Thread: Declare a template class as a friend?

I'm feeling really silly as this is just a syntax detail, but I have not managed to find a decent example on the web or in my textbooks.

How do you define a template class declared outside the class as a friend? I'm working on a linked list container class, and defining my List class as a friend of Iterator, which looks something like this:

iterator.h

Code:

#include "node.h"

template <class T>
class Iterator
{
	// declare class List as a friend somewhere?
	public:
		typedef *Node<T> nodePtr;

		T& operator*();
		bool operator==();
		//etc
	private:
		nodePtr dataNode;
		void setNext();
		void setPrev();
		void getNext();
		// etc accessors/mutators for node manipulation
}

list.h

Code:

#include "iterator.h"

template <class T>
class List
{
	public:
		typedef Iterator<T> iterator;

		void push_back(const T& val);	// functions need access to Iterator's 
		void push_front(const T& val);	// private methods to change sequence
		void insert(iterator pos);	// of list.
	private:
		iterator head;
		iterator tail;
};

They're may be some syntax errors. I don't have the code in front of me, so I'm just typing this in notepad from memory.

I looked at the STL list header, and they got around this buy declaring their iterator inside the list function, but I'm not sure thats appropriate for this project. I found plenty of examples on how to declare a non-template class as a friend, and I experimented with possible ways of writing a template version, but my compiler didn't like any of them. In case it effects the template syntax, I'm currenty using MS VC++ 6.0 SP 4.

Thank you,
AH_Tze

Code:

template <class T>
class List
{
    friend class Iterator<T>;
...
};

gg

Just plugged that into Dev-C++, although I switched the friend, so within Iterator class definition it declares "friend class List<T>".

Compiles with an "List is not a template" error. After looking at non-template friend examples, I think I need some placeholder code in iterator.h which tells the compiler a class List will be defined, but I'm not sure how to write this for a template class.

I wrote a quicky test program in which class List tries to access class Iterator's private member function. Hopefully this will illustrate my problem.

main.cpp

Code:

#include "list.h"

int main(int argc, char *argv[])
{
    List<int> testList;
    int testVal;
    
    testVal = testList.getData();
  
  system("PAUSE");	
  return 0;
}

list.h

Code:

#ifndef LIST_H
#define LIST_H

#include "iterator.h"

template <class T>
class List
{
  
    private:
        Iterator<T> data;

    public:
                
        T getData() {
                return *(data.getdata());   // access private member of data
        }
};

#endif

iterator

Code:

#ifndef ITERATOR_H
#define ITERATOR_H

template <class T>
class Iterator
{
    //friend class List<T>; // compile error
    
    private:
        T* dataPtr;

        T* getdata() {
                return dataPtr;
        }

        public:
        Iterator() {
                dataPtr = NULL;
        }

};

#endif

ahh... forward declaration. I think that was what I was looking for.

In iterator.h, before the class, I added

Code:

template<class T>
class List;

And now it works like I wanted it to. The depressing thing is that I think I did that exact same thing earlier in VC++ 6.0 and it gave me an error. Or maybe it was just a typo on my part.

Either way, thank you Prelude and Codeplug.

Code:

template <class T> class B; // forward decl.

template <class T>
class A
{
    T t;

public:
    void Test(const B<T> &b) {t = b.t;}
};//A

template <class T>
class B
{
    friend class A<T>; // friend decl.
    T t;
};//B

Now you have an example of a forward and friend template class declaration.

gg

I did the example in VC++ 6.0

gg

If you break it up into a driver and seperate headers for each class it no longer works.

Code:

// a.h
template <class T> class B; // forward decl.

template <class T>
class A
{
    T t;

public:
    void Test(const B<T> &b) {t = b.t;}
};

***************************************************

// b.h
template <class T> class A; // forward decl.

template <class T>
class B
{
    friend class A<T>;
    T t;
};

***************************************************

// main.cpp
#include "b.h"
#include "a.h"

int main()
{
    A<int> a;
    B<int> b;

    a.Test(b);

    return 0;
}//main

Works fine.

gg