Down Hierarchy with Non-Polymorphic Types

Question

It is strict in C.

Say you are given one basic structure and two other derived structures. These derived structures are not produced in the classical sense (i.e.: A: B), but rather contain only structures of the base type. So, if struct A is the base, and B is one of 2 derived structures, B will have a member of type A. Like this:

struct A {
     // blah blah...
};

struct B {
     A part_a;
     // more stuff...
}

struct C {
     A part_a;
     // SO MUCH STUFF
}

Say you have an A_downcast_B function, something like this:

B * A_downcast_B(A * a)
{
     // downcast the A* here somehow
}

You want this function to return, 0or -1if you were 'a'not able to flush down to the type structure B. So, for example, if a "derived" type structure Chad a pointer to a type of type A*, and this pointer was passed to this function, the function would return 0, -1or null.

? , .

Answer 1

- struct A. , - , , "" .

struct RTTI {
  const char *typename;
  // etc.
};

struct A {
  const RTTI *rtti;
  // rest of A
};

struct B {
  A part_a;
  ...
};

const RTTI RTTI_B = {"B"};

struct C {
  A part_a;
  ...
};

const RTTI RTTI_C = {"C"};

void make_B(B *b)
{
  b->part_a.rtti = &RTTI_B;
  // make the rest of B
}

void make_C(C *c)
{
  c->part_a.rtti = &RTTI_C;
  // make the rest of C
}

B * A_downcast_B(A * a)
{
   return (a->rtti == &RTTI_B) ? (B*)a : NULL;
}

, ++ dynamic_cast, . , , - vtable.