Forward declaring an enum in C++

Question

I'm trying to do something like the following:

enum E;

void Foo(E e);

enum E {A, B, C};

which the compiler rejects. I've had a quick look on Google and the consensus seems to be "you can't do it", but I can't understand why. Can anyone explain?

Clarification 2: I'm doing this as I have private methods in a class that take said enum, and I do not want the enum's values exposed - so, for example, I do not want anyone to know that E is defined as

enum E {
    FUNCTIONALITY_NORMAL, FUNCTIONALITY_RESTRICTED, FUNCTIONALITY_FOR_PROJECT_X
}

as project X is not something I want my users to know about.

So, I wanted to forward declare the enum so I could put the private methods in the header file, declare the enum internally in the cpp, and distribute the built library file and header to people.

As for the compiler - it's GCC.

Answer 1

Forward declaration of enums is possible since C++11. Previously, the reason enum types couldn't be forward declared was because the size of the enumeration depended on its contents. As long as the size of the enumeration is specified by the application, it can be forward declared:

enum Enum1;                     // Illegal in C++03 and C++11; no size is explicitly specified.
enum Enum2 : unsigned int;      // Legal in C++11.
enum class Enum3;               // Legal in C++11, because enum class declarations have a default type of "int".
enum class Enum4: unsigned int; // Legal C++11.
enum Enum2 : unsigned short;    // Illegal in C++11, because Enum2 was previously declared with a different type.