Is in-class enum forward declaration possible? [duplicate]

Question

I know that in C++11 it's possible to forward declare an enum type (if storage type is provided) e.g.

enum E : short; void foo(E e);  ....  enum E : short {     VALUE_1,     VALUE_2,     .... } 

But I would like to forward declare an enum defined within a class e.g.

enum Foo::E : short; void foo(E e);  ....  class Foo {     enum E : short     {         VALUE_1,         VALUE_2,     ....     } } 

Is something like this possible in C++11 ?

Answer 1

No, such a forward declaration isn't possible. [decl.enum]/5 (bold emphasis mine):

If the enum-key is followed by a nested-name-specifier, the enum-specifier shall refer to an enumeration that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the enum-specifier shall appear in a namespace enclosing the previous declaration.

(In this case the nested-name-specifier would be the name of your class followed by a ::.)
You could, though, put the enumeration outside and use an opaque-enum-declaration.

Answer 2

As @Columbo says, you can't declare it in the form you specify.

You can, however, forward declare the nested enum inside the class declaration:

class Foo {     enum E : short; };  void foo(Foo::E e);  enum Foo::E : short {      VALUE_1,      VALUE_2,     .... }; 

Whether you gain any benefit by doing so depends, of course, on the circumstances.