Here’s a common error: you define a class-internal type, say an enum or typedef, and then try to use it as the return type of that class’s member function:
class A{public: enum sizes {small, medium, large}; sizes default_size() const{ return medium;}};
Up until now, all is fine. However, at a later stage you decide to move the body of default_size() outside the class:
sizes A::default_size() const{ return medium;}
All of a sudden, the compiler is moaning and bitching at you that ‘sizes’ is an unknown type. The problem is that outside the scope of class, you must use a qualified name to refer to an internal type, as follows:
A::sizes A::default_size() const{ return medium;}
remember that the compiler recognizes the non-qualified names defined within a class only within that class’s scope.