Can't a class have static constexpr member instances of itself?

Question

This code is giving me incomplete type error. What is the problem? Isn't allowed for a class to have static member instances of itself? Is there a way to achieve the same result?

struct Size {     const unsigned int width;     const unsigned int height;      static constexpr Size big = { 480, 240 };      static constexpr Size small = { 210, 170 };  private:      Size( ) = default; }; 

Answer 1

A class is allowed to have a static member of the same type. However, a class is incomplete until the end of its definition, and an object cannot be defined with incomplete type. You can declare an object with incomplete type, and define it later where it is complete (outside the class).

struct Size {     const unsigned int width;     const unsigned int height;      static const Size big;     static const Size small;  private:      Size( ) = default; };  const Size Size::big = { 480, 240 }; const Size Size::small = { 210, 170 }; 

see this here: http://coliru.stacked-crooked.com/a/f43395e5d08a3952

This doesn't work for constexpr members, however.

Answer 2

Is there a way to achieve the same result?

By "the same result", do you specifically intend the constexpr-ness of Size::big and Size::small? In that case maybe this would be close enough:

struct Size {     const unsigned int width = 0;     const unsigned int height = 0;      static constexpr Size big() {         return Size { 480, 240 };     }      static constexpr Size small() {         return Size { 210, 170 };     }  private:      constexpr Size() = default;     constexpr Size(int w, int h )     : width(w),height(h){} };  static_assert(Size::big().width == 480,""); static_assert(Size::small().height == 170,"");