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POWhy does reallocating a vector copy instead of moving the elements?
 

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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/8001823/how-to-enforce-move-semantics-when-a-vector-grows">How to enforce move semantics when a vector grows?</a> </p> </blockquote> <p><code>insert</code>, <code>push_back</code> and <code>emplace</code>(<code>_back</code>) can cause a reallocation of a <code>std::vector</code>. I was baffled to see that the following code <em>copies</em> the elements instead of <em>moving</em> them while reallocating the container.</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; struct foo { int value; explicit foo(int value) : value(value) { std::cout &lt;&lt; "foo(" &lt;&lt; value &lt;&lt; ")\n"; } foo(foo const&amp; other) noexcept : value(other.value) { std::cout &lt;&lt; "foo(foo(" &lt;&lt; value &lt;&lt; "))\n"; } foo(foo&amp;&amp; other) noexcept : value(std::move(other.value)) { other.value = -1; std::cout &lt;&lt; "foo(move(foo(" &lt;&lt; value &lt;&lt; "))\n"; } ~foo() { if (value != -1) std::cout &lt;&lt; "~foo(" &lt;&lt; value &lt;&lt; ")\n"; } }; int main() { std::vector&lt;foo&gt; foos; foos.emplace_back(1); foos.emplace_back(2); } </code></pre> <p>On my specific machine using my specific compiler (GCC 4.7) this prints the following:</p> <pre><code>foo(1) foo(2) foo(foo(1)) ~foo(1) ~foo(1) ~foo(2) </code></pre> <p>However, when deleting the copy constructor (<code>foo(foo const&amp;) = delete;</code>), the following (expected) output is generated:</p> <pre><code>foo(1) foo(2) foo(move(foo(1)) ~foo(1) ~foo(2) </code></pre> <p>Why is that? Would’t moving generally be more efficient, or at least not much less efficient, than copying?</p> <p>It bears noting that <a href="http://ideone.com/2EPxu" rel="nofollow noreferrer">GCC 4.5.1 does the expected thing</a> – is this a regression in GCC 4.7 or is it some deviously clever optimisation because the compiler sees that my object is cheap to copy (but how?!)?</p> <p>Also note that I made sure that this <em>is</em> caused by reallocation, by experimentally putting a <code>foos.reserve(2);</code> in front of the insertions; this causes neither copy nor move to be executed.</p>
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    1. COIt's not a regression, it's a bug fix. The standard specifies that `std::vector` will only prefer an element move constructor which is non-throwing.
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    2. CO@BenVoigt: Not at all a duplicate. This is asking a specific implementation related question.
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    3. CO@DeadMG: Yes it is. The answers to the other question explain this completely.
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