StackOverflow2013

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15529908
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2013-03-20T16:56:14.017
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2013-03-20T16:56:14.017
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The cast in effect here is a <code>static_cast<Derived&></code>, governed by the following rule (§5.2.9 Static cast): <blockquote> An lvalue of type "cv1 <code>B</code>," where <code>B</code> is a class type, can be cast to type "reference to cv2 <code>D</code>," where <code>D</code> is a class derived (Clause 10) from <code>B</code>, if a valid standard conversion from "pointer to <code>D</code>" to "pointer to B" exists (4.10), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and <code>B</code> is neither a virtual base class of <code>D</code> nor a base class of a virtual base class of <code>D</code>. The result has type "cv2 <code>D</code>." </blockquote> The cast "pointer to <code>D</code>" to "pointer to <code>B</code>" is a valid standard conversion (§4.10): <blockquote> A prvalue of type "pointer to cv <code>D</code>", where <code>D</code> is a class type, can be converted to a prvalue of type "pointer to cv <code>B</code>", where <code>B</code> is a base class (Clause 10) of <code>D</code>. </blockquote> However, just because the cast works, doesn't mean it's okay to do. Note (§5.2.9): <blockquote> If the object of type "cv1 <code>B</code>" is actually a subobject of an object of type <code>D</code>, the result refers to the enclosing object of type <code>D</code>. Otherwise, the result of the cast is undefined. </blockquote> So this code results in undefined behaviour. You can cast from a base class to any of its derived classes, but you only have define behaviour if it truly is an object of that derived type. So to answer the questions: <ol> <li>It's a <code>static_cast<Derived&></code>.</li> <li>Yes, because that's what's happening.</li> <li>It doesn't work anyway because it results in undefined behaviour.</li> <li>I can't be certain what the point was here. I'd expect that it might continue to perform as expected if you don't use any <code>Derived</code> specific features inside <code>foo</code>. However, if that's the case, just take a <code>Base&</code>.</li> </ol>
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1. USJoseph Mansfield
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1. COThank you for you answer. It was exactly the explanation I was looking for.
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