Why can I not convert a reverse iterator to a forward iterator?

Question

Well, I know why, it\'s because there isn\'t a conversion, but why isn\'t there a conversion? Why can forward iterators be turned to reverse iterators but not the other way roun

Answer 1

You could write a helper function. One particularity of reverse_iterator is that base() gives a forward iterator that is next from the value that the reverse iterator dereferences to. This is because a reverse iterator physically points to the element after the one it logically points to. So to have the forward iterator to the same item as your reverse_iterator, you'll need to decrement the result of base() by one, or you could increment the reverse iterator first, then take the .base() of that.

Both examples are shown below:

#include 
#include 
#include 

//result is undefined if passed container.rend()
template 
typename ReverseIterator::iterator_type make_forward(ReverseIterator rit)
{
    return --(rit.base()); // move result of .base() back by one.
    // alternatively
    // return (++rit).base() ;
    // or
    // return (rit+1).base().
}

int main()
{
    std::vector vec(1, 1);
    std::vector::reverse_iterator rit = vec.rbegin();
    std::vector::iterator fit = make_forward(rit);
    std::cout << *fit << ' ' << *rit << '\n';
} 

Warning: this behavior is different from that of the reverse_iterator(iterator) constructor.