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POstd::pair move not elided on definition?
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  1. POstd::pair move not elided on definition?
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    copied!<p>I noticed something very strange with Visual Studio 2012: Defining a pair object like so:</p> <pre><code> auto objp = pair&lt;int, LogMe&gt;(); </code></pre> <p>will <em>not</em> elide the copy/move of the pair in VC11, this call will print:</p> <pre><code>LogMe::LogMe - def.ctor! LogMe::LogMe - move.ctor! LogMe::~LogMe - dtor! </code></pre> <p>that is, a temporary pair <em>will</em> be created and then moved into the objp variable. (declaring it as <code>pair&lt;...&gt; obj;</code> only logs the default ctor)</p> <p>I have cross checked with my LogMe test object alone:</p> <pre><code>cout &lt;&lt; "# Construct Object via auto obj = ...\n"; auto obj = LogMe(); # Construct Object via auto obj = ... LogMe::LogMe - def.ctor! </code></pre> <p>and here the assignment will be elided.</p> <p>This seems to be specific to VC11, as <a href="http://ideone.com/wXuU6o" rel="noreferrer">testing it in IDEOne</a> (which uses gcc 4.8.1) shows that the extraneous move is elided always there.</p> <p><strong>What's going on here?</strong> <em>Not being able to rely on the initialization copy being elided makes me nervous.</em></p> <p>Note: Tests for release vs. debug version show the same result. (Which I would have expected, as copy-elision is performed independently of optimization flags in MSVC.)</p> <hr> <p>Full sourcecode to test (See also the <a href="http://ideone.com/wXuU6o" rel="noreferrer">ideone link</a>):</p> <pre><code>#include "stdafx.h" #include &lt;iostream&gt; #include &lt;map&gt; using namespace std; struct LogMe { std::string member; LogMe() { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - def.ctor!" &lt;&lt; endl; } ~LogMe() { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - dtor!" &lt;&lt; endl; } LogMe(LogMe const&amp;) { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - cpy.ctor!" &lt;&lt; endl; } LogMe&amp; operator=(LogMe const&amp;) { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - cpy.assign.op!" &lt;&lt; endl; return *this; } LogMe(LogMe&amp;&amp;) { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - move.ctor!" &lt;&lt; endl; } LogMe&amp; operator=(LogMe&amp;&amp;) { cout &lt;&lt; __FUNCTION__ &lt;&lt; " - move.assign.op!" &lt;&lt; endl; return *this; } }; int _tmain(int argc, _TCHAR* argv[]) { { cout &lt;&lt; "# Construct Object via auto obj = ...\n"; auto obj = LogMe(); cout &lt;&lt; "# Construct pair&lt;int, object&gt; via auto objp = ...\n"; auto objp = pair&lt;int, LogMe&gt;(); cout &lt;&lt; "# Construct pair&lt;int, object&gt; via pair objp2; ...\n"; pair&lt;int, LogMe&gt; p2; } return 0; </code></pre>
 

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