Re: quetsion about c++ object memory layout
"andb" <twicethinking@gmail.com> wrote in message news:a0827e7f-89aa-478d-b20a-7364f07a7e5c@v31g2000vbs.googlegroups.com...
class A
{
virtual a() {}
};
class B
{
virtual b() {}
};
class C : public A, public B
{
};
class D : virtual public A, virtual public B
{
};
sizeof(A) = 4
sizeof(B) = 4
sizeof(C) = 8
sizeof(D) = 12
address(C) = 00381148
address(D) = 00381180
address(A in C) = 00381148
address(B in C) = 0038114C
address(A in D) = 00381184
address(B in D) = 00381188
what i'm confused is what the leading 4 bytes contains in object C?
Typically, polymorphism in C++ classes is implemented by a vtable, essentially an array of function pointers to that class's virtual methods. Each instantiated object of that class contains a vptr, a pointer to that class's vtable, one vptr for each polymorphic base class.
Apparently, you ran this on a system using 32-bit pointers. sizeof(A)=4 is actually the sizeof the vptr in A, not just the empty object allocation. Similarly for sizeof(B).
Object C, to answer your question finally, contains both a vptr to class A's vtable, and a vptr to class B's vtable.
If you meant, instead, to ask about the additional 4-bytes in class D, they have to do with the virtual inheritance. It is coming on close to 20 years since I last read Stroustrup's, and no longer recall the fine complexities and details needed to implement virtual inheritance. I'll be pleased if you would post what you find as a refresher for us older folks.
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