Re: A question about Template parameter

Francesco S. Carta <entuland@gmail.com> wrote:


  There are situations where you want to declare (not define, but declare)
a template class. When you are doing such a declaration, you don't need the
template parameter for anything, and thus you can omit it.

  One practical situation where you may need to do that is when declaring
a template class to be a friend of another class (something which
surprisingly few C++ programmers know how to do). In other words, something
like:

    class A
    {
        template<typename> friend class B;
        ...
    };

  Here we are declaring that class B, which is a template class, is a
friend class of A. You could specify a name for the template parameter,
but since it's not used in the declaration, it can be omitted.

  Complete example:

#include <iostream>

class A
{
    template<typename> friend class B;
    int i;

 public:
    A(int value): i(value) {}
};

template<typename T>
class B
{
 public:
    void foo(const A& obj) { std::cout << obj.i << std::endl; }
};

int main()
{
    A a(5);
    B<int> b;
    b.foo(a);
}