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1085. BidirectionalIterator concept should provide default implementation for post-decrement

Section: 24.3.5.6 [bidirectional.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 258 [CD1]

A default implementation should be supplied for the post-decrement 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 postdecrement_result invalid, see 1084(i).

Proposed resolution:

Change 24.3.5.6 [bidirectional.iterators]: 

concept BidirectionalIterator<typename X> : ForwardIterator<X> { 
  MoveConstructible postdecrement_result; 
  requires HasDereference<postdecrement_result> 
        && Convertible<HasDereference<postdecrement_result>::result_type, const value_type&> 
        && Convertible<postdecrement_result, const X&>; 
  X& operator--(X&); 
  postdecrement_result operator--(X& r, int); {
     X tmp = r;
     --r;
     return tmp;
  }
}