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<title>Issue 2875: shared_ptr::shared_ptr(Y*, D, [&hellip;]) constructors 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="2875"><a href="lwg-defects.html#2875">2875</a>. <code>shared_ptr::shared_ptr(Y*, D, [&hellip;])</code> constructors 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 126</b></p>

<p>Paragraph 8: 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 pre-Kona]</i></p>

<p>
See US 125: LWG <a href="lwg-defects.html#2874" title="Constructor shared_ptr::shared_ptr(Y*) should be constrained (Status: C++17)">2874</a><sup><a href="https://cplusplus.github.io/LWG/issue2874" title="Latest snapshot">(i)</a></sup>.
</p>

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

<ol style="list-style-type:upper-alpha">
<li><p>If the proposed resolution of LWG <a href="lwg-defects.html#2802" title="shared_ptr constructor requirements for a deleter (Status: C++17)">2802</a><sup><a href="https://cplusplus.github.io/LWG/issue2802" title="Latest snapshot">(i)</a></sup> <em>has been accepted</em> modify p8 as shown:</p>

<blockquote>
<pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template &lt;class D&gt; shared_ptr(nullptr_t p, D d);
template &lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre>
<blockquote>
<p>
-8- <i>Requires:</i> <del><code>D</code> shall be <code>MoveConstructible</code> and c</del><ins>C</ins>onstruction of 
<code>d</code> and a deleter of type 
<code>D</code> initialized with <code>std::move(d)</code> shall not throw exceptions. The expression <code>d(p)</code> 
<del>shall be well formed,</del> shall have well-defined behavior, and shall not throw exceptions. <code>A</code> 
shall be an allocator (17.5.3.5). <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>
</blockquote>
</blockquote>
</li>

<li><p>If the proposed resolution of LWG <a href="lwg-defects.html#2802" title="shared_ptr constructor requirements for a deleter (Status: C++17)">2802</a><sup><a href="https://cplusplus.github.io/LWG/issue2802" title="Latest snapshot">(i)</a></sup> <em>has not been accepted</em> modify p8 as shown:</p>

<blockquote>
<pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template &lt;class D&gt; shared_ptr(nullptr_t p, D d);
template &lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre>
<blockquote>
<p>
-8- <i>Requires:</i> <del><code>D</code> shall be <code>CopyConstructible</code> and such 
c</del><ins>C</ins>onstruction <ins>of <code>d</code> and a copy of <code>d</code></ins> shall not throw exceptions. 
The destructor of <code>D</code> shall not throw exceptions. The expression <code>d(p)</code> 
<del>shall be well formed,</del> shall have well defined behavior, and shall not throw exceptions. <code>A</code> 
shall be an allocator (17.5.3.5). The copy constructor and destructor of <code>A</code> 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>
</blockquote>
</blockquote>
</li>
</ol>
</li>
</ol>

<ol>
<li><p>In either case, add a <i>Remarks</i> paragraph after p11:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> If LWG <a href="lwg-defects.html#2802" title="shared_ptr constructor requirements for a deleter (Status: C++17)">2802</a><sup><a href="https://cplusplus.github.io/LWG/issue2802" title="Latest snapshot">(i)</a></sup> is <em>not</em> accepted, replace
<code>is_move_constructible_v</code> with <code>is_copy_constructible_v</code>.]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template &lt;class D&gt; shared_ptr(nullptr_t p, D d);
template &lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre>
<blockquote>
<p>
-8- <i>Requires:</i> [&hellip;]
<p/>
[&hellip;]
<p/>
-11- <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 <code>is_move_constructible_v&lt;D&gt;</code>
is <code>true</code>, the expression <code>d(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
<code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the
expression <code>d(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-2875"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4640">N4640</a>
as modified by the proposed resolution of LWG <a href="lwg-defects.html#2802" title="shared_ptr constructor requirements for a deleter (Status: C++17)">2802</a><sup><a href="https://cplusplus.github.io/LWG/issue2802" title="Latest snapshot">(i)</a></sup>.
</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>
<pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template &lt;class D&gt; shared_ptr(nullptr_t p, D d);
template &lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre>
<blockquote>
<p>
-8- <i>Requires:</i> <del><code>D</code> shall be <code>MoveConstructible</code> and c</del><ins>C</ins>onstruction of 
<code>d</code> and a deleter of type 
<code>D</code> initialized with <code>std::move(d)</code> shall not throw exceptions. The expression <code>d(p)</code> 
<del>shall be well formed,</del> shall have well-defined behavior, and shall not throw exceptions. <code>A</code> 
shall be an allocator (17.5.3.5). <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>
</blockquote>
</blockquote>
</li>

<li><p>Add a <i>Remarks</i> paragraph after p11:</p>

<blockquote>
<pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template &lt;class D&gt; shared_ptr(nullptr_t p, D d);
template &lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre>
<blockquote>
<p>
-8- <i>Requires:</i> [&hellip;]
<p/>
[&hellip;]
<p/>
-11- <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 <code>is_move_constructible_v&lt;D&gt;</code>
is <code>true</code>, the expression <code>d(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
<code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the
expression <code>d(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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