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POMove constructor seemingly not executed
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  1. POMove constructor seemingly not executed
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    copied!<p>This is my first experiment with C++0x rvalue references and something strange seems to be going on.</p> <p>In the code sample below the factory function <code>MakeWindow</code> returns a Window object by value. The caller uses it to initialize a Window object. If I understood correctly, this should invoke the move constructor. In order to detect this I throw an exception there. On top of that I disabled the copy constructor:</p> <pre><code>#include &lt;iostream&gt; // Fake WinAPI typedef void* HWND; HWND CreateWindow() { return (void*)1; } void DestroyWindow(HWND) { } // End WinAPI // C++ WinAPI Wrapper Library class Window { public: Window(HWND inHandle) : mHandle(inHandle) { std::cout &lt;&lt; "Window constructor. Handle: " &lt;&lt; inHandle &lt;&lt; std::endl; } Window(Window &amp;&amp; rhs) : mHandle(rhs.mHandle) { std::cout &lt;&lt; "Window move constructor. Handle: " &lt;&lt; mHandle &lt;&lt; std::endl; rhs.mHandle = 0; throw 1; // this is my "breakpoint" } ~Window() { std::cout &lt;&lt; "Window destructor. Handle: " &lt;&lt; mHandle &lt;&lt; std::endl; if (mHandle) { DestroyWindow(mHandle); } } private: Window(const Window&amp;); Window&amp; operator=(const Window&amp;); HWND mHandle; }; // Factory function Window MakeWindow() { return Window(CreateWindow()); } int main() { { Window window(MakeWindow()); } std::cout &lt;&lt; "Everything is OK." &lt;&lt; std::endl; return 0; } </code></pre> <p>However the code runs fine without this exception being thrown. This is the console output:</p> <pre><code>Window constructor. Handle: 0x1 Window destructor. Handle: 0x1 Everything is OK. </code></pre> <p>If I comment out the move constructor then compilation fails with the following errors:</p> <pre><code>MysteryMove.cpp: In function 'Window MakeWindow()': MysteryMove.cpp:39:5: error: 'Window::Window(const Window&amp;)' is private MysteryMove.cpp:49:33: error: within this context MysteryMove.cpp: In function 'int main()': MysteryMove.cpp:39:5: error: 'Window::Window(const Window&amp;)' is private MysteryMove.cpp:57:35: error: within this context make: *** [all] Error 1 </code></pre> <p>It doesn't seem to make sense. Can anyone explain what is going on?</p> <h3>Update</h3> <p>Thanks to @Philipp I learned that move constructors can also be omitted. This is described in §12.8/34 and footnote 124 of the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3126.pdf" rel="noreferrer">N3126 draft standard</a>.</p> <p>It is there also mentioned that RVO is only allowed for non-volatile objects. This means I can get around it writing the factory function like this:</p> <pre><code>// Factory function Window MakeWindow() { volatile Window window(CreateWindow()); return const_cast&lt;Window&amp;&amp;&gt;(window); } </code></pre> <p>And indeed it works:</p> <pre><code>Window constructor. Handle: 0x1 Window move constructor. Handle: 0x1 terminate called after throwing an instance of 'int' Abort trap </code></pre>
 

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