Question

I’m experiencing some problems with breaking my code to reusable parts using templates and inheritance. I’d like to achieve that my tree class and avltree class use the same node class and that avltree class inherits some methods from the tree class and adds some specific ones. So I came up with the code below. Compiler throws an error in tree.h as marked below and I don’t really know how to overcome this. Any help appreciated! 🙂

node.h:

#ifndef NODE_H
#define NODE_H
#include "tree.h"

template <class T>
class node
{
T data;
    ...

node()
    ... 

  friend class tree<T>;
};

#endif

tree.h

#ifndef DREVO_H
#define DREVO_H

#include "node.h"

template <class T>
class tree
{
public: //signatures
    tree();
...

    void insert(const T&);
private:
    node<T> *root; //missing type specifier - int assumed. Note: C++ does not support default-int


};
//implementations

#endif

avl.h

#ifndef AVL_H
#define AVL_H

#include "tree.h"
#include "node.h"

template <class T>
class avl: public tree<T>
{
public: //specific
    int findMin() const;
...

protected:
    void rotateLeft(node<T> *)const;
private:
    node<T> *root;

};

#endif

avl.cpp (I tried separating headers from implementation, it worked before I started to combine avl code with tree code)

#include "drevo"
#include "avl.h"
#include "vozlisce.h"

template class avl<int>; //I know that only avl with int can be used like this, but currently this is doesn't matter :)
//implementations
...

Answer 1

Both tree.h and node.h try to include each other, the include guards will prevent one of them from seeing the other.

Instead of #include "tree.h" try forward declaring tree like:

template <class T>
class tree;

in node.h

EDIT: As sbi suggested in a comment, it makes more sense to forward declare tree in node.h than the other way around, since it’s about granting tree access to node through a friend declaration.