Binary Search Tree Destructor

You can have a recursive destructor; what you can't do is delete the same object twice.

A typical way to delete a tree in C++ might be something like this:

BinSearchTree::~BinSearchTree()
{
   delete _rootNode;  // will recursively delete all nodes below it as well
}

tNode::~tNode()
{
   delete left;
   delete right;
}

Regarding the unresolved external error -- is that error thrown when you try to compile/link the program? If so, it's probably because the code for the tNode class (and in particular the tNode destructor, if you declared one) doesn't exist or isn't getting compiled into your project.

Previous answers have pointed out that the unresolved external error is likely caused by a tNode destructor that is declared but not defined in a translation unit that the linker can see.

However, you have a second error: You appear to believe that setting n to null does something it doesn't do. The pointer value n is passed by value, not by reference, such that changing its value (e.g. by assigning NULL) has no effect after the function returns.

This will probably give you errors when you clear the tree and expect the root node pointer to have been set to NULL, when it remains a dangling pointer to freed memory. The result will be runtime error, not your linker error.

void BinSearchTree::Clear(tNode **N)
{
    tNode * n = *N;
    if (n->left != NULL)
        Clear(n->left);
    if (n->right != NULL)
        Clear(n->right);
    delete n;
    *N = NULL;
    size--;
}

Will do what you expect.