虚函数和多态-代码说明

Question

#include <iostream>
using namespace std;

class GrandParent {
 public:
      int variable1;
      static int variable2;
      int virtual show() { return 10; }
};

int GrandParent :: variable2 = 9;

class Parent1 : public virtual GrandParent {
 public:
      int show() { return 1; }  
};

class Parent2 : public virtual GrandParent {
 public:
      int show() { return 9; }  
};

class Child : public Parent1, public Parent2 {
 public:
      int variable1;
      Child(){variable1 = 10; GrandParent::variable2 = 3;}
      int show() { return 7; }  
};

int main() {
      GrandParent *objGrand = new Child();
      GrandParent *objGrand1 = new Parent1();
      Parent2 *objP = new Child();
      Child *objChild = new Child();
      int a = (objGrand->show())*(objP->show())*(objChild->show())*(objGrand1->show());
      int b = (objGrand->variable1)*(objP->variable1)*(objChild->variable1);
      int c = (objGrand->variable2)*(objP->variable2)*(objChild->variable2);

      cout << a + b * c << endl;
      return 0;
}

Can someone please help me understand why (objGrand->variable1) = 0 & (objP->variable1) = 0 and not 10? This is a part of an online course I am doing. They posted the solution and an explanation. However, I did not find their explanation convincing so needed some help.

Answer 1

Child re-declares the member variable variable1 . This gives this class its own variable with that name, shadowing the variable inherited from the GrandParent class. So when its constructor does:

variable1 = 10;

it's assigning to Child::variable1 , not GrandParent::variable1 .

Member variables aren't accessed virtually, this is only done for virtual member functions. So if you then use

objGrand->variable1

it refers to GrandParent::variable1 , because objGrand is GrandParent* , even though objGrand points to a Child object. Since GrandParent has no constructor that initializes variable1 , this is uninitialized, not 0 , and using it results in undefined behavior.

To get Child::variable1 you need to downcast it:

dynamic_cast<Child*>(objGrand)->variable1

The same is true for objP for the same reasons.