I am new to C++ so please bear with me. I am trying to understand STL iterator_traits. In the book "The C++ Standard Library" the structure iterator_traits is defined as follows:
template <class T>
struct iterator_traits {
typedef typename T::value_type value_type;
typedef typename T::difference_type difference_type;
typedef typename T::iterator_category iterator_category;
typedef typename T::pointer pointer;
typedef typename T::reference reference;
};
So it seems to me that it is re-exposing the subtypes that T already exposes. Moving ahead further, the book gives an example of how to use it, which is something like the following
template <class MyIterator>
void do_something(MyIterator start, MyIterator end) {
typedef typename iterator_traits<MyIterator>::value_type value_type;
value_type v = *start;
.....
}
My question is why do I need this iterator_traits structure here, if the idea was to obtain the value_type, couldn't I have obtained it from MyIterator directly ? My confusion seems to arise from my (surely incorrect) understanding that the information of the subtypes have to be sourced from the template <class T> used to instantiate the iterator_trait. So if you could explain, and preferably with an example why and where would I need iterator_traits that would be very helpful.