Question

So I have a function called find, which has two versions:

template <typename T> 
typename btree<T>::iterator btree<T>::find(const T& elem)
{
    //Implementation
}

and the other is the const_iterator version:

template <typename T> 
typename btree<T>::const_iterator btree<T>::find(const T& elem) const
{
    //Implementation
}

In my test file when I do

btree<char>::iterator it = myTree.find('M');

Everything works fine, however when I use the const_iterator version:

btree<char>::const_iterator it = myTree.find('M');

It gives me the error

error: conversion from ‘btree_iterator’ to non-scalar type ‘const_btree_iterator’ requested

Which obviously means that find is only ever using the iterator (non const) version. I know that C++ is supposed to call the const_iterator version automatically – if I had done everything right. So the question is, what might I be doing wrong?

The iterator classes are:

class btree_iterator and class const_btree_iterator which is just a copy paste of btree_iterator with the names changed

Here are the full source code:
btree_iterator.h (includes const_btree_iterator) http://pastebin.com/zQnj9DxA
btree.h http://pastebin.com/9U5AXmtV
btree.tem http://pastebin.com/U9uV3uXj

Answer 1

All the standard containers implement conversion of non-const to const iterators (as specified in the requirements for the Container concept):

The type of iterator used to iterate through a container’s elements. The iterator’s value type is expected to be the container’s value type. A conversion from the iterator type to the const iterator type must exist.

You need conversion constructor like so:

class btree_iterator;
class const_btree_iterator
{
       // ....
       public:
              const_btree_iterator(const btree_iterator& rhs) { /* .... */ }
//optionally: const_btree_iterator& operator=(const btree_iterator& rhs) { /* .... */ }
};

I threw in the assignment operator too but I suppose it is redundant