C++ :: Automatic Conversion For Function Parameters?

So I'm writing a data structure from scratch as part of a university assignment in c++, and I have to write an iterator for it. The problem involves comparison between constant iterators and normal iterators, and I'm going about it in this way: I wrote a constructor of normal iterator which takes a const iterator as its only parameter, hoping that the comparison operator between two normal iterators will be enough:

btree_iterator(const_btree_iterator<T>&conv):_target(conv._target),_index(conv._index),_last(conv._last){}

(and somewhere else)

template <typename T>
bool operator!=(btree_iterator<T> one,btree_iterator<T> other){
return !(other == one);
}

and I'm testing it in this way:

btree<int> bl(5);//the data structure
auto iter = bl.begin();
iter != bl.cend(); //trying to compare iterator with const iterator

but apparently this is wrong, since the compiler tells me something along the line of "no function 'operator!=' which takes ......" It seems the constructor is alright, since the following works:

btree<int>::iterator i(bl.cend());

Am I getting something fundamentally wrong about it? How is this functionality actually implemented in C++ library containers like vector?