Friends and templates (C++ only)

There are four kinds of relationships between classes and their friends when templates are involved:

• One-to-many: A non-template function may be a friend to all template class instantiations.
• Many-to-one: All instantiations of a template function may be friends to a regular non-template class.
• One-to-one: A template function instantiated with one set of template arguments may be a friend to one template class instantiated with the same set of template arguments. This is also the relationship between a regular non-template class and a regular non-template friend function.
• Many-to-many: All instantiations of a template function may be a friend to all instantiations of the template class.

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();
};

• Function e() has a one-to-many relationship with class A. Function e() is a friend to all instantiations of class A.
• Function f() has a one-to-one relationship with class A. The compiler will give you a warning for this kind of declaration similar to the following:

  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.

• Function g() has a one-to-one relationship with class A. Function g() is a function template. It must be declared before here or else the compiler will not recognize g<T> as a template name. For each instantiation of A there is one matching instantiation of g(). For example, g<int> is a friend of A<int>.
• Function h() has a many-to-many relationship with class A. Function h() is a function template. For all instantiations of A all instantiations of h() are friends.
• Function j() has a many-to-one relationship with class B.

These relationships also apply to friend classes.