in the translation unit where it is defined it can even be used as a constant expression:

lib.h:

extern const int a;

lib.cpp:

#include "lib.h"

const int a = 10;

int b[a] = {1, 2, 3};   // OK in this translation unit

In C++17, there is a new feature "inline variables" which lets you say:

inline constexpr int a = 10;

And this is an "inline definition" that can appear repeatedly, and each definition defines the same entity (just like all the other "inline" entities in the language).