Question

In C++, whose responsibility is it to delete members of a class: the class, or the creator of an instance of that class?
For example, in the following code:

class B {
  public:
    B(int x) { num = x; }
    int num;
};

class A {
  public:
    A(B* o) { obj = o; }
    B* obj;
};

int main(void) {
  A myA(new B(3));
  return 0;
}

Should main delete the instance of B, or should A‘s destructor delete its local variable obj? Is this true in most cases, and in which cases if any is it not?

Answer 1

This is a basic question of ownership. If every A should own a B (as in, there should be a new B created when the A is created, that should also be destroyed be the A is destroyed, then you’d normally make A responsible for creating and destroying the instance of B:

class B { 
    int num;
public:
    B(int x) : num(x) {}
};

class A { 
    B *obj;
public:
    A(int value) : obj(new B(value)) {}
    ~A() { delete B; }
};

In such a case, however, chances are pretty good that A should just be written like:

class A {
    B obj;
public:
    A(int v) : obj(v) {}
};

int main() { 
    A a(3);
    return 0;
}

This way, the instance of B will be created and destroyed automatically, without any intervention on your part at all.