How to optimize object's memory size while using multiple inheritance with visual studio compiler?

问题

#include <iostream>
class A {};
class B {};
class C : public A, public B {
    int a;
};

int main() {
    std::cout << sizeof(C) << std::endl;
    return 0;
}

If I compile above code using cl, the output is '8'. While compiling with g++, the output will be '4'.

Without multiple inheritance the output will be '4' with both compilers.

Thanks.

回答1:

Here is the answer why it is 8-byte: Why empty base class optimization is not working?

The solution is to chain all base classes. To be elegant, we could write like this:

template <class Base = empty_base>
class A1 : public Base {};

template <class Base = empty_base>
class A2 : public Base {};

template <class Base =empty_base>
class A3 : public Base {};

class C : public A1<A2<A3> > { int c; };

You can find more code in this pattern in "boost/operators.hpp"

来源：https://stackoverflow.com/questions/24722070/how-to-optimize-objects-memory-size-while-using-multiple-inheritance-with-visua