array of class element as a static constexpr member

Question

I have a bootstrap problem with a constexpr static member of a class Bar which is an array of Bar itself. Consider the following perfectly correct code:

struct Foo {
  int i;
  static const std::array<Foo, 2> A;
};
const std::array<Foo, 2> Foo::A {{{1},{2}}};

Now I'd like to have Foo::A not only const but also constexpr. I'm faced with the problem that static constexpr member initialization must be done inside the class declaration. However, since the declaration is not yet finished, the compiler doesn't yet know the size of an instance and therefore refuse to make the array. For example

 struct Bar {
   int i;
   constexpr static const std::array<Bar, 2> A{{{1},{2}}};
 };

is refused with

/usr/include/c++/4.8/array: In instantiation of ‘struct std::array<Bar, 2ul>’:
ess.cpp:14:56:   required from here
/usr/include/c++/4.8/array:97:56: error: ‘std::array<_Tp, _Nm>::_M_elems’ has incomplete type
       typename _AT_Type::_Type                         _M_elems;

Is there a way to solve that ? Or a workaround ?

Answer 1

This is not possible at the moment, the compiler cannot know in advance if the constexpr is actually allowed / possible. Replace the member A with a function and it should work:

struct Bar
{
    int i;
    constexpr static std::array<Bar, 2> get_A()
    {
        return {{{1}, {2}}};
    }
};

Related (almost duplicate): static constexpr member of same type as class being defined