CODEWITHSUNDEEP
c++multiple-inheritancesizeof
ownfos
ownfos

Reputation: 155

Size of child class with diamond inheritance structure seems strange

I have 4 classes A, B, C, D. B and C inherit A, and D inherit B and C.

If A, B, C, D doesn't have any member variable, sizeof(D) returns 1(which is expected). But when B has one integer as member variable, sizeof(D) suddenly changes to 8.

Since sizeof(int) is 4, shouldn't sizeof(D) return 4 as well?

class A
{
};

class B : public A
{
    int data;
};

class C : public A
{
};

class D : public B, public C
{
};

int main()
{
    sizeof(A); // 1
    sizeof(B); // 4
    sizeof(C); // 1
    sizeof(D); // 8?
}

Upvotes: 3

Views: 148

Answers (1)

Caleth
Caleth

Reputation: 62719

You have 2 A subobjects in D, and they must have distinct representations. The lower bound on sizeof(D) is therefore 1 + sizeof(int).

The implementation you used chose to size D such that (sizeof(D) % alignof(D)) == 0.

Your implementation is non-conforming in the entirely empty case, as it has put two distinct As in the same storage.

Upvotes: 4

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