What is an iterator's default value?

Question

For any STL container that I\'m using, if I declare an iterator (of this particular container type) using the iterator\'s default constructor, what will the iterator be init

Answer 1

An updated answer.

Up to and including C++11: a default- and value-initialized iterator may contain a singular value. Technically it may not be compared, nor dereferenced. See [iterator.requirements.general]/p5.

By convention however, STL implementations used to initialize such an iterator as a past-the-end iterator.

Starting from C++14: a value-initialized forward iterator compares equal to a past-the-end iterator. See [iterators.forward.iterators]/p2:

... value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type. [ Note: Value-initialized iterators behave as if they refer past the end of the same empty sequence.  — end note ]

Therefore:

std::list::iterator iter {}; should work as a past-the-end iterator.

std::list::iterator iter; is dangerous as iter will be initialized only if one has a non-trivial default constructor. Though for std::list that will probably be the case, and so should also work.