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    <p>Are you asking why the compiler does the access check? 12.8/14 in C++03:</p> <blockquote> <p>A program is ill-formed if the copy constructor or the copy assignment operator for an object is implicitly used and the special member function is not accessible</p> </blockquote> <p>When the implementation "omits the copy construction" (permitted by 12.8/15), I don't believe this means that the copy ctor is no longer "implicitly used", it just isn't executed.</p> <p>Or are you asking why the standard says that? If copy elision were an exception to this rule about the access check, your program would be well-formed in implementations that successfully perform the elision, but ill-formed in implementations that don't.</p> <p>I'm pretty sure the authors would consider this a Bad Thing. Certainly it's easier to write portable code this way -- the compiler tells you if you write code that <em>attempts</em> to copy a non-copyable object, even if the copy happens to be elided in your implementation. I suspect that it could also inconvenience implementers to figure out whether the optimization will be successful <em>before</em> checking access (or to defer the access check until after the optimization is attempted), although I have no idea whether that warranted consideration.</p> <blockquote> <p>Could this behavior be dangerous? I mean, I might do some other useful thing in the copy-ctor, but if it doesn't call it, then does it not alter the behavior of the program?</p> </blockquote> <p>Of course it could be dangerous - side-effects in copy constructors occur if and only if the object is actually copied, and you should design them accordingly: the standard says copies can be elided, so don't put code in a copy constructor unless you're happy for it to be elided under the conditions defined in 12.8/15:</p> <pre><code>MyObject(const MyObject &amp;other) { std::cout &lt;&lt; "copy " &lt;&lt; (void*)(&amp;other) &lt;&lt; " to " &lt;&lt; (void*)this &lt;&lt; "\n"; // OK std::cout &lt;&lt; "object returned from function\n"; // dangerous: if the copy is // elided then an object will be returned but you won't see the message. } </code></pre>
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    1. POStrange behavior of copy-initialization, doesn't call the copy-constructor!
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    1. USSteve Jessop
    1. USSteve Jessop
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    1. POStrange behavior of copy-initialization, doesn't call the copy-constructor!
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    1. COSteve, can you explain your code-snippet a bit more? I didn't really understand it, and when you said `you should design them accordingly`. How exactly?
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      1. USNawaz
    2. CO+1 for attempting to answer my question `"Could this behavior be dangerous?"`.
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    3. COIsn't the basic question here about accessibility?
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      1. USNeil Butterworth
 

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