类模板的NonTemplate函数之友

Question

On Lippman p656 I read:

A nontemplate function or class can be a friend to a class template:

template<class Type> class Bar {
    friend class Foobar;
    friend void fcn();
}; 

I wonder what all this means. If fcn is made a friend it is becuase you want it to access the private members of Bar , but how can it access them it if has not got any Bar object passed into as a parameter?

Can somebody please enlighten me on this?

Answer 1

Being a friend of a class X means the friend (whether it's function, or class) has access to all private and protected members of the class X .

In your example, the class Foobar and the function fcn has access to private and protected members of the class Bar .

Now the question is:

how can it access them it if has not got any Bar object passed into as a parameter?

Well, it can access if it has instance of Bar . For example.

void fcn()
{
   Bar<int> bar;
   bar.PrivateFun(); //okay even if PrivateFun is a private function of Bar
   bar.PrivateData = 10; //okay even if PrivateData is a private data of Bar
}

Just to emphasize the difference, consider this another function:

void g()
{
   Bar<int> bar;
   bar.PrivateFun(); //compilation error - g() is not a friend of Bar!
   bar.PrivateData = 10; //compilation error - g() is not a friend of Bar!
}

Hope it helps you understanding what it means to have access to private members of a class, and what it means to be a friend of a class!

Answer 2

Perhaps there is a global Bar<T> which it can access- just because there is no obvious parameter doesn't mean that it can't access a Bar<T> . Also, that's ill-formed syntax.