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19304610
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2013-10-10T19:30:29.620
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2013-10-10T20:46:23.267
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2013-10-10T20:46:23.267
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It appears it is not the move ctor, nor the templated move ctor that is causing the problem, but the presence of <code>enable_if<is_convertable<...</code> in the templated move ctor: Testing with just an object, throwing <code>auto</code> and <code>pair</code> out of the test: <ul> <li>OK, copy/move elided: <pre><code> cout << "# Construct Object: auto obj = LogMe();\n"; LogMe obj = LogMe(); LogMe(LogMe&&) { cout << __FUNCTION__ ... } </code></pre></li> </ul> And, with a test like so: <pre><code> cout << "# Construct Object: LogMeTempl obj = LogMeTempl();\n"; LogMeTempl obj = LogMeTempl(); cout << "# Construct Object: LogMeTempl obj2;\n"; LogMeTempl obj2; </code></pre> <ul> <li>OK, copy move also elided: <pre><code>template<class Other> LogMeTempl(Other&& rhs // , typename enable_if<is_convertible<Other, LogMeTempl>::value, void>::type ** = 0 ) { cout << __FUNCTION__ << ...; } </code></pre></li> <li>Fail! Move ctor invoked! <pre><code>template<class Other> LogMeTempl(Other&& rhs , typename enable_if<is_convertible<Other, LogMeTempl>::value, void>::type ** = 0 ) { cout << __FUNCTION__ << ...; } </code></pre> And note that the enable_if can be reduced to <code>enable_if<true, void>::type** = 0</code> - if fact any additional defaulted parameter will do (e.g. <code>, int defaulted_param_on_move_ctor = 0</code> and it will still prevent the move elision). This also extends to a type with a copy-ctor only that has a default argument. It won't be elided either. A quick <a href="http://ideone.com/dBXckh" rel="nofollow">cross-check with gcc</a> shows there doesn't seem to be any such problem there.</li> </ul> <h3>Short Answer</h3> Types with defaulted arguments in their copy/move ctor don't have their initialization copy/move elided. I have added a <a href="https://connect.microsoft.com/VisualStudio/feedback/details/805051/visual-c-11-vs-2012-copy-constructor-with-defaulted-second-argument-prevents-copy-elision" rel="nofollow">bug on MS.connect</a> for this issue. I have also added a test-case for (N)RVO <a href="http://ideone.com/9XcO6w" rel="nofollow">to IDEone</a>. Even without the default argument, *N*RVO seems to work better in gcc than VC++.
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1. POstd::pair move not elided on definition?
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1. USMartin Ba
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1. CONeat! So two problems: first, they have to use the hacky "default nullptr with contrived type" to enable SFINAE, and second because the elision optimization is blocked by additional default parameters (incorrectly, I think). In C++11 (and in MSVC2013) they can use default template parameters in the ctor to do this without a default argument to the constructor. I do not know if the MSVC2013 std C++ library has this improvement or not, but MSVC 2013 has compiler support for it http://blogs.msdn.com/b/vcblog/archive/2013/06/28/c-11-14-stl-features-fixes-and-breaking-changes-in-vs-2013.aspx
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 USYakk - Adam Nevraumont

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