Question

I’m having trouble understanding the value of this in the following example:

struct A {
  int i;

  void bar() {
    cout << this << endl;
  }
};

struct B : public A {
  virtual void foo() = 0;
};

struct C : public B {
  void foo() {
    printf("hello world!\n");
  }
};

int main (int argc, const char* argv[]) {
  C* c = new C; 
  cout << c << endl;
  c->bar();
  return 0;
}

Both times I print the pointer to the console, I get different values. I would expect it to be the same, as they refer to the same instance both times?!

If I remove either the virtual function or the int i in A it goes away. Why?

Answer 1

But it refers to differnt things.

void bar() {
    cout << this << endl;
  }

Here this has a type A*. It points to the part of the object that is A.

out << c << endl;

Here c is a C* and points to the part of the object that is C.

If the A object lines up exactly with the C object the pointers are the same. If they do not (like when C contains other members (hidden pointer to the vtable) and thus the ‘A’ part is offset from the start of the larger object) then the pointers are not necessarily the same.