CODEWITHSUNDEEP
c++iteratorc++20
Bonita Montero
Bonita Montero

Reputation: 3101

Obtaining a raw pointer through an iterator with C++20

Sometimes I used &*it to get a raw pointer from an iterator. But this doesn't always work when I'm using interator-debugging, consider getting a raw pointer of &*myVec.end(). AFAIK there's a templated C++20 function to get an address from an iterator, but I can't remember it anymore. Can anyone help me ?

Upvotes: 2

Views: 640

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473242

For any iterator i which is a contiguous_iterator, the expression to_address(i) shall result in a pointer to a location in the range of the iterator, or a past-the-end pointer if it is the past-the-end iterator of the range. This can be provided via a specialization of pointer_traits<I> for the iterator type I, but for many iterators, calling operator->() is adequate to the task.

This is only supported for contiguous iterators because... well, it doesn't make much sense otherwise. Assuming we're talking about a C++20 iterator that is not a proxy iterator (ie: reference is exactly value_type&), it is possible to get a pointer to the underlying value. But there is no requirement that the end iterator give you a pointer that is in any way meaningful.

This is only meaningful for contiguous iterators, because contiguous iterators have to point to an actual C++ array of value_types. As such, there is a pointer which is a past-the-end pointer to that array. Even if you could extract some kind of pointer from the end iterator of a non-contiguous iterator, it wouldn't be meaningful because non-contiguous iterators don't (necessarily) point to arrays.

So if you're working with a non-contiguous iterator, just use &*i. It won't work on non-dereferenceable iterators, but nothing could work on those, since there's no pointer it could return.

Upvotes: 3

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