virtual inheritance and sizes

[ahmad alghadban]

I know why the size of Derived1 is 1
but why the size of Derived3 is 8
what does virtual keyword do in this place regardless of preventing double inheritance ?

#include <iostream>
using namespace std;

class Empty
{};

class Derived1 : public Empty
{};

class Derived3 : virtual public Empty
{
	char c;
};

int main()
{
	cout << "sizeof(Empty) " << sizeof(Empty) << endl;
	cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
	cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;

	return 0;
}

[Ganado]

I am assuming you are compiling this as a 32-bit program such that your pointer sizes are usually 4 (i.e. 32-bit addresses).

The reason it is 8 is because 1 byte is for the char, and 4 bytes is for the address of the shared base class object. This is 5 bytes in total, but due to padding/aliasing rules, the compiler rounds up to 8, with 3 bytes of padding.

The way that it handles "double inheritance" (a.k.a. multiple inheritance, or the "diamond problem") is by having both subclass objects point to the same base class object.

[ahmad alghadban]

Thank you