Type of auto reference

Question

If I have the following two loops:

std::vector<int> v;

for(auto i : v)
  //do something with i

for(auto& j : v)
  //do something with j

When I hover over i, intellisense shows it as int i (as expected). However, when I hover over j I don't get int& as I expected to, but rather

std::_Simple_types<std::_Wrap_alloc<std::_Vec_base_types<int, std::allocator<int> >::_Alloc>::value_type>::value_type &j

What is this complicated definition? Is it the same as int&? If not, what is it? And if it is, why can it deduce just int for i, but not int& for j?

Answer 1

The standard states in 6.5.4 [stmt.ranges]:

For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses

( expression )

In each case, a range-based for statement is equivalent to

{
    auto && __range = range-init;
    for ( auto __begin = begin-expr,  __end = end-expr;  __begin != __end; ++__begin ) {
        for-range-declaration = *__begin;
        statement
    }
}

So you can see that in your case the types of i and j are deduced from the type of *it where it is a std::vector iterator. std::vector iterators are implementation defined, however the result of *it is not.

---

As indicated in the comments, a std::vector iterator is a forward iterator, and after 24.2.5/1 [forward.iterators]:

A class or pointer type X satisfies the requirements of a forward iterator if

• ...
• if X is a mutable iterator, reference is a reference to T; if X is a const iterator, reference is a reference to const T,

Here reference is used in 24.4.4/2 [iterator.iterators] to indicate the return type of *it.

---

Thus for your case the standard requires the type of i to be int and the type of j to be int&. This is probably the case for MSVC++, and intellisense is just not able to correctly resolve the type.

---

Edit: Fixed the answer regarding the return type when dereferencing iterators.

Answer 2

IntelliSense is giving you a peek into the implementation of std::vector. That is a complicated way of saying reference to std::vector<int>::value_type or, in this case, reference to int.