constexpr initializing static member using static function

Question

Requirements

I want a constexpr value (i.e. a compile-time constant) computed from a constexpr function. And I want both of these scoped

Answer 1

The Standard requires (section 9.4.2):

A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression.

In your "second attempt" and the code in Ilya's answer, the declaration doesn't have a brace-or-equal-initializer.

Your first code is correct. It's unfortunate that gcc 4.6 isn't accepting it, and I don't know anywhere to conveniently try 4.7.x (e.g. ideone.com is still stuck on gcc 4.5).

This isn't possible, because unfortunately the Standard precludes initializing a static constexpr data member in any context where the class is complete. The special rule for brace-or-equal-initializers in 9.2p2 only applies to non-static data members, but this one is static.

The most likely reason for this is that constexpr variables have to be available as compile-time constant expressions from inside the bodies of member functions, so the variable initializers are completely defined before the function bodies -- which means the function is still incomplete (undefined) in the context of the initializer, and then this rule kicks in, making the expression not be a constant expression:

an invocation of an undefined constexpr function or an undefined constexpr constructor outside the definition of a constexpr function or a constexpr constructor;

Consider:

class C1
{
  constexpr static int foo(int x) { return x + bar; }
  constexpr static int bar = foo(sizeof(int));
};