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2437. iterator_traits<OutIt>::reference can and can't be void

Section: 24.3.5.2 [iterator.iterators] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 3

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Discussion:

24.3.5.2 [iterator.iterators]/2 requires an Iterator's *r to return reference, i.e. iterator_traits<X>::reference according to 24.3.1 [iterator.requirements.general]/11.

24.3.5.4 [output.iterators]/1 requires an OutputIterator's *r = o to do its job, so *r clearly can't return void.

24.3.2.3 [iterator.traits]/1 says: "In the case of an output iterator, the types

iterator_traits<Iterator>::difference_type
iterator_traits<Iterator>::value_type
iterator_traits<Iterator>::reference
iterator_traits<Iterator>::pointer

may be defined as void."

This is contradictory. I suggest fixing this by moving the offending requirement down from Iterator to InputIterator, and making Iterator say that *r returns an unspecified type. This will have the following effects:

• Output-only iterators will inherit Iterator's "*r returns unspecified" requirement, while 24.3.2.3 [iterator.traits]/1 clearly permits reference/etc. to be void.

• Input-or-stronger iterators (whether constant or mutable) are unaffected — they still have to satisfy "*r returns reference", they're just getting that requirement from InputIterator instead of Iterator.

[2015-02 Cologne]

EF: This is related to 2438⁽ⁱ⁾. MC: I'd like to take up 2438 right after this.

AM: Does anyone think this is wrong?

GR: Why do we give output iterators to have reference type void? AM: we've mandated that certain output iterators define it as void since 1998. GR: Oh OK, I'm satisfied.

Accepted. And 2438⁽ⁱ⁾ is already Ready.

Proposed resolution:

This wording is relative to N3936.

1. In 24.3.5.2 [iterator.iterators] Table 106 "Iterator requirements" change as indicated:

   Table 106 — Iterator requirements

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   *r	referenceunspecified		pre: r is dereferenceable.
   …

2. In 24.3.5.3 [input.iterators] Table 107 "Input iterator requirements" change as indicated:

   Table 107 — Input iterator requirements (in addition to Iterator)

   Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
   …
   *a	reference, convertible to T		[…]
   …