Question on Friend Class

[khal]

Hi, I have two simple class that I made just to understand how friend class works. I am confused as to why this doesn't compile and also would the Linear class have access to the struct inside Queues class?

Linear.h

template<typename k, typename v >
class Linear
{
	public:
        //Can I create a instance of Queues here if the code did compile? 
	
	private:
};

Linear.cpp

#include "Linear.h" 

Queues.h

#include "Linear.h"

template<typename k, typename v >
class Linear;

template<typename x>
class Queues
{
	public:
	
	private:
		struct Nodes{
			int x;
		};
        //Does this mean I am giving Linear class access to all of my Queues class variable or is it the opposite ? 
	friend class Linear<k,v>;
};

#include"Queues.h" 

My errors are
Queues.h:15: error: `k' was not declared in this scope
Queues.h:15: error: `v' was not declared in this scope
Queues.h:15: error: template argument 1 is invalid
Queues.h:15: error: template argument 2 is invalid
Queues.h:15: error: friend declaration does not name a class or function

[maeriden]

I'm not an expert on templates but here's what I've got

The error comes from the fact that class Queues doesn't know what k and v are. Linear does, but you have to tell that to Queues. You can do

template<typename k, typename v >
class Linear;

template<typename x, typename k, typename v>
class Queues
{
	public:

	private:
		struct Nodes{
			int n; // You declared a variable that had the same name as the template type. That's a no-no
		} list; // You must create an instance of Nodes or Linear will have access to an empty class
        // Linear will have access to protected/private members of Queues, so it will be able to access list
	friend class Linear<k, v>;
};

To create data of type Queues inside Linear you need to put the declaration of Queues before the declaration of Linear, or it will not know that it is a type. So it's not Queues.h that needs to include Linear.h, it's the opposite (by the way, you still need to forward declare Linear to be able to make it a friend class).