c++ forward declare enum of class

Question

Suppose I have the following enum inside class

class A
{
    enum E {a,b};
};

I know that since c++11 we can forward declare enum, because it's default type is int. But how can I forward declare enum E of class A in case class A is only declared and not defined?

And if it is not possible then why?

see https://stackoverflow.com/a/1280969/4117728 "As long as the size of the enumeration is specified by the application, it can be forward declared" you cannot forward declare your `E` even if it was outside of the class — 463035818_is_not_an_ai, Aug 31 '22 at 14:37
*I know that since c++11 we can forward declare enum, because it's default type is int.* That is not correct, you'd need to explicitly declare the backing store type: `enum E : int {a,b};` — Eljay, Aug 31 '22 at 14:38
see also https://stackoverflow.com/a/72599/4117728 for why not — 463035818_is_not_an_ai, Aug 31 '22 at 14:38
It can be done `class A{ enum E: int; };` and then definition `enum A::E: int {};` — user7860670, Aug 31 '22 at 14:38
@user7860670 whats the point? You define a different `A::E`. OPs `A::E` cannot be forward declared — 463035818_is_not_an_ai, Aug 31 '22 at 14:40
@user7860670 But OP mentioned that he wanted this forward declaration to be done when A is also forward declared (not defined yet) — Fareanor, Aug 31 '22 at 14:40
@Fareanor Yes, is is not possible to forward declare any class members without defining class therefore it was a comment, not an answer. — user7860670, Aug 31 '22 at 14:42
@user7860670 what prevents the compiler from accepting? wherever I use this enum, the compiler will know what exact size to allocate for it. — Vahag Chakhoyan, Aug 31 '22 at 14:54
Because C++ requires for `A` to be defined when accessing some member using `A::E` syntax. — user7860670, Aug 31 '22 at 14:59

463035818_is_not_an_ai · Answer 1 · 2022-08-31T14:52:44.897

I know that since c++11 we can forward declare enum, because it's default type is int.

Not quite. You can only forward declare an enum when the underlying type is explicitly specified. This is not allowed:

enum A;
enum A { X,Y};   // not allowed

With gcc the error message is still a little misleading, stating that it would be disallowed to forward declare enums in general which isn't correct anymore (https://godbolt.org/z/xYb5cKEWP).

This is ok

enum A : int;
enum A : int { X,Y};

Note that the default is not simply int. From cppreference:

[...] the underlying type is an implementation-defined integral type that can represent all enumerator values; this type is not larger than int unless the value of an enumerator cannot fit in an int or unsigned int. If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0. If no integral type can represent all the enumerator values, the enumeration is ill-formed

I don't think this answers the question. Now try this when, as the question describes, the enum is an inner class member. "But how can I forward declare enum E of class A in case class A is only declared and not defined?" — Sam Varshavchik, Aug 31 '22 at 16:38
@Sam the answer is no because you cannot forward declare `enum E {a,b}` whether inside a class or not. Though I agree that this answer fails to explain the case of `class A { enum E : int {a,b} };`. Just cannot fix it now — 463035818_is_not_an_ai, Aug 31 '22 at 16:42

c++ forward declare enum of class

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