understanding c++ pointers to parent/base classes

Question

I was asked this interview question and I got it wrong. "What is the output": My answer was 135, the actual output is 136. This implies that the pointer's to the two parent classes are not equal even though they pass the prior test of being equal to the child class. I thought I understood c++ pointers, but this has me stumped for an explanation. Although I think I see what's going on, I'm not sure why. Any c++ experts out there that can offer a technical explanation? It appears the first two comparisons are more logical in nature, while the last comparison is more literal...

#include <iostream>

class A
{
    public:
    A() : m_i(0) { }

    protected:
    int m_i;
};

class B
{
    public:
    B() : m_d(0.0) { }

    protected:
    double m_d;
};

class C
    : public A, public B
{
    public:
    C() : m_c('a') { }

    private:
    char m_c;
};

int main()
{
    C c;
    A *pa = &c;
    B *pb = &c;

    const int x = (pa == &c) ? 1 : 2;
    const int y = (pb == &c) ? 3 : 4;
    const int z = (reinterpret_cast<char*>(pa) == reinterpret_cast<char*>(pb)) ? 5 : 6;

    std::cout << x << y << z << std::endl;
    return 0;
}

Answer 1

Perhaps if you print out pa and pb it will be more clear what is going on.

std::cout << "A: " << pa << std::endl;
std::cout << "B: " << pb << std::endl;
std::cout << "C: " << &c << std::endl;

Running this at the end of main gives me

A: 0xbfef54e0
B: 0xbfef54e4
C: 0xbfef54e0

(your output may be different, the important thing is that they aren't all equal)

This is because of how a C object is represented in memory. Since a C is both an A and a B, it needs to have the data members from each part. The real layout of C is something like this (ignoring padding):

int    A::m_i;
double B::m_d;
char   C::m_c;

When you convert a C* to an A*, the compiler knows that the A part starts at offset 0, so the pointer value doesn't change. For C* to B* it needs to offset by sizeof(int) (plus padding). This offset handling is done automatically for you for calculating x and y. For z it is bypassed since you use reinterpret_cast