How to retrieve value type from iterator in C++?

Question

Hi everyone, my question is sure a simple one for anybody familiar with C++ syntax. I'm just learning c++ and this is some sort of homework.

template<typename Iter>
void quickSort(Iter begin, Iter end)
{        
    //..
    auto pivot = * ( begin + (end - begin)/2 );
    //..
}

pivot is supposed to contain value from the center of [begin, end] interval. Ite The code i wrote there works, but auto is a keyword from the new standart of language. How to do it the old-way? What to write instead of auto?

Thanks, Ilya.

Answer 1

typename std::iterator_traits<Iter>::value_type

This will work if your template is instantiated with Iter as a pointer type.

By the way, typename isn't part of the type itself. It tells the compiler that value_type really is a type. If it was the name of a function or a static data member, then that affects the syntax. The compiler doesn't necessarily know what it is, since the specialization of iterator_traits for Iter might not be visible when the template is compiled.

Answer 2

The answer of Steve is right; in C++98, you have to use std::iterator_traits or you can use Iter::value_type if you know that the iterator has this typedef (e.g. is derived from std::iterator). However, there is another problem in your code: you normally can't simply divide iterators. This works with pointers, of course, but not in the more general case. A more general approach would be:

Iter itPivot = std::advance(begin, std::distance(begin, end)/2);