Re: Double abstract class Inheritance concern.

[Nathan Sidwell <nathan@codesourcery.com>]

Chris Lattner wrote:
> Tiago Stein wrote:
> 
>>class A{};
>>class B :public A{};
>>class C: public A{ int g; };
>>class D: public B, public C {
>>  unsigned int foo;
>>};
>>Why sizeof class D is 12? Should not be 8?
> 
> 
> I believe the reason is that the ABI does not permit multiple different
> instances of A to be located at the same address, even if they have zero
> size.  If the size of the structure was 8, all of "A", "B", and "A" would
> have offset zero.
> 
> I'm not sure WHY this is required, but this is the reason it happens at
> least.  A declaration of "A X[100];" allocates 100 bytes as
> a result of the same rule.

It is a C++ language requirement that no two logically distinct objects
of the same type have the same address.

BEWARE. GNU C has an empty struct extension to C, that does not obey
the same rules.

nathan

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