为什么我的模板化函数需要从一个迭代器到另一个迭代器的转换？

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?

Answer 1

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.