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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-42313] Fix forwarding reference issue in `ScopeGuard`</title>
                <link>https://jira.mongodb.org/browse/SERVER-42313</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;```&lt;br/&gt;
template&amp;lt; typename T &amp;gt;&lt;/p&gt;

&lt;p&gt;void&lt;br/&gt;
f( T &amp;amp;&amp;amp;t )&lt;/p&gt;
{
&#160; T x= std::move( t );
}
&lt;p&gt;```&lt;/p&gt;

&lt;p&gt;This is somewhat incorrect (even though this has a `const T &amp;amp;` overload in `ScopeGuard` &#8211; `std::forward&amp;lt; T &amp;gt;` is what should be used.&lt;/p&gt;

&lt;p&gt;We should also consider removing the move operations entirely from `ScopeGuard` in favour of using the pure MCE/NRV/RVO transform.&lt;/p&gt;</description>
                <environment></environment>
        <key id="870122">SERVER-42313</key>
            <summary>Fix forwarding reference issue in `ScopeGuard`</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="adam.martin@mongodb.com">ADAM Martin</assignee>
                                    <reporter username="adam.martin@mongodb.com">ADAM Martin</reporter>
                        <labels>
                    </labels>
                <created>Mon, 22 Jul 2019 16:13:13 +0000</created>
                <updated>Sun, 29 Oct 2023 22:18:56 +0000</updated>
                            <resolved>Fri, 26 Jul 2019 19:05:29 +0000</resolved>
                                                    <fixVersion>4.3.1</fixVersion>
                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="2346170" author="xgen-internal-githook" created="Fri, 26 Jul 2019 19:05:03 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;ADAM David Alan Martin&apos;, &apos;username&apos;: &apos;adamlsd&apos;, &apos;email&apos;: &apos;adam.martin@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-42313&quot; title=&quot;Fix forwarding reference issue in `ScopeGuard`&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-42313&quot;&gt;&lt;del&gt;SERVER-42313&lt;/del&gt;&lt;/a&gt; Fix forwarding in `ScopeGuard` ctor.&lt;/p&gt;

&lt;p&gt;No need to have two distinct ctors.  Just create one and use forwarding.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/c44cf6f5f299991ec6819b5933b081476dc65a27&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/c44cf6f5f299991ec6819b5933b081476dc65a27&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2337055" author="jason.carey" created="Mon, 22 Jul 2019 17:46:40 +0000"  >&lt;p&gt;I&apos;m strongly in favor of dumping the move opts.  We can&apos;t stop RVO (so you have to think about it regardless), might as well let that be the way you factory construct a scope guard as well&lt;/p&gt;</comment>
                            <comment id="2336926" author="adam.martin" created="Mon, 22 Jul 2019 16:56:08 +0000"  >&lt;p&gt;Indeed, there&apos;s no code using `F &amp;amp;&amp;amp;` as a forwarding reference, but this means that the conversion happens in the `makeScopeGuard` operation, rather than making it in the ctor itself.&#160; It basically eliminates a ctor, the way I&apos;ve done it.&lt;/p&gt;

&lt;p&gt;MCE - Mandatory Copy Elision&lt;br/&gt;
NRV - Named Return Value Transform&lt;br/&gt;
RVO - Return Value Optimization&lt;/p&gt;

&lt;p&gt;These three appear overlap or be the same, but do different, interlinking things.&lt;/p&gt;

&lt;p&gt;MCE is when the standard says that a copy or move can be entirely eliminated, due to seeing how the object flows through the code.&#160; Often allows direct construction into a paramater slot, or from a return slot, bypassing any need for a copy or move ctor.&lt;/p&gt;

&lt;p&gt;NRV is the transformation the standard allows when all common paths through a function after the declaration of a named variable wind up returning that same named variable.&#160; The Named Return Value can be constructed in the return-slot for the function.&lt;/p&gt;

&lt;p&gt;RVO is the optimization path whereby the return slot of a function can be pointed at the memory where you want the returned object to be stored.&lt;/p&gt;

&lt;p&gt;They cooperate to give the best return performance.&lt;/p&gt;</comment>
                            <comment id="2336864" author="billy.donahue" created="Mon, 22 Jul 2019 16:30:51 +0000"  >&lt;p&gt;Description needs to be clearer. What&apos;s &quot;the pure MCE/NRV/RVO transform&quot; ?&lt;/p&gt;


&lt;p&gt;There&apos;s also no code in ScopeGuard that matches the opening snippet:&lt;/p&gt;


&lt;p/&gt;
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&lt;p/&gt;

&lt;p&gt;`std::forward` is for a forwarding reference. The `ScopeGuard&amp;lt;F&amp;gt;` constructor&apos;s use of `F&amp;amp;&amp;amp;` isn&apos;t a deduced template parameter type of the constructor, it&apos;s a parameter of the class. The constructor is not a template.&lt;/p&gt;</comment>
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