You cannot forward declare a nested class. For this reason, the following code will not compile:
// assuming class B is nested in class A int main() { class A::B; // error; cannot fwd-declare a nested class A::B ptr; // error }
The only way to convince the compiler to accept this code is by making the declaration of class A visible, i.e., by #including the appropriate header before main():
// file a.h class A { public: class B {/*..*/}; // nested }; #include "a.h" int main() { class A::B; // fine but redundant A::B ptr; // OK }
However, once you #include the declaration of A, the forward declaration in main() becomes redundant.
Remember that nested classes are meant to hide implementation details. If you need to access a nested class anywhere outside its enclosing class, perhaps it shouldn’t be a nested class in the first place.