Question

The following situation:

class Main{
   void MainMethod(){
      C * c;
      B * b = new B();
      b->fillC(c);
   }
};

class B{
   void fillC(C* c){
      c = new C();
   }
};

class C{
};

In my software I have this situation. At the end of the program c is still empty for class Main. Why is this?

Answer 1

The pointer c is copied into the fillC function – this is known as passing by value. You then assign to that copy. The original c variable from MainMethod is left unaffected. You need to pass the pointer by reference if you want to change it:

class B{
  void fillC(C*& c){
    c = new C();
  }
};

Now the c inside fillC refers to the same pointer as the c in MainMethod. Changing the value of one affects the value of the other.

You could alternatively change the parameter of fillC to type C**, call it with fillC(&c) and assign to it with *c = new C() – but you’re already using a mishmash of idioms from other languages, so I wouldn’t recommend this.

Don’t forget to do delete c at some point!