Question

You’re given two structs. One of type ‘A’, another of type ‘B’.
The one of type ‘B’ has a struct of type ‘A’ in it. So something like this:

struct A {
     void print_stuff(A * a){ puts("From A"); }
};

struct B {
     A a_part;
     char * s;
     void print_stuff(B * b){
          printf("From B\n\t%s\n", b->s);
     }
};

B * B_new(char * str) {
     B * b = (B*)malloc(sizeof(struct B));
     b->s = strdup(str);
     return b; 
}

You want to be able to call struct B’s ‘print_stuff’ function, even with code like this:

A * aptr = (A*) B_new("foo");
aptr->print_stuff(aptr);

You are NOT allowed to use inheritance, C++ containers or classes. Struct B MUST have a_part.

How would you write your code so that regardless of the type of the pointer, the correct print_stuff function is called (in this case, if a pointer to a struct of type B was casted to A*, how would you guarantee that the print_stuff function in B is called)?

Answer 1

You could make print_stuff a function pointer, but then you need to manually hook it up to the correct function. In this case though, you’re just replicating C++ inheritance mechanisms so it’s pointless.

The A object can’t automatically know where it’s contained, or if it’s contained at all.

So the bottom line is, just use C++ inheritance.