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2967. std::equal on empty ranges

Section: 27.6.13 [alg.equal] Status: NAD Submitter: Gennaro Prota Opened: 2017-05-26 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

The description of the std::equal() algorithm in the standard doesn't make clear what the result of it is on empty ranges:

std::equal(first, first, second) ; // what does this return?

It should IMHO return true (two empty ranges are always equal).

[2017-07 Toronto Monday issue prioritization]

Closing as NAD; the existing wording covers empty sequences

Proposed resolution:

This wording is relative to N4659.

1. Edit 27.6.13 [alg.equal] as indicated:

   [Drafting note: The current wording presented below uses two times the unusual phrase "[…] return […]" instead of "[…] returns […]". The project editor is kindly asked to consider to replace these unusual wording forms by the usual one. — end drafting note]

   template<class InputIterator1, class InputIterator2>
     bool equal(InputIterator1 first1, InputIterator1 last1,
                InputIterator2 first2);
   […]
   

   -1- Remarks: If last2 was not given in the argument list, it denotes first2 + (last1 - first1) below.

   -2- Returns: If [first1, last1) and [first2, last2) are both empty, returns true. If last1 - first1 != last2 - first2, return false. Otherwise return true if for every iterator i in the range [first1, last1) the following corresponding conditions hold: *i == *(first2 + (i - first1)), pred(*i, *(first2 + (i - first1))) != false. Otherwise, returns false.

   […]