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<title>Issue 1084: Concept ForwardIterator should provide default implementation for post-increment</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a> status.</em></p>
<h3 id="1084"><a href="lwg-closed.html#1084">1084</a>. Concept <code>ForwardIterator</code> should provide default implementation for post-increment</h3>
<p><b>Section:</b> 24.3.5.5 <a href="https://wg21.link/forward.iterators">[forward.iterators]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2009-03-22 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#forward.iterators">issues</a> in [forward.iterators].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Concepts">NAD Concepts</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses UK 250 [CD1]</b></p>

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

<p>
Copy the Effects clause into the concept description as the default
implementation. Assumes a default value for <code>postincrement_result</code>.
</p>

<p><i>[
Summit:
]</i></p>

<blockquote><p>
Howard will open an issue.
</p></blockquote>

<p><i>[
2009-06-07 Daniel adds:
]</i></p>


<blockquote><p>
This issue cannot currently be resolved as suggested, because
that would render auto-detection of the return type
<code>postincrement_result</code> 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.
</p></blockquote>


<p id="res-1084"><b>Proposed resolution:</b></p>
<p><i>[
This wording assumes the acceptance of UK 251 / <a href="lwg-closed.html#1009" title="InputIterator post-increment dangerous (Status: NAD)">1009</a><sup><a href="https://cplusplus.github.io/LWG/issue1009" title="Latest snapshot">(i)</a></sup>.  Both
wordings change the same paragraphs.
]</i></p>


<p>
Change 24.3.5.5 <a href="https://wg21.link/forward.iterators">[forward.iterators]</a>:
</p>

<blockquote>
<pre>
concept ForwardIterator&lt;typename X&gt; : InputIterator&lt;X&gt;, Regular&lt;X&gt; { 

  MoveConstructible postincrement_result;
  requires HasDereference&lt;postincrement_result&gt;
        &amp;&amp; Convertible&lt;HasDereference&lt;postincrement_result&gt;::result_type, const value_type&amp;&gt;;

  postincrement_result operator++(X&amp; r, int)<del>;</del> <ins>{
     X tmp = r;
     ++r;
     return tmp;
  }</ins>

  axiom MultiPass(X a, X b) { 
    if (a == b) *a == *b; 
    if (a == b) ++a == ++b; 
  } 
}
</pre></blockquote>






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