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<title>Issue 478: Should forward iterator requirements table have a line for r->m?</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#CD1">CD1</a> status.</em></p>
<h3 id="478"><a href="lwg-defects.html#478">478</a>. Should forward iterator requirements table have a line for r->m?</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#CD1">CD1</a>
 <b>Submitter:</b> Dave Abrahams <b>Opened:</b> 2004-07-11 <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#CD1">CD1</a> status.</p>
<p><b>Duplicate of:</b> <a href="lwg-closed.html#477" title="Operator-> for const forward iterators (Status: Dup)">477</a></p>
<p><b>Discussion:</b></p>
<p>
The Forward Iterator requirements table contains the following:
</p>
<pre>
 expression  return type         operational  precondition
                                  semantics
  ==========  ==================  ===========  ==========================
  a->m        U&amp; if X is mutable, (*a).m       pre: (*a).m is well-defined.
              otherwise const U&amp;

  r->m        U&amp;                  (*r).m       pre: (*r).m is well-defined.
</pre>

<p>The second line may be unnecessary.  Paragraph 11 of
  [lib.iterator.requirements] says:
</p>

<blockquote><p>
   In the following sections, a and b denote values of type const X, n
   denotes a value of the difference type Distance, u, tmp, and m
   denote identifiers, r denotes a value of X&amp;, t denotes a value of
   value type T, o denotes a value of some type that is writable to
   the output iterator.
</p></blockquote>

<p>
Because operators can be overloaded on an iterator's const-ness, the
current requirements allow iterators to make many of the operations
specified using the identifiers a and b invalid for non-const
iterators.</p>

<p>Related issue: <a href="lwg-closed.html#477" title="Operator-> for const forward iterators (Status: Dup)">477</a><sup><a href="https://cplusplus.github.io/LWG/issue477" title="Latest snapshot">(i)</a></sup></p>


<p id="res-478"><b>Proposed resolution:</b></p>

<p>Remove the "r->m" line from the Forward Iterator requirements
table. Change</p>
<blockquote><p>
    "const X"
</p></blockquote>

<p> to </p>

<blockquote><p>
    "X or const X" 
</p></blockquote>

<p>in paragraph 11 of [lib.iterator.requirements].</p>




<p><b>Rationale:</b></p>
<p>
This is a defect because it constrains an lvalue to returning a modifiable lvalue.
</p>





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