Question

Is decltype(*it) the value type of the iterator, or an lvalue reference to that, or something else?

I think it is an lvalue reference, because *it is an lvalue, but I’m not sure.

Note: In my case, it is a BidirectionalIterator, but feel free to answer the general case.

Answer 1

*it is not necessarily an lvalue. Only forward iterators have that requirement.

Iterators (§24.2.2) are required to have *it be a valid expression that returns iterator_traits<Iterator>::reference (and other irrelevant things). Nothing else is said about this and reference does not have to be a reference type†.

Input iterators (§24.2.3) are required to have *it be a valid expression that returns something convertible to the value type.

Forward iterators, however, have the following requirement (§24.2.5 paragraph 1):

— 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 T is the iterator’s value type)

This requires *it to be a reference, which means it has to be a glvalue (i.e. cannot be a prvalue but can be an xvalue like it is the case with move iterators).

The higher iterator categories do not add any relevant requirements.

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† reference is defined to be the type of *it which makes it a bit of a circular definition, but poses no restrictions.