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<title>Issue 741: Const-incorrect get_deleter function for shared_ptr</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">NAD</a> status.</em></p>
<h3 id="741"><a href="lwg-closed.html#741">741</a>. Const-incorrect <code>get_deleter</code> function for <code>shared_ptr</code></h3>
<p><b>Section:</b> 20.3.2.2.11 <a href="https://wg21.link/util.smartptr.getdeleter">[util.smartptr.getdeleter]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2007-09-27 <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#util.smartptr.getdeleter">issues</a> in [util.smartptr.getdeleter].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following issue was raised by Alf P. Steinbach in c.l.c++.mod:
</p>

<p>
According to the recent draft N2369, both the header memory synopsis
of 20.2 <a href="https://wg21.link/memory">[memory]</a> and 20.3.2.2.11 <a href="https://wg21.link/util.smartptr.getdeleter">[util.smartptr.getdeleter]</a> declare:
</p>

<blockquote><pre>
template&lt;class D, class T&gt; D* get_deleter(shared_ptr&lt;T&gt; const&amp; p);
</pre></blockquote>

<p>
This allows to retrieve the pointer to a mutable deleter of a <code>const
shared_ptr</code> (if that owns one) and therefore contradicts the usual
philosophy that associated functors are either read-only (e.g.
<code>key_comp</code> or <code>value_comp</code> of <code>std::map</code>) or do at least reflect
the mutability of the owner (as seen for the both overloads of
<code>unique_ptr::get_deleter</code>).
Even the next similar counter-part of <code>get_deleter</code> - the two
overloads of <code>function::target</code> in the class template function
synopsis 22.10.17.3 <a href="https://wg21.link/func.wrap.func">[func.wrap.func]</a> or in 22.10.17.3.6 <a href="https://wg21.link/func.wrap.func.targ">[func.wrap.func.targ]</a> - do
properly mirror the const-state of the owner.
</p>

<p><b>Possible proposed resolutions:</b></p>

<p>
Replace the declarations of <code>get_deleter</code> in the header <code>&lt;memory&gt;</code>
synopsis of 20.2 <a href="https://wg21.link/memory">[memory]</a> and in 20.3.2.2.11 <a href="https://wg21.link/util.smartptr.getdeleter">[util.smartptr.getdeleter]</a> by one of the
following alternatives (A) or (B):
</p>

<ol style="list-style-type:upper-alpha">
<li>
Provide <b>only</b> the immutable variant. This would reflect the
current praxis of <code>container::get_allocator()</code>, <code>map::key_comp()</code>, or
<code>map::value_comp</code>.

<blockquote><pre>
template&lt;class D, class T&gt; const D* get_deleter(shared_ptr&lt;T&gt; const&amp; p);
</pre></blockquote>
</li>
<li>
Just remove the function.
</li>
</ol>

<p>
Alberto Ganesh Barbati adds:
</p>

<ol style="list-style-type:upper-alpha" start="3">
<li>
<p>
Replace it with two functions:
</p>
<blockquote><pre>
template &lt;class D, class T&gt; D get_deleter(shared_ptr&lt;T&gt; const&amp;);
template &lt;class D, class T&gt; bool has_deleter(shared_ptr&lt;T&gt; const&amp;);
</pre></blockquote>

<p>
The first one would throw if <code>D</code> is the wrong type, while the latter would
never throw. This approach would reflect the current praxis of
<code>use_facet/has_facet</code>, with the twist of returning the deleter by value as
<code>container::get_allocator()</code> do.
</p>
</li>
</ol>

<p>
Peter Dimov adds:
</p>

<blockquote>
<p>
My favorite option is "not a defect". A, B and C break useful code.
</p>
</blockquote>

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


<blockquote><p>
Concern this is similar to confusing "pointer to const" with "a constant pointer".
</p></blockquote>


<p id="res-741"><b>Proposed resolution:</b></p>
<p>
</p>





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