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POWhen will a C++11 compiler make RVO and NRVO outperform move semantics and const reference binding?
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copied!<p>Consider the case when "whole" objects with move semantics enabled are returned from functions, as with <code>std::basic_string<></code>:</p> <pre><code>std::wstring build_report() const { std::wstring report; ... return report; } </code></pre> <p>Can I then realistically be expected to make the "best" choice whether to use the returned string with move semantics, as in</p> <pre><code>const std::wstring report(std::move(build_report())); </code></pre> <p>or if I should rely on (N)RVO to take place with</p> <pre><code>const std::wstring report(build_report()); </code></pre> <p>or even bind a const reference to the temporary with</p> <pre><code>const std::wstring& report(build_report()); </code></pre> <p><strong>What scheme is there to make a deterministic choice of these options, if any?</strong></p> <p><strong>EDIT 1:</strong> Note that the usage of <code>std::wstring</code> above is just an example of a move semantics enabled type. It just as well be swapped for your <code>arbitrary_large_structure</code>. :-)</p> <p><strong>EDIT 2:</strong> I checked the generated assembly when running a speed-optmized release build in VS 2010 of the following:</p> <pre><code>std::wstring build_report(const std::wstring& title, const std::wstring& content) { std::wstring report; report.append(title); report.append(content); return report; } const std::wstring title1(L"title1"); const std::wstring content1(L"content1"); const std::wstring title2(L"title2"); const std::wstring content2(L"content2"); const std::wstring title3(L"title3"); const std::wstring content3(L"content3"); int _tmain(int argc, _TCHAR* argv[]) { const std::wstring report1(std::move(build_report(title1, content1))); const std::wstring report2(build_report(title2, content2)); const std::wstring& report3(build_report(title3, content3)); ... return 0; } </code></pre> <p>The 2 most interesting outcomes:</p> <ul> <li>Explicitly calling <code>std::move</code> for <code>report1</code> to use the move constructor <strong>triples</strong> the instruction count.</li> <li>As noted by James McNellis in <a href="https://stackoverflow.com/questions/6531700/when-will-a-c0x-compiler-make-rvo-and-nrvo-outperform-move-semantics-and-const/6531758#6531758">his answer below</a>, <code>report2</code> and <code>report3</code> does indeed generate identical assembly with 3 times fewer instructions than explicitly calling <code>std::move</code>.</li> </ul>

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