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copied!<p>What compiler are you using? A static cast from a base type to a derived type might result in an adjustment to the pointer - especially likely if multiple inheritance is involved (which doesn't seem to be the case in your situation from your description). However, it's still possible without MI.</p> <p>The standard indicates that if a null pointer value is being cast that the result will be a null pointer value (5.2.9/8 Static cast). However, I think that on many compilers most downcasts (especially when single inheritance is involved) don't result in a pointer adjustment, so I could imagine that a compiler might have a bug such that it wouldn't make the special check for null that would be required to avoid 'converting' a zero value null pointer to some non-zero value senseless pointer. I would assume that for such a bug to exist you must be doing something unusual to get the compiler to have to adjust the pointer in the downcast.</p> <p>It might be interesting to see what kind of assembly code was generated for your example.</p> <p>And for detailed information about how a compiler might layout an object that might need pointer adjustment with static casts, <a href="http://www.informit.com/store/product.aspx?isbn=0201834545" rel="nofollow noreferrer">Stan Lippman's "Inside the C++ Object Model"</a> is a great resource.</p> <p><a href="http://www.usenix.org/publications/compsystems/1989/fall_stroustrup.pdf" rel="nofollow noreferrer">Stroustrup's paper on Multiple Inheritance for C++</a> (from 1989) is also a good read. It's too bad if a C++ compiler has a bug like I speculate about here - Stroustrup discusses the null pointer issue explicitly in that paper (4.5 Zero Valued Pointers).</p> <p>For your second question:</p> <blockquote> <p>Q2. Is static_casting from B to C/D/E valid?</p> </blockquote> <p>This is perfectly valid as long as when you perform the cast of the B pointer to a C/D/E pointer the B pointer is actually pointing to the B sub-object of a C/D/E object (respectively) and B isn't a virtual base. This is mentioned in the same paragraph of the standard (5.2.9/8 Static cast). I've highlighted the sentences of the paragraph most relevant to your questions:</p> <blockquote> <p>An rvalue of type “pointer to cv1 B”, where B is a class type, can be converted to an rvalue of type “pointer to cv2 D”, where D is a class derived (clause 10) from B, if a valid standard conversion from “pointer to D” to “pointer to B” exists (4.10), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and B is not a virtual base class of D. <strong>The null pointer value (4.10) is converted to the null pointer value of the destination type. If the rvalue of type “pointer to cv1 B” points to a B that is actually a sub-object of an object of type D, the resulting pointer points to the enclosing object of type D.</strong> Otherwise, the result of the cast is undefined.</p> </blockquote> <p>As a final aside, you can workaround the problem using something like:</p> <pre><code>Set1(pEntity ? static_cast<C*>(pEntity) : 0); </code></pre> <p>which is what the compiler should be doing for you.</p>

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