Question

I was writing an operator== between two kinds of smart pointer and thought I should run a quick sanity check. I’m suprised by the result…

In the snippet below how is it that all variants of f and b end up with the same value?

struct Foo {
    int x;
};

struct Bar : public Foo {
    int y;
};

#include <iostream>

int main ()
{
    Bar bar;

    Foo * f = &bar;
    Bar * b = &bar;
    std :: cout << f << " " << b << " " << (f == b) << "\n";

    void * fv = f;
    void * bv = b;
    std :: cout << fv << " " << bv << " " << (fv == bv) << "\n";

    int fi = reinterpret_cast <int> (f);
    int bi = reinterpret_cast <int> (b);
    std :: cout << fi << " " << bi << " " << (fi == bi) << "\n";
}

Answer 1

About the only time that a base class object won’t have the same address as its subclass object is when multiple inheritance is involved.

In the above example memory probably looks like this:

                               / --------- \
                              /  | x     |  > This is the Foo portion of bar
This is the whole Bar object <   --------- /  
                              \  | y     |  
                               \ ---------  

Both views of the object have the same starting point, so a pointer to either view will have the same value.

In multiple inheritance, things get more complicated. Say you have:

struct Foo1{ int x; };
struct Foo2{ int y; };
struct Bar : public Foo1, public Foo2 { int z; };
Bar bar;

Now the memory will have to be laid out something like this:

                                / --------- \
                               /  | x     |  > This is the Foo1 portion of bar
                              /   --------- / \ 
This is the whole Bar object <    | y     |    > This is the Foo2 portion of bar  
                              \   ---------   /
                               \  | z     |  
                                \ ---------  

So &bar and (Foo1*)&bar will have the same value, while (Foo2*)&bar will have a different value, since it the Foo2 portion of the object starts at a higher address.