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POEfficient push_back of classes and structs
 

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    <p>I've seen my colleague do the second snippet quite often. Why is this? I've tried adding print statements to track the ctors and dtors, but both seem identical.</p> <pre><code> std::vector&lt;ClassTest&gt; vecClass1; ClassTest ct1; ct1.blah = blah // set some stuff ... vecClass1.push_back(ct1); </code></pre> <p></p> <pre><code> std::vector&lt;ClassTest&gt; vecClass2; vecClass2.push_back(ClassTest()); ClassTest&amp; ct2 = vecClass2.back(); ct2.blah = blah // set some stuff ... </code></pre> <p>PS. I'm sorry if the title is misleading.</p> <p><strong>Edit:</strong></p> <p>Firstly, thank you all for your responses.</p> <p>I've written a small application using <code>std::move</code>. The results are surprising to me perhaps because I've done something wrong ... would someone please explain why the "fast" path is performing significantly better.</p> <pre><code>#include &lt;vector&gt; #include &lt;string&gt; #include &lt;boost/progress.hpp&gt; #include &lt;iostream&gt; const std::size_t SIZE = 10*100*100*100; //const std::size_t SIZE = 1; const bool log = (SIZE == 1); struct SomeType { std::string who; std::string bio; SomeType() { if (log) std::cout &lt;&lt; "SomeType()" &lt;&lt; std::endl; } SomeType(const SomeType&amp; other) { if (log) std::cout &lt;&lt; "SomeType(const SomeType&amp;)" &lt;&lt; std::endl; //this-&gt;who.swap(other.who); //this-&gt;bio.swap(other.bio); this-&gt;who = other.who; this-&gt;bio = other.bio; } SomeType&amp; operator=(SomeType&amp; other) { if (log) std::cout &lt;&lt; "SomeType::operator=()" &lt;&lt; std::endl; this-&gt;who.swap(other.who); this-&gt;bio.swap(other.bio); return *this; } ~SomeType() { if (log) std::cout &lt;&lt; "~SomeType()" &lt;&lt; std::endl; } void swap(SomeType&amp; other) { if (log) std::cout &lt;&lt; "Swapping" &lt;&lt; std::endl; this-&gt;who.swap(other.who); this-&gt;bio.swap(other.bio); } // move semantics SomeType(SomeType&amp;&amp; other) : who(std::move(other.who)) , bio(std::move(other.bio)) { if (log) std::cout &lt;&lt; "SomeType(SomeType&amp;&amp;)" &lt;&lt; std::endl; } SomeType&amp; operator=(SomeType&amp;&amp; other) { if (log) std::cout &lt;&lt; "SomeType::operator=(SomeType&amp;&amp;)" &lt;&lt; std::endl; this-&gt;who = std::move(other.who); this-&gt;bio = std::move(other.bio); return *this; } }; int main(int argc, char** argv) { { boost::progress_timer time_taken; std::vector&lt;SomeType&gt; store; std::cout &lt;&lt; "Timing \"slow\" path" &lt;&lt; std::endl; for (std::size_t i = 0; i &lt; SIZE; ++i) { SomeType some; some.who = "bruce banner the hulk"; some.bio = "you do not want to see me angry"; //store.push_back(SomeType()); //store.back().swap(some); store.push_back(std::move(some)); } } { boost::progress_timer time_taken; std::vector&lt;SomeType&gt; store; std::cout &lt;&lt; "Timing \"fast\" path" &lt;&lt; std::endl; for (std::size_t i = 0; i &lt; SIZE; ++i) { store.push_back(SomeType()); SomeType&amp; some = store.back(); some.who = "bruce banner the hulk"; some.bio = "you do not want to see me angry"; } } return 0; } </code></pre> <p>Output:</p> <pre><code>dev@ubuntu-10:~/Desktop/perf_test$ g++ -Wall -O3 push_back-test.cpp -std=c++0x dev@ubuntu-10:~/Desktop/perf_test$ ./a.out Timing "slow" path 3.36 s Timing "fast" path 3.08 s </code></pre>
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    1. USbruce.banner
    1. USbruce.banner
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    1. COPremature optimization, I guess.
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    2. CONote that in C++, "classes and structs" is redundant, since classes and structs are the same thing.
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      1. USJames McNellis
    3. CORegarding your edit. Things to check first -- does your implementation have an efficient `move` for `std::string`, and does your implementation have `std::vector::push_back(T&&)`. If the answer to either is "no", then the `std::move` code is doing a copy (because your implementation's C++11 support is incomplete).
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      1. USSteve Jessop
 

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