How to define and use a friend function in the temlate class with the same template?

Question

I wrote the following code:

#include <iostream>
using namespace std;

template <class T>
class AA
{
  T a;

public:
AA()
{
 a = 7;
}

friend void print(const AA<T> & z);
};

template <class T>
void print(const AA<T> & z)
{
    cout<<"Print: "<<z.a<<endl;
}

void main()
{
AA<int> a;
print<int>(a);
}

And getting the following error:

error C2248: 'AA<T>::a' : cannot access private member declared in class 'AA<T>'
1>        with
1>        [
1>            T=int
1>        ]
1>        c:\users\narek\documents\visual studio 2008\projects\aaa\aaa\a.cpp(7) : see declaration of 'AA<T>::a'
1>        with
1>        [
1>            T=int
1>        ]
1>        c:\users\narek\documents\visual studio 2008\projects\aaa\aaa\a.cpp(30) : see reference to function template instantiation 'void print<int>(const AA<T> &)' being compiled
1>        with
1>        [
1>            T=int
1>        ]

What's wrong?

PS I am using Visual Studio 2008.

Answer 1

The problem is that when you do something like

template<class T>
class AA {friend void print(const AA<T>&);};

and you create an instance AAas follows

AA<int> a;

friend’s ad will be created in such a way as

friend void print(const AA<int>&);

which is a function without a template! This means that the compiler will not match a friend's declaration with the function print.

print AA , . :

#include <iostream>
using namespace std;

//forward declare AA because print needs it
template<class T>
class AA;

//declare print before AA to make the friend declaration
//match with this function
template<class T>
void print(const AA<T> & z);

template <class T>
class AA
{
        //the <> is needed to make sure the compiler knows we're
        //dealing with a template function here
        friend void print<>(const AA<T> & z);

    public:
        AA() {a = 7;}

    private:
        T a;
};

//implement print
template<class T>
void print(const AA<T> & z)
{
    cout<<"Print: "<<z.a<<endl;
}

int main()
{
    AA<int> a;
    print(a);
}

, , <> . . ? , print,

void print(const AA<int>&);

. print ( , ), , friend. , .

Answer 2

:

#include <iostream>
using namespace std;

template <class T>
class AA
{
    T a;
  public:
    AA()
    {
      a = 7;
    }

    friend void print(const AA<T> &z)
    {
      cout<<"Print: "<<z.a<<endl;
    }
};

int main()
{
  AA<int> a;
  print(a);
}