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<title>Issue 536: Container iterator constructor and explicit convertibility</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#Dup">Dup</a> status.</em></p>
<h3 id="536"><a href="lwg-closed.html#536">536</a>. Container iterator constructor and explicit convertibility</h3>
<p><b>Section:</b> 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a> <b>Status:</b> <a href="lwg-active.html#Dup">Dup</a>
 <b>Submitter:</b> Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz <b>Opened:</b> 2005-12-17 <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#container.requirements">issues</a> in [container.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Dup">Dup</a> status.</p>
<p><b>Duplicate of:</b> <a href="lwg-defects.html#589" title="Requirements on iterators of member template functions of containers (Status: CD1)">589</a></p>
<p><b>Discussion:</b></p>
<p>
The iterator constructor X(i,j) for containers as defined in 23.1.1 and
23.2.2 does only require that i and j be input iterators but
nothing is said about their associated value_type. There are three
sensible
options:
</p>
<ol>
<li>iterator's value_type is exactly X::value_type (modulo cv).</li>
<li>iterator's value_type is *implicitly* convertible to X::value_type.</li>
<li>iterator's value_type is *explicitly* convertible to X::value_type.</li>
</ol>
<p>
The issue has practical implications, and stdlib vendors have
taken divergent approaches to it: Dinkumware follows 2,
libstdc++ follows 3.
</p>
<p>
The same problem applies to the definition of insert(p,i,j) for
sequences and insert(i,j) for associative contianers, as well as
assign.
</p>

<p><i>[
The following added by Howard and the example code was originally written by
Dietmar.
]</i></p>

<p>
Valid code below?
</p>

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

struct foo 
{ 
    explicit foo(int) {} 
}; 

int main() 
{ 
    std::vector&lt;int&gt; v_int; 
    std::vector&lt;foo&gt; v_foo1(v_int.begin(), v_int.end()); 
    std::vector&lt;foo&gt; v_foo2((std::istream_iterator&lt;int&gt;(std::cin)), 
                             std::istream_iterator&lt;int&gt;()); 
} 
</pre></blockquote>
<p><i>[
Berlin: Some support, not universal, for respecting the explicit qualifier.
]</i></p>




<p id="res-536"><b>Proposed resolution:</b></p>






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