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    <p>The C code is not valid (nor would the equivalent C++ code in a context where the class definition is not visible) because what C does in this case is the equivalent of a <code>reinterpret_cast</code>. Note that in a simple situation like yours it will likely "work" because most compilers will put the single base object at the beginning of the derived object, so a pointer adjustment is not necessary. However, in the general case (especially when using multiple inheritance), the pointer will have to be adjusted to point to the correct subobject, and since C does not know how to do that, the cast is wrong.</p> <p>So what is meant with "pointer adjustment"? Consider the following situation:</p> <pre><code>class A { virtual ~A(); int i; ... }; class B { virtual ~B(); int j; ... }; class C: public A, public B { ... }; </code></pre> <p>Now the layout of <code>C</code> may be as follows:</p> <pre><code>+----------------------------+----------------------------+ | A subobject (containing i) | B subobject (containing j) | +----------------------------+----------------------------+ </code></pre> <p>where the virtual pointers of both the <code>A</code> and <code>B</code> subobjects point to <code>C</code>.</p> <p>Now imagine you've got a <code>C*</code> which you want to convert to a <code>B*</code>. Of course the code which receives the <code>B*</code> may not know about the existence of <code>C</code>; indeed, it may have been compiled before <code>C</code> was even written. Therefore the <code>B*</code> must point to the <code>B</code> subobject of the <code>C</code> object. In other words, on conversion from <code>C*</code> to <code>B*</code>, the size of the <code>A</code> subobject has to be added to the address stored into the pointer. If you do not do this, the <code>B*</code> will actually point to the <code>A</code> subobject, which clearly is wrong.</p> <p>Now without access to the class definition of <code>C</code>, there's of course no way to know that there even <em>is</em> an <code>A</code> subobject, not to mention how large it is. Therefore it is impossible to do a correct conversion from <code>C*</code> to <code>B*</code> if the class definition of <code>C</code> is not available.</p>
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    1. POWill polymorphism hold for C++ object references passed around in C?
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    1. COI'm a little confused about what you're saying here. Could you explain why that's wrong?
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    2. COSee my edit; I hope that makes it clear.
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    3. COAha, thanks for the update. Do you know if the standard guarantees the behavior I'm looking for if I'm only using single inheritance? And either way, is there a better way to do what I'm looking for? Good answer, though, +1 from me.
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