Is it possible to manually calculate the byte-offset of a class member?

Question

That is, what is the standard a compiler uses to generate a class? For example, let's say that I have class C with members x, y, and z, and I want to know the offset of z within that class. Can I just add up the data-type sizes of the other members, like I would for a structure?

Answer 1

No You cannot.
Compilers are free to align the members as they chose to.It is an implementation detail of the compilers.

If you're working with a POD, then you can use the offsetof macro.

But If working with a Non-POD, then I suppose there won't be any portable method to do so.

Answer 2

You can do it programatically like this in a method of the class. Not generic but works.

offset = (unsigned char*)&(this->z) - (unsigned char*)this;

Full working example

#include <iostream>

class C
{
public:
    int x;
    char y;
    int z;
    size_t offset() const
    {
        return (unsigned char*)&(this->z) - (unsigned char*)this;
    }
};

int main()
{
    C c;
    std::cerr << "Offset(cast): " << c.offset() << "\n";
}