Referencing a pointer in a C++ member function

Question

I'm writing a member function that uses a member variable pointer as an iterator. However I want to reference the pointer within the function purely for readability's sake. Like so:

/* getNext will return a pos object each time it is called for each node
 * in the tree. If all nodes have been returned it will return a Pos
 * object (-1, -1).
 * TODO: Add a lock boolean to tree structure and assert unlocked for
 *       push/pop.
 */
Pos BTree::getNext () const
{
    BTreeNode*& it = this->getNextIter;

    while (it)
    {
        if (it->visited)
        {
            /* node has been visited already, visit an unvisited right
             * child node, or move up the tree
             */
            if (   it->child [BTREE_RIGHT] != NULL
                && !it->child [BTREE_RIGHT]->visited)
            {
                it = it->child [BTREE_RIGHT];
            }
            else
            {
                it = it->parent;
            }
        }
        else
        {
            /* if unvisited nodes exist on the left branch, iterate
             * to the smallest (leftmost) of them.
             */
            if (   it->child [BTREE_LEFT] != NULL
                && !it->child [BTREE_LEFT]->visited)
            {
                for (;
                     it->child [BTREE_LEFT] != NULL;
                     it = it->child [BTREE_LEFT]) {}
            }
            else
            {
                it->visited = 1;
                return it->pos;
            }
        }
    }

    it = this->root;
    this->setTreeNotVisited (this->root);
    return Pos (-1, -1);
}

This is basically what I'm going for, where this->getNextIter is a BTreeNode*. However I get the error:

    btree.cpp:238: error: invalid initialization of reference of type
'DataTypes::BTreeNode*&' from expression of type 'DataTypes::BTreeNode* const'

What is the appropriate syntax for this kind of thing?

Cheers,

Rhys

Answer 1

Your member function is const-qualified, so you cannot modify the member variable getNextIter. You need to use a const reference:

BTreeNode * const & it = getNextIter;

However, in your function, you modify it, so instead you probably need to remove the const-qualification from the member function or make the getNextIter member variable mutable.

When you have a member function that is const-qualified, all non-mutable member variables are const-qualified inside of the member function, hence why the compiler reports that when you try to use getNextIter inside of getNext(), it has a type of DataTypes::BTreeNode* const (note the const).