Question

This is my first foray into C++ templates, and I’m trying to construct a BinaryTree template to help me with a Project Euler problem; however, I seem to be getting an error where BinaryTree class doesn’t recognize all the constructors of the BinaryTreeNode! Here’s a snippet of the code.

template <class T>
class BinaryTreeNode
{
private:
    BinaryTreeNode<T>* _left;
    BinaryTreeNode<T>* _right;
    T* _value;

public:
    BinaryTreeNode();
    explicit BinaryTreeNode(const T& value) : _value(&(T(value))) {}
    BinaryTreeNode(BinaryTreeNode<T>& left, BinaryTreeNode<T>& right, const T& value) :
        _left(&left), _right(&right), _value(&(T(value))){}
};

The BinaryTree class

#include "BinaryTreeNode.h"
template <class T>
class BinaryTree
{
private:
    BinaryTreeNode<T>* _root;
    BinaryTreeNode<T>* _current;
    unsigned int size;

public:
    BinaryTree() : size(0), _root(0), _current(0) { }
    explicit BinaryTree(BinaryTree<T>& leftTree, BinaryTree<T>& rightTree, const T& value) : 
        size(leftTree.Size() + rightTree.Size() + 1), _root(leftTree.Root(), rightTree.Root(), value), _current(_root) {}
    explicit BinaryTree(const T& value) : size(1), _root(value) {}
    const BinaryTreeNode<T>& Root() const { return *_root;}
};

I’m getting these errors.

error C2359: 'BinaryTree<T>::_root' : member of non-class type requires single initializer expression
error C2440: 'initializing' : cannot convert from 'const int' to 'BinaryTreeNode<T> *'
error C2439: 'BinaryTree<T>::_root' : member could not be initialized

The BinaryTreeNode constructor of (BinaryTreeNode<T>&, BinaryTreeNode<T>&, const T& value) works when I include it in my main code, but it doesn’t seem to work under my BinaryTree template. Anyone know why?

Answer 1

In your initialization expression _root(leftTree.Root(), rightTree.Root(), value), _root is a pointer. You can only initialize it to another pointer. Perhaps you mean to initialize it to a pointer to a new node constructed on those arguments?

This could be done like this: (updated after your edit)

_root(new BinaryTreeNode<T>(leftTree.Root(), rightTree.Root(), value))

However, this is very dangerous (think about an exception in the allocation), and you should probably avoid using raw pointers in your class design and instead use smart managing pointers.

Similarly, the initializer _root(value) does the wrong thing, you might want:

_root(new BinaryTreeNode<T>(value))

(Also note that you should initialize members in their order of declaration.)

Update: I changed the first constructor call following your edit, but as @Luc says, your constructors take non-const arguments but Root() only provides a const reference, so you still need to fix that.