You can define an
enum tag without specifying its possible values. This results in an incomplete type, much like what you get if you write
struct foo without describing the elements. A later declaration that does specify the possible values completes the type.
You cannot allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of
enum more consistent with the way
union are handled.
This extension is not supported by GNU C++.