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1084. Concept ForwardIterator should provide default implementation for post-increment

Section: 24.3.5.5 [forward.iterators] Status: NAD Concepts Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 250 [CD1]

A default implementation should be supplied for the post-increment operator to simplify implementation of iterators by users.

Copy the Effects clause into the concept description as the default implementation. Assumes a default value for postincrement_result.

[ Summit: ]

Howard will open an issue.

[ 2009-06-07 Daniel adds: ]

This issue cannot currently be resolved as suggested, because that would render auto-detection of the return type postincrement_result invalid, see [concept.map.assoc]/4+5. The best fix would be to add a default type to that associated type, but unfortunately any default type will prevent auto-deduction of types of associated functions as quoted above. A corresponding core issue is in preparation.

Proposed resolution:

[ This wording assumes the acceptance of UK 251 / 1009(i). Both wordings change the same paragraphs. ]

Change 24.3.5.5 [forward.iterators]: 

concept ForwardIterator<typename X> : InputIterator<X>, Regular<X> { 

  MoveConstructible postincrement_result;
  requires HasDereference<postincrement_result>
        && Convertible<HasDereference<postincrement_result>::result_type, const value_type&>;

  postincrement_result operator++(X& r, int); {
     X tmp = r;
     ++r;
     return tmp;
  }

  axiom MultiPass(X a, X b) { 
    if (a == b) *a == *b; 
    if (a == b) ++a == ++b; 
  } 
}