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POCopy elision on Visual C++ 2010 Beta 2
 

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    <p>I was reading <a href="http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/" rel="nofollow noreferrer"><em>Want Speed? Pass by Value</em></a> on the <a href="http://cpp-next.com" rel="nofollow noreferrer">C++ Next blog</a> and created <a href="http://pastebin.com/f39c826c6" rel="nofollow noreferrer">this program</a> to get a feel for copy elision and move semantics in C++0x:</p> <pre><code>#include &lt;vector&gt; #include &lt;iostream&gt; class MoveableClass { public: MoveableClass() : m_simpleData(0), instance(++Instances) { std::cout &lt;&lt; "Construct instance " &lt;&lt; instance &lt;&lt; " (no data)" &lt;&lt; std::endl; } MoveableClass(std::vector&lt;double&gt; data) : m_data(std::move(data)), m_simpleData(0), instance(++Instances) { std::cout &lt;&lt; "Construct instance " &lt;&lt; instance &lt;&lt; " (with data)" &lt;&lt; std::endl; } MoveableClass(int simpleData) : m_simpleData(simpleData), instance(++Instances) { std::cout &lt;&lt; "Construct instance " &lt;&lt; instance &lt;&lt; " (with simple data)" &lt;&lt; std::endl; } MoveableClass(const MoveableClass&amp; other) : m_data(other.m_data), m_simpleData(other.m_simpleData), instance(++Instances) { std::cout &lt;&lt; "Construct instance " &lt;&lt; instance &lt;&lt; " from a copy of " &lt;&lt; other.instance &lt;&lt; std::endl; Elided = false; } MoveableClass(MoveableClass&amp;&amp; other) : m_data(std::move(other.m_data)), m_simpleData(other.m_simpleData), instance(++Instances) { std::cout &lt;&lt; "Construct instance " &lt;&lt; instance &lt;&lt; " from a move of " &lt;&lt; other.instance &lt;&lt; std::endl; Elided = false; } MoveableClass&amp; operator=(MoveableClass other) { std::cout &lt;&lt; "Assign to instance " &lt;&lt; instance &lt;&lt; " from " &lt;&lt; other.instance &lt;&lt; std::endl; other.Swap(*this); return *this; } ~MoveableClass() { std::cout &lt;&lt; "Destroy instance " &lt;&lt; instance &lt;&lt; std::endl; --Instances; } void Swap(MoveableClass&amp; other) { std::swap(m_data, other.m_data); std::swap(m_simpleData, other.m_simpleData); } static int Instances; static bool Elided; private: int instance; int m_simpleData; std::vector&lt;double&gt; m_data; }; int MoveableClass::Instances = 0; bool MoveableClass::Elided = true; std::vector&lt;double&gt; BunchOfData() { return std::vector&lt;double&gt;(9999999); } int SimpleData() { return 9999999; } MoveableClass CreateRVO() { return MoveableClass(BunchOfData()); } MoveableClass CreateNRVO() { MoveableClass named(BunchOfData()); return named; } MoveableClass CreateRVO_Simple() { return MoveableClass(SimpleData()); } MoveableClass CreateNRVO_Simple() { MoveableClass named(SimpleData()); return named; } int main(int argc, char* argv[]) { std::cout &lt;&lt; "\nMove assign from RVO: " &lt;&lt; '\n'; { MoveableClass a; a = CreateRVO(); } std::cout &lt;&lt; "Move elided: " &lt;&lt; (MoveableClass::Elided ? "Yes" : "No") &lt;&lt; '\n'; MoveableClass::Elided = true; // reset for next test std::cout &lt;&lt; "\nMove assign from RVO simple: " &lt;&lt; '\n'; { MoveableClass a; a = CreateRVO_Simple(); } std::cout &lt;&lt; "Move elided: " &lt;&lt; (MoveableClass::Elided ? "Yes" : "No") &lt;&lt; '\n'; MoveableClass::Elided = true; // reset for next test std::cout &lt;&lt; "\nMove assign from NRVO: " &lt;&lt; '\n'; { MoveableClass a; a = CreateNRVO(); } std::cout &lt;&lt; "Move elided: " &lt;&lt; (MoveableClass::Elided ? "Yes" : "No") &lt;&lt; '\n'; MoveableClass::Elided = true; // reset for next test std::cout &lt;&lt; "\nMove assign from NRVO simple: " &lt;&lt; std::endl; { MoveableClass a; a = CreateNRVO_Simple(); } std::cout &lt;&lt; "Move elided: " &lt;&lt; (MoveableClass::Elided ? "Yes" : "No") &lt;&lt; '\n'; MoveableClass::Elided = true; // reset for next test } </code></pre> <p>Here is the output I get when compiled in release mode on Visual C++ 10.0 (Beta 2):</p> <blockquote> <p>Move assign from RVO:<br> Construct instance 1 (no data)<br> Construct instance 2 (with data)<br> Construct instance 3 from a move of 2<br> Destroy instance 2<br> Assign to instance 1 from 3<br> Destroy instance 3<br> Destroy instance 1<br> Move elided: No </p> <p>Move assign from RVO simple:<br> Construct instance 1 (no data)<br> Construct instance 2 (with simple data)<br> Assign to instance 1 from 2<br> Destroy instance 2<br> Destroy instance 1<br> Move elided: Yes </p> <p>Move assign from NRVO:<br> Construct instance 1 (no data)<br> Construct instance 2 (with data)<br> Assign to instance 1 from 2<br> Destroy instance 2<br> Destroy instance 1<br> Move elided: Yes </p> <p>Move assign from NRVO simple:<br> Construct instance 1 (no data)<br> Construct instance 2 (with simple data)<br> Assign to instance 1 from 2<br> Destroy instance 2<br> Destroy instance 1<br> Move elided: Yes</p> </blockquote> <p>However, I am perplexed by one thing. As you can see, all of the moves are elided except for the first one. Why can't the compiler perform RVO with a MoveableClass(std::vector) at line 86, but can with a MoveableClass(int) at line 97? Is this just a bug with MSVC or is there a good reason for this? And if there is a good reason, why can it still perform NRVO on a MoveableClass(std::vector) at line 91?</p> <p>I'd like to understand it so I can go to sleep happy. :)</p>
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    1. COA very good question. For what it's worth, g++ 4.3.3 does elide all these moves, even with the `-O0` flag.
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    2. COThanks Thomas. It's interesting that it does work on GCC. Maybe that suggests is something wrong with the MSVC implementation.
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    3. COI think this just underscores how vast the chasm is between "the compiler should" and "the compiler does."
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