There are four kinds of relationships between classes and their friends when
templates are involved:
The following example demonstrates these relationships:
class B{ template<class V> friend int j(); } template<class S> g(); template<class T> class A { friend int e(); friend int f(T); friend int g<T>(); template<class U> friend int h(); };
The friend function declaration "f" will cause an error when the enclosing template class is instantiated with arguments that declare a friend function that does not match an existing definition. The function declares only one function because it is not a template but the function type depends on one or more template parameters.
These relationships also apply to friend classes.
Related References