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

Section: 25.3.5.5 [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. Both wordings change the same paragraphs. ]

Change 25.3.5.5 [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;
>       }
>     }