C++ – C-Style upcast and downcast involving private inheritance


Consider the following piece of code :-

class A {};

class B : private A {};

B* bPtr1 = new B;
// A* aPtr1 = bPtr1; // error
// A* aPtr2 = static_cast<A*>(bPtr1); // error
A* aPtr3 = (A*)bPtr1;
B* bPtr2 = (B*)aPtr3;

The C-style cast discards the private inheritance while both the implicit and static_cast fail (also dynamic_cast). Why ?
If C-style casts are just bit-fiddling, how are C++ casts implemented i.e. how do they know the type of inheritance from memory footprint?

After bPtr1 is casted to aPtr3, i will have to use another C-style cast to downcast to B as again both static_cast and dynamic_cast fail. So, is bPtr2 guaranteed to be good?

Thanks in advance

Best Solution

The standard states in 5.4.7 that C-style casts can actually do more than any sequence of new-style casts can do -- specifically including casting from a pointer-to-derived to pointer-to-base even when the base class is inaccessible, which is precisely what happens here with private inheritance. (Why this should be allowed, and in particular why it should be allowed only for C-style casts, is utterly beyond me; but it's undeniably allowed.)

So dribeas is right, compilers are obliged to handle the OP's C-style pointer conversion correctly, even when B inherits from multiple base classes. My own testing with MSVC++8 and MinGW confirms his results in practice -- when B inherits from multiple base classes, the compiler will adjust pointers when converting a B* to an A* or vice versa so that the correct object or subobject is identified.

I stand by my assertion that you ought to derive B publicly from A if you ever intend to treat a B as an A, since using private inheritance instead necessitates using C-style casts.

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