Definition different from declaration for constexpr static member [duplicate]

Question

Consider the code

#include <iostream>

struct Foo
{
    constexpr static int n = 42;
};

const int Foo::n; // note const, NOT constexpr

int main()
{
    std::cout << Foo::n;
}

The definition of the static member is different from the in-class declaration, i.e. const is used instead of constexpr. Is the code above legal, and if yes, why? It compiles with both gcc and clang. It also compiles if we interchanged const and constexpr in the definition and declaration, respectively. I know that constexpr implies const on variables, but not the other way around.

Answer 1

My two cents, while looking at some documentation like that one.

Actually, it could be valid.

In fact, the difference between constexpr and const mainly relies into their purposes, but the former implies the latter as a side effect.

There is also a more subtle difference: constexpr is a specifier, while const is a type qualifier.

In particular:

const’s primary function is to express the idea that an object is not modified through an interface

On the other side:

constexpr’s primary function is to extend the range of what can be computed at compile time, making such computation type safe and also usable in compile-time contexts

Or even more concise from here:

constexpr - specifies that the value of a variable or function can appear in constant expressions

Anyway it happens that:

constexpr in front of a variable definition [...] implies const

So, even though the reason for which one should use constexpr instead of const is clear, and everybody tends to remember that:

constexpr is not a general purpose replacement for const (or vice versa)

It's a fact that using constexpr you are actually saying that this member is implicitly const (also you have a set of more specific constraints on how that member can be defined).

Anyway, once defined, the member is nothing more than a data member having the const type qualifier (which can be used in constant expressions), that is what you are declaring out of your class.