[temp.mem.class] (original) (raw)

A member class of a class template may be defined outside the class template definition in which it is declared.

[Note 1:

The member class must be defined before its first use that requires an instantiation ([temp.inst]).

For example,template<class T> struct A { class B;}; A<int>::B* b1; // OK, requires A to be defined but not A​::​B template<class T> class A<T>::B { }; A<int>::B b2; // OK, requires A​::​B to be defined

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