C++ Objects Part 2: Single Inheritance

In my previous article I described the basic layout of a C++ object in Metrowerks CodeWarrior, with a pointer in the object to a static table of function pointers and a class description structure. Now we can look at how single inheritance is implemented.

Simple inheritance is, well, simple: the derived class simply contains the first class at the beginning and new data later in memory. The same thing is true for the vtable - new entries are tacked on the end.

(Please note that this is not true for virtual base classes - I'll get to that later.)

This meets our rule: a pointer to X must be laid out the same. Since derived class Y contains X at its beginning, the first bytes of Y look like an X.

Here's subclassing from the last post...

class bar : public foo {
virtual void b(float); // ovrrride
virtual void new_method(int);
char * new_member;
};

virtual base class - only one copy of the base no matter how we derive. How?

struct bar_vtable {
foo_vtable parent;
void (* new_method_func)(int);
};

void bar_b(float);
void bar_new_method(int);

static type_id bar_type_id = { ... };
static bar_vtable sbar_vtable = { &bar_type_id, 0, foo_a, bar_b, bar_new_method };

struct bar {
foo parent; /* superclass goes first. */
char * new_member; /* then new members. */
};

Note that a new vtable is made for the derived class, which may contain a mix of functions - here we have the "a" virtual function from the superclass, an overridden "b" function (bar_b, an override, goes into a slot in foo's vtable), and then a new virtual function in the end.

In fact, bar_b in a slot in a foo_vtable is where the "magic" of overrides happen. This is what lets us call a method from a foo * and get a method from a bar object. Because an object points to its classes vtable, we get that class's behavior.