C++ Primer Plus

Then the compiler substitutes int for TT and generates the following class definition:

class HasFriendT

{

...

    friend void counts();

    friend void report<>(HasFriendT &);

};

One specialization is based on TT, which becomes int, and the other is based on HasFriendT, which becomes HasFriendT. Thus, the template specializations counts() and report >() are declared as friends to the HasFriendT class.

The third requirement the program must meet is to provide template definitions for the friends. Listing 14.23 illustrates these three aspects. Note that Listing 14.22 has one count() function that is a friend to all HasFriend classes, whereas Listing 14.23 has two count() functions, one of which is a friend to each of the instantiated class types. Because the count() function calls have no function parameter from which the compiler can deduce the desired specialization, these calls use the count() and count() forms to indicate the specialization. For the calls to report(), however, the compiler can use the argument type to deduce the specialization. You could use the <> form to the same effect:

report >(hfi2);  // same as report(hfi2);

Listing 14.23. tmp2tmp.cpp

// tmp2tmp.cpp -- template friends to a template class

#include

using std::cout;

using std::endl;

// template prototypes

template void counts();

template void report(T &);

// template class

template

class HasFriendT

{

private:

    TT item;

    static int ct;

public:

    HasFriendT(const TT & i) : item(i) {ct++;}

    ~HasFriendT() { ct--; }

    friend void counts();

    friend void report<>(HasFriendT &);

};

template

int HasFriendT::ct = 0;

// template friend functions definitions

template

void counts()

{

    cout << "template size: " << sizeof(HasFriendT) << "; ";

    cout << "template counts(): " << HasFriendT::ct << endl;

}

template

void report(T & hf)

{

    cout << hf.item << endl;

}

int main()

{

    counts();

    HasFriendT hfi1(10);

    HasFriendT hfi2(20);

    HasFriendT hfdb(10.5);

    report(hfi1);  // generate report(HasFriendT &)

    report(hfi2);  // generate report(HasFriendT &)

    report(hfdb);  // generate report(HasFriendT &)

    cout << "counts() output:\n";

    counts();

    cout << "counts() output:\n";

    counts();

    return 0;

}

Here is the output of the program in Listing 14.23:

template size: 4; template counts(): 0

10

20

10.5

counts() output:

template size: 4; template counts(): 2

counts() output:

template size: 8; template counts(): 1

As you can see, counts reports a different template size from counts, demonstrating that each T type now gets its own count() friend.

Unbound Template Friend Functions to Template Classes

The bound template friend functions in the preceding section are template specializations of a template declared outside a class. An int class specialization gets an int function specialization, and so on. By declaring a template inside a class, you can create unbound friend functions for which every function specialization is a friend to every class specialization. For unbound friends, the friend template type parameters are different from the template class type parameters:

template

class ManyFriend

{

...

    template friend void show2(C &, D &);

};

Listing 14.24 shows an example that uses an unbound friend. In it, the function call show2(hfi1, hfi2) gets matched to the following specialization:

void show2 &, ManyFriend &>

          (ManyFriend & c, ManyFriend & d);

Because it is a friend to all specializations of ManyFriend, this function has access to the item members of all specializations. But it only uses access to ManyFriend objects.

Similarly, show2(hfd, hfi2) gets matched to this specialization:

void show2 &, ManyFriend &>

          (ManyFriend & c, ManyFriend & d);

It, too, is a friend to all ManyFriend specializations, and it uses its access to the item member of a ManyFriend object and to the item member of a ManyFriend object.

Listing 14.24. manyfrnd.cpp

// manyfrnd.cpp -- unbound template friend to a template class

#include

using std::cout;

using std::endl;

template

class ManyFriend

{

private:

    T item;

public:

    ManyFriend(const T & i) : item(i) {}

    template friend void show2(C &, D &);

};

template void show2(C & c, D & d)

{

    cout << c.item << ", " << d.item << endl;

}

int main()

{