C++ 空类与多重继承的空类占用内存空间

#include <iostream>

using namespace std;

class A {};

class A2 {};

class B : public A {};

class C : public A, public A2 {};

class D : public virtual B {};


int main()
{
    cout << sizeof(A) << endl;
    cout << sizeof(B) << endl;
    cout << sizeof(C) << endl;
    cout << sizeof(D) << endl;    
    
    return 0;

}

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