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<title>Issue 2874: Constructor shared_ptr::shared_ptr(Y*) should be constrained</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++17">C++17</a> status.</em></p>
<h3 id="2874"><a href="lwg-defects.html#2874">2874</a>. Constructor <code>shared_ptr::shared_ptr(Y*)</code> should be constrained</h3>
<p><b>Section:</b> 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#util.smartptr.shared.const">active issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.const">issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US 125</b></p>

<p>Paragraph 4: This constructor should not participate in overload 
resolution unless the <i>Requires</i> clause is satisfied. Note that this would 
therefore apply to some assignment operator and <code>reset</code> overloads, via 
<i>Effects:</i> equivalent to some code wording.</p>

<p>
Proposed change:
</p>
<p>
Add a <i>Remarks:</i> clause to constrain this constructor 
not to participate in overload resolution unless the <i>Requires</i>
clause is satisfied.
</p>

<p><i>[2017-02-23, Jonathan provides wording]</i></p>

 
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>

<p>
This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.
</p>

<ol>
<li><p>Modify 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> as indicated:</p>
<blockquote class="note">
<p>
[<i>Drafting note:</i> This also adds a hyphen to "well defined"]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class Y&gt; explicit shared_ptr(Y* p);
</pre>
<blockquote>
<p>
-4- <i>Requires:</i> <code>Y</code> shall be a complete type. The expression <code>delete[] p</code>, when <code>T</code> 
is an array type, or <code>delete p</code>, when <code>T</code> is not an array type, <del>shall be well formed,</del> shall 
have well<del> </del><ins>-</ins>defined behavior, and shall not throw exceptions. <del>When <code>T</code> is <code>U[N]</code>, 
<code>Y(*)[N]</code> shall be convertible to <code>T*</code>; when <code>T</code> is <code>U[]</code>, <code>Y(*)[]</code> shall be convertible to 
<code>T*</code>; otherwise, <code>Y*</code> shall be convertible to <code>T*</code>.</del>
<p/>
-5- <i>Effects:</i> [&hellip;]
<p/>
-6- <i>Postconditions:</i> [&hellip;]
<p/>
-7- <i>Throws:</i> [&hellip;]
<p/>
<ins>-?- <i>Remarks:</i> When <code>T</code> is an array type, this constructor shall not
participate in overload resolution unless the expression <code>delete[] p</code> is
well-formed and either <code>T</code> is <code>U[N]</code> and <code>Y(*)[N]</code> is convertible to <code>T*</code>, or
<code>Y(*)[]</code> is convertible to <code>T*</code>. When <code>T</code> is not an array type, this
constructor shall not participate in overload resolution unless the
expression <code>delete p</code> is well-formed and <code>Y*</code> is convertible to <code>T*</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>

<p><i>[Kona 2017-02-27: Jonathan updates wording after LWG review]</i></p>


<p><i>[Kona 2017-02-27]</i></p>

<p>Accepted as Immediate to resolve NB comment.</p>


<p id="res-2874"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.
</p>

<ol>
<li><p>Modify 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> as indicated:</p>
<blockquote class="note">
<p>
[<i>Drafting note:</i> This also adds a hyphen to "well defined"]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class Y&gt; explicit shared_ptr(Y* p);
</pre>
<blockquote>
<p>
-4- <i>Requires:</i> <code>Y</code> shall be a complete type. The expression <code>delete[] p</code>, when <code>T</code> 
is an array type, or <code>delete p</code>, when <code>T</code> is not an array type, <del>shall be well formed,</del> shall 
have well<del> </del><ins>-</ins>defined behavior, and shall not throw exceptions. <del>When <code>T</code> is <code>U[N]</code>, 
<code>Y(*)[N]</code> shall be convertible to <code>T*</code>; when <code>T</code> is <code>U[]</code>, <code>Y(*)[]</code> shall be convertible to 
<code>T*</code>; otherwise, <code>Y*</code> shall be convertible to <code>T*</code>.</del>
<p/>
-5- <i>Effects:</i> [&hellip;]
<p/>
-6- <i>Postconditions:</i> [&hellip;]
<p/>
-7- <i>Throws:</i> [&hellip;]
<p/>
<ins>-?- <i>Remarks:</i> When <code>T</code> is an array type, this constructor shall not
participate in overload resolution unless the expression <code>delete[] p</code> is
well-formed and either <code>T</code> is <code>U[N]</code> and <code>Y(*)[N]</code> is convertible to <code>T*</code>, or
<code>T</code> is <code>U[]</code> and <code>Y(*)[]</code> is convertible to <code>T*</code>. When <code>T</code> is not an array type, this
constructor shall not participate in overload resolution unless the
expression <code>delete p</code> is well-formed and <code>Y*</code> is convertible to <code>T*</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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