Forward declaring an enum in C++

The reason the enum can't be forward declared is that, without knowing the values, the compiler can't know the storage required for the enum variable. C++ compilers are allowed to specify the actual storage space based on the size necessary to contain all the values specified. If all that is visible is the forward declaration, the translation unit can't know what storage size has been chosen – it could be a char, or an int, or something else.

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From Section 7.2.5 of the ISO C++ Standard:

The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration. It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be 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. The value of sizeof() applied to an enumeration type, an object of enumeration type, or an enumerator, is the value of sizeof() applied to the underlying type.

Since the caller to the function must know the sizes of the parameters to correctly set up the call stack, the number of enumerations in an enumeration list must be known before the function prototype.

Update:

In C++0X, a syntax for forward declaring enum types has been proposed and accepted. You can see the proposal at Forward declaration of enumerations (rev.3)

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.