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POIs this correct usage of C++ 'move' semantics?
 

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  1. POIs this correct usage of C++ 'move' semantics?
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    <p>Tonight I've been taking a look at some code I've been working on over the last few days, and began reading up on move semantics, specifically std::move. I have a few questions to ask you pros to ensure that I am going down the right path and not making any stupid assumptions!</p> <p>Firstly:</p> <p><strong>1) Originally, my code had a function that returned a large vector:</strong></p> <pre><code>template&lt;class T&gt; class MyObject { public: std::vector&lt;T&gt; doSomething() const; { std::vector&lt;T&gt; theVector; // produce/work with a vector right here return(theVector); }; // eo doSomething }; // eo class MyObject </code></pre> <p>Given "theVector" is temporary in this and "throw-away", I modified the function to:</p> <pre><code> std::vector&lt;T&gt;&amp;&amp; doSomething() const; { std::vector&lt;T&gt; theVector; // produce/work with a vector right here return(static_cast&lt;std::vector&lt;T&gt;&amp;&amp;&gt;(theVector)); }; // eo doSomething </code></pre> <p>Is this correct? Any pitfalls in doing it this way?</p> <p><strong>2) I noticed in a function I have that returns <code>std::string</code> that it automatically called the move constructor. Debugging in to Return of the String (thankyou, Aragorn), I noticed it called an explicit move constructor. Why is there one for the string class and not vector?</strong></p> <p>I didn't have to make any modifications to this function to take advantage of move semantics:</p> <pre><code>// below, no need for std::string&amp;&amp; return value? std::string AnyConverter::toString(const boost::any&amp; _val) const { string ret; // convert here return(ret); // No need for static_cast&lt;std::string&amp;&amp;&gt; ? }; // eo toString </code></pre> <p><strong>3) Finally, I wanted to do some performance tests, is the amazingly-fast results I got because of std::move semantics or did my compiler (VS2010) do some optimizing too?</strong></p> <p>(Implementation of <code>_getMilliseconds()</code> omitted for brevity)</p> <pre><code>std::vector&lt;int&gt; v; for(int a(0); a &lt; 1000000; ++a) v.push_back(a); std::vector&lt;int&gt; x; for(int a(0); a &lt; 1000000; ++a) x.push_back(a); int s1 = _getMilliseconds(); std::vector&lt;int&gt; v2 = v; int s2 = _getMilliseconds(); std::vector&lt;int&gt; v3 = std::move(x); int s3 = _getMilliseconds(); int result1 = s2 - s1; int result2 = s3 - s2; </code></pre> <p>The results were, obviously, awesome. result1, a standard assignment, took 630ms. The second result, was 0ms. Is this a good performance test of these things?</p> <p>I know some of this is obvious to a lot of you, but I want to make sure I understand the semantics right before I go blazer on my code.</p> <p>Thanks in advance!</p>
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    1. COWhy are you using `static_cast<T&&>` instead of `std::move`?
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    2. CO@GMan - This is why I am asking these questions. My motivation originally came from here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm#string Motivation which admittedly I might have misread, hence this post :)
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    3. COI've added a link to my answer for you to read if you really want to learn rvalues.
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