14 Templates [temp]

Friend classes or functions can be declared within a class template. When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation.

As with non-template classes, the names of namespace-scope friend functions of a class template specialization are not visible during an ordinary lookup unless explicitly declared at namespace scope ([class.friend]). Such names may be found under the rules for associated classes ([basic.lookup.argdep]).142 [ Example:

template<typename T> struct number {
  number(int);
  friend number gcd(number x, number y) { return 0; };
};

void g() {
  number<double> a(3), b(4);
  a = gcd(a,b);     // finds gcd because number<double> is an
                    // associated class, making gcd visible
                    // in its namespace (global scope)
  b = gcd(3,4);     // ill-formed; gcd is not visible
}

 — end example ]