Why can't I get value_type from iterator_traits?

Question

I'm doing this:

const int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 };
const auto foo = cbegin(arr);
const typename iterator_traits<decltype(foo)>::value_type bar = 1;

I would have expected bar to have the type int. But instead I'm getting an error:

error C2039: value_type: is not a member of std::iterator_traits<_Ty *const >

Is this a problem with the const do I need to strip that or something?

Answer 1

Indeed the const is problematic, you do basically:

std::iterator_traits<const int* const>::value_type // incorrect due to the last const

You might fix it by changing it to

std::iterator_traits<const int*>::value_type // Correct

You might use std::decay or std::remove_cv for that:

const typename std::iterator_traits<std::remove_cv_t<decltype(foo)>>::value_type

(or drop const from foo if relevant).

Answer 2

The issue here is with the line

const auto foo = cbegin(arr);

cbegin(arr) is going to return a int const * (pointer to const int) so applying const to that with const auto foo means foo is a int const * const (const pointer to const int)

std::iterator_traits is only specialized for a T* or T const* so giving it a T* const fails since there is no valid specialization.

You can fix this by removing the constness in the declaration of bar with

const typename std::iterator_traits<std::remove_cv_t<decltype(foo)>>::value_type

or you can change foo to

auto foo = std::cbegin(arr);

if you are okay with it not being const.

Answer 3

Declaring a const qualified iterator const auto foo = cbegin(arr); is questionable. What use do you have for an iterator on which you cannot apply operator++()? Also, the Iterator requirement requires the type int const *const to be Copy Assignable; as such, the variable foo does not satisfy the Iterator requirement. So strictly speaking, foo is not an Iterator.