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<title>Issue 1334: Insert iterators are broken for some proxy containers compared to C++03</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#C++11">C++11</a> status.</em></p>
<h3 id="1334"><a href="lwg-defects.html#1334">1334</a>. Insert iterators are broken for some proxy containers compared to C++03</h3>
<p><b>Section:</b> 24.5.2.2.2 <a href="https://wg21.link/back.insert.iter.ops">[back.insert.iter.ops]</a>, 24.5.2.3.2 <a href="https://wg21.link/front.insert.iter.ops">[front.insert.iter.ops]</a>, 24.5.2.4.2 <a href="https://wg21.link/insert.iter.ops">[insert.iter.ops]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2010-03-28 <b>Last modified:</b> 2023-02-07</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#back.insert.iter.ops">issues</a> in [back.insert.iter.ops].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In C++03 this was valid code:
</p>

<blockquote><pre>
#include &lt;vector&gt;
#include &lt;iterator&gt;

int main() {
  typedef std::vector&lt;bool&gt; Cont;
  Cont c;
  std::back_insert_iterator&lt;Cont&gt; it = std::back_inserter(c);
  *it = true;
}
</pre></blockquote>

<p>
In C++0x this code does no longer compile because of an ambiguity error for this
<code>operator=</code> overload pair:
</p>

<blockquote><pre>
back_insert_iterator&lt;Container&gt;&amp;
operator=(typename Container::const_reference value);

back_insert_iterator&lt;Container&gt;&amp;
operator=(typename Container::value_type&amp;&amp; value);
</pre></blockquote>

<p>
This is so, because for proxy-containers like <code>std::vector&lt;bool&gt;</code>
the <code>const_reference</code> usually is a non-reference type and in this case
it's identical to <code>Container::value_type</code>, thus forming the ambiguous
overload pair
</p>

<blockquote><pre>
back_insert_iterator&lt;Container&gt;&amp;
operator=(bool value);

back_insert_iterator&lt;Container&gt;&amp;
operator=(bool&amp;&amp; value);
</pre></blockquote>

<p>
The same problem exists for <code>std::back_insert_iterator</code>,
<code>std::front_insert_iterator</code>, and <code>std::insert_iterator</code>.
</p>

<p>
One possible fix would be to require that <code>const_reference</code> of a proxy
container must not be the same as the <code>value_type</code>, but this would break
earlier valid code. The alternative would be to change the first signature to
</p>

<blockquote><pre>
back_insert_iterator&lt;Container&gt;&amp;
operator=(const typename Container::const_reference&amp; value);
</pre></blockquote>

<p>
This would have the effect that this signature <em>always</em> expects an lvalue
or rvalue, but it would not create an ambiguity relative to the second form with
rvalue-references. [For all non-proxy containers the signature will be the same
as before due to reference-collapsing and const folding rules]
</p>


<p><i>[
Post-Rapperswil
]</i></p>


<p>
This problem is not restricted to the unspeakable <code>vector&lt;bool&gt;</code>, but is already existing for other proxy
containers like gcc's <code>rope</code> class. The following code does no longer work ([Bug libstdc++/44963]):
</p>
<blockquote><pre>
#include &lt;iostream&gt;
#include &lt;ext/rope&gt;

using namespace std;

int main()
{
     __gnu_cxx::crope line("test");
     auto ii(back_inserter(line));

     *ii++ = 'm'; // #1
     *ii++ = 'e'; // #2

     cout &lt;&lt; line &lt;&lt; endl;
}
</pre></blockquote>
<p>
Both lines marked with #1 and #2 issue now an error because the library has properly implemented the current
wording state (Thanks to Paolo Calini for making me aware of this real-life example).
</p>
<p>
The following P/R is a revision of the orignal P/R and was initially suggested by Howard
Hinnant. Paolo verified that the approach works in gcc.
</p>

<blockquote><p>
Moved to Tentatively Ready with revised wording after 6 positive votes on c++std-lib.
</p></blockquote>

<p><i>[
Adopted at 2010-11 Batavia
]</i></p>




<p id="res-1334"><b>Proposed resolution:</b></p>
<p><i>The wording refers to N3126.</i></p>

<ol>
<li>Change [back.insert.iterator], class <code>back_insert_iterator</code> synopsis as indicated:
<blockquote><pre>
template &lt;class Container&gt;
class back_insert_iterator :
 public iterator&lt;output_iterator_tag,void,void,void,void&gt; {
protected:
 Container* container;
public:
 [..]
 back_insert_iterator&lt;Container&gt;&amp;
   operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
 back_insert_iterator&lt;Container&gt;&amp;
   operator=(typename Container::value_type&amp;&amp; value);
 [..]
};
</pre></blockquote>
</li>
<li>Change [back.insert.iter.op=] before p. 1 as indicated:
<blockquote><pre>
back_insert_iterator&lt;Container&gt;&amp;
   operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
</pre>
<blockquote><p>
1 <em>Effects</em>: <code>container-&gt;push_back(value)</code>;<br/>
2 <em>Returns</em>: <code>*this</code>.
</p></blockquote></blockquote>
</li>
<li>Change [front.insert.iterator], class <code>front_insert_iterator</code> synposis as indicated:
<blockquote><pre>
template &lt;class Container&gt;
class front_insert_iterator :
 public iterator&lt;output_iterator_tag,void,void,void,void&gt; {
protected:
 Container* container;
public:
 [..]
 front_insert_iterator&lt;Container&gt;&amp;
   operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
 front_insert_iterator&lt;Container&gt;&amp;
   operator=(typename Container::value_type&amp;&amp; value);
 [..]
};
</pre></blockquote>
</li>
<li>Change [front.insert.iter.op=] before p.1 as indicated:
<blockquote><pre>
front_insert_iterator&lt;Container&gt;&amp;
   operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
</pre>
<blockquote><p>
1 <em>Effects</em>: <code>container-&gt;push_front(value)</code>;<br/>
2 <em>Returns</em>: <code>*this</code>.
</p></blockquote></blockquote>
</li>
<li>Change [insert.iterator], class insert_iterator synopsis as indicated:
<blockquote><pre>
template &lt;class Container&gt;
   class insert_iterator :
     public iterator&lt;output_iterator_tag,void,void,void,void&gt; {
   protected:
     Container* container;
     typename Container::iterator iter;
   public:
     [..]
     insert_iterator&lt;Container&gt;&amp;
       operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
     insert_iterator&lt;Container&gt;&amp;
       operator=(typename Container::value_type&amp;&amp; value);
     [..]
   };
</pre></blockquote>
</li>
<li>Change [insert.iter.op=] before p. 1 as indicated:
<blockquote><pre>
insert_iterator&lt;Container&gt;&amp;
    operator=(<ins>const</ins> typename Container::<del>const_reference</del><ins>value_type&amp;</ins> value);
</pre>
<blockquote><p>
1 <em>Effects</em>:
</p><blockquote><pre>
  iter = container-&gt;insert(iter, value);
  ++iter;
</pre></blockquote><p>
2 <em>Returns</em>: <code>*this</code>.
</p></blockquote></blockquote>
</li>
</ol>






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