C++11 - Can't define constexpr literal using constexpr function?

Question

I've run into what seems a counterintuitive error, namely, the inability to assign the value of a constexpr function to a constexpr literal (hope I'm using the language right). Here's the example:

class MyClass {
 public:
  static constexpr int FooValue(int n) { return n + 5; }
  static constexpr int Foo5 = FooValue(5);  // compiler error
  static constexpr int Foo5Alt(void) { return FooValue(5); }  // OK
};

In GCC 4.8.4, Foo5 is flagged for field initializer is not constant . Found this thread suggesting that the older version of GCC might be the culprit. So I plugged it into Coliru (GCC 6.2.0) and got the error 'static constexpr int MyClass::FooValue(int)' called in a constant expression before its definition is complete . I added Foo5Alt() which returns its value as a constexpr function rather than literal, and that compiles fine.

I guess I'm not following why FooValue(5) can't be used as the initializer for Foo5 . The definition for FooValue(int n) is complete, isn't it? { return n + 5; } { return n + 5; } is the entire definition. constexpr denotes an expression that can be fully evaluated at compile time, so why can it not be used to define the return value of a constexpr literal?

What subtlety of C++ am I missing?

Answer 1

In C++, inline definitions of member functions for a class are only parsed after the declaration of the class is complete.

So even though the compiler "knows" about MyClass::FooValue(int) , it hasn't "seen" its definition yet, and hence it can't be used in a constexpr expression.

A general workaround for this is to stick to constexpr member functions, or declare constexpr constants outside the class.

Answer 2

According to the standard, MyClass is considered an incomplete type when you try to invoke FooValue to initialize Foo5 . Therefore, you cannot use its members as you did.
The type is considered a completely-defined object type (or complete type) at the closing } .
On the other side, the class is regarded as complete within function bodies . That's why Foo5Alt compiles just fine.
See [class.mem]/6 for further details.