Question

I have to write definition of the code below. I undarstand some basics of OOP Cpp, I know what is *x or &x, but that’s not enough… The main fact is that i cant understand line B* p = new D, m, *k; i cant understand what m and *k stand for.

class B {
  public: 
    virtual void msg() { cout << "classB"; }
};

class D: public B {
   public: 
     virtual void msg() { cout << "classD"; }
};

int main() {
  B* p = new D, m, *k;
  p->msg(); k = &m; k->msg();
  . . .
}

Help, if you can explain how (and why so) this code will work.

thanks, for your time.

Answer 1

It declares multiple variables at once. This is basically the same as:

B *p = new D;
B  m;
B *k;

So p is a pointer to an instance of D allocated with new. m is a local instance of class B and k is a pointer to B that is later assigned to point at m.

The msg function is called on both allocated objects via the pointers p and k.

Note that * applies to each variable declaration separately. So B* a, b; doesn’t declare two pointers, but instead declares one pointer and one local object. This is the reason, that many people prefer to write the * directly in front of the variable name: B *a, b makes this a bit more obvious.