C++ Forward declaring class scoped enumeration

Question

I'm wondering if it's possible to forward declare an enum that's defined within another class scope. For example, consider the following:

//A.h
class A
{
public:
    enum class type: unsigned long { /*some stuff*/ }
};

Now, in another header I'd like to forward declare the 'type' enum (suppose class B below has a function that does something with A::type)

//B.h
enum A::type; //use of undefined type 'A'

class B
{
public:
    UseTypeEnum(A::Type&);
};

This doesn't work either:

//B.h
class A;
enum A::type; //still use of undefined type

class B...

There's no problem if the enum is declared at global scope in A.h.

Is there any way to do this?

enumerations should be declared as static members, are you sure you cannot write the program with leaving out the enums forward declaration? — Syntactic Fructose, Dec 12 '12 at 14:41
for `enum`: http://stackoverflow.com/questions/2238170/forward-declaration-of-nested-enum — Ciro Santilli OurBigBook.com, Dec 18 '16 at 11:27

score 29 · Accepted Answer · answered Dec 12 '12 at 14:43

29

You can't declare nested types outside the class definition.

If you need to use them outside the class, you will have to either include the class definition, or move them into a namespace.

answered Dec 12 '12 at 14:43

Mike Seymour

5

The sad part is when it's about external libraries though. – Predelnik Feb 16 '14 at 10:59

1 Answers1