How is C++'s multiple inheritance implemented?

Single inheritance is easy to implement. For example, in C, the inheritance can be simulated as:

struct Base { int a; }
struct Descendant { Base parent; int b; }

But with multiple inheritance, the compiler has to arrange multiple parents inside newly constructed class. How is it done?

The problem I see arising is: should the parents be arranged in AB or BA, or maybe even other way? And then, if I do a cast:

SecondBase * base = (SecondBase *) &object_with_base1_and_base2_parents;

The compiler must consider whether to alter or not the original pointer. Similar tricky things are required with virtuals.

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And then, if I do a cast:
SecondBase base = (SecondBase *) object_with_base1_and_base2_parents;
The compiler must consider whether to alter or not the original pointer. Similar tricky things with virtuals.

With non-virutal inheritance this is less tricky than you might think - at the point where the cast is compiled, the compiler knows the exact layout of the derived class (after all, the compiler did the layout). Usually all that happens is a fixed offset (which may be zero for one of the base classes) is added/subtracted from the derived class pointer.

With virutal inheritance it is maybe a bit more complex - it may involve grabbing an offset from a vtbl (or similar).

Stan Lippman's book, "Inside the C++ Object Model" has very good descriptions of how this stuff might (and often actually does) work.

answered 2009-06-16T16:44:10.657

How is C++'s multiple inheritance implemented?

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