Embedded structs and correct function calls

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 guarant开发者_如何学Cee that the print_stuff function in B is called)?

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.

This is really horrible. But yes, there is a way to do it. Put a bool field in A, and in the same place in B, to indicate the real type. The code below should give you an appreciation of C++ (and really shouldn't be used for any other purpose).

struct A {
  bool isB;
  A()
  {
    isB=false;
  }
  void print_stuff(A * a);
};

struct B {
  bool trueB;
  A a_part;
  char * s;

  B()
  {
    isB=true;
  }

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

void A::print_stuff(A * a)
  {

    if(isB)
      {
        B* b = (B*)(this);
        b->print_stuff(b);
      }
    else
      puts("From A");
  }

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