Why does my templated function require a conversion from one iterator to the other?

Question

I am trying to implement a binary search tree container. At the moment I have to implement a find() function that is able to return an iterator or a constant iterator. I choose to overload the find function to accomodate both possibilities

MyIterator<treepair> find(const key& x)
{
    return tree_search<MyIterator<treepair>>(root,x);   
}
const_MyIterator<treepair> find(const key& x) const
{
    return tree_search<const_MyIterator<treepair>>(root,x); 
}

Then the function tree_search recursively finds the node that contains the wanted key by percurring the tree:

template<typename iterator>
iterator tree_search(Node<treepair>* x, const key& y) const
{
    if(x == nullptr)
    {
        std::cout<<"element not found"<<std::endl;
        iterator x = end();//HERE I HAVE A PROBLEM
        return x;
    }
    else if (y == x->value.first)
    {
        iterator i{x,tree_maximum()};
        std::cout<<"element found"<<std::endl;
        return i;
    }
    if(y < x->value.first) 
        return tree_search<iterator>(x->left,y);
    else return tree_search<iterator>(x->right,y);
}

Now the end() function is overloaded to give both a const_iterator and a regular iterator:

MyIterator<treepair> end(){
return MyIterator<treepair>{nullptr,tree_maximum()};
}

const_MyIterator<treepair> end() const{
return const_MyIterator<treepair>{nullptr,tree_maximum()};
}

however I receive this error

test_on_iterators.cc:508:12: error: conversion from ‘const_MyIterator<std::pair<int, int> >’ to non-scalar type ‘MyIterator<std::pair<int, int> >’ requested
      iterator x = end();

Is this error due to a requirement in the conversion between the types? Isn't the compiler supposed to choose the wanted end() function according to the iterator type it has to produce?

Does this answer your question? [C++ iterator and const\_iterator problem for own container class](https://stackoverflow.com/questions/2844339/c-iterator-and-const-iterator-problem-for-own-container-class) — Maciej Dziuban, Feb 13 '20 at 23:53

score 0 · Answer 1 · answered Feb 14 '20 at 00:02

Isn't the compiler supposed to choose the wanted end() function according to the iterator type it has to produce?

No.

tree_search() is a const method. That means its this pointer is pointing at a const object (even if the object tree_search() is called on is not really const).

As such, when tree_search() internally calls end(), it calls the overload that is callable on a const object. That overload returns a const_MyIterator. Then tree_search() tries to assign that const_MyIterator to a non-const MyIterator, which is where you get the error since there is no conversion defined from const_MyIterator to MyIterator.

You need to make x be a const_MyIterator to match what the const version of end() returns.

You should also make tree_search() return a const_MyIterator as well, instead of returning a non-const iterator. If you want tree_search() to return a non-const Iterator, then don't declare it as a const method.

Why does my templated function require a conversion from one iterator to the other?

1 Answers1