Is the following code valid and well-defined?
auto start = std::string::const_iterator();
auto end = std::string::const_iterator();
auto output = std::string(start, end);
(The expected output being an empty string.)
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user673679
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Possible duplicate of [What is an iterator's default value?](https://stackoverflow.com/questions/3395180/what-is-an-iterators-default-value) – apple apple May 07 '19 at 12:19
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1I guess there is no guarantee two default constructed (if possible) iterator should compare equal. – apple apple May 07 '19 at 12:20
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According to cppreference.com, a random access iterator, of which a string iterator is one, meets all requirements of a bidirectional iterator.
Furthermore, a bidirectional iterator meets all requirements of a forward iterator.
Finally, since C++14, a forward iterator can be value-initialized, and will compare equal to all other value-initialized forward iterators of the same type:
A value-initialized LegacyForwardIterator behaves like the past-the-end iterator of some unspecified empty container: it compares equal to all value-initialized LegacyForwardIterators of the same type.
Based on that, I believe this is well-defined at least as of C++14.
Sam Varshavchik
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Good to know that singular iterators are required by C++14 standard, +1. – Maxim Egorushkin May 07 '19 at 12:30
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I thought there was a requirement that iterators in a range point into the same container. – Richard Critten May 07 '19 at 12:32
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@RichardCritten The cited wording abides by such a requirement; well, at least, if we assume that we always get the _same_ unspecified empty container. Heh. – Lightness Races in Orbit May 07 '19 at 12:34
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1Addedndum: This guarantee does not extend to default-initialized iterators. – eerorika May 07 '19 at 12:37