Example 1. without virtual keyword / Using scope resolution
#include <iostream>

using namespace std;

class classA {
    public:
    int a;
};

class classB : public classA {
    public:
    int b;
};
class classC : public classA {
    public:
    int c;
};
class classD : public classB, public classC {
    public:
    int d;
    
};

int main() {
   
   classD objD;
   //objD.a; Error
   
   objD.classB::a = 100; // this way is correct
   objD.classC::a = 200;
   
   cout<<"A from classB = "<<objD.classB::a<<endl;
   cout<<"A from classc = "<<objD.classC::a<<endl;
   objD.b = 10;
   objD.c = 20;
   objD.d = 30;
   cout<<"B = "<<objD.b<<endl;
   cout<<"C = "<<objD.c<<endl;
   cout<<"D = "<<objD.d<<endl;
   
    
    return 0;
}
//Output:
A from classB = 100
A from classc = 200
B = 10
C = 20
D = 30

_______________________________________________________________-
  
  Example 2. Using Virtual Kwyeord
  #include <iostream>

using namespace std;

class classA {
    public:
    int a;
};

class classB : virtual public classA {
    public:
    int b;
};
class classC : virtual public classA {
    public:
    int c;
};
class classD : public classB, public classC {
    public:
    int d;
    
};

int main() {
   
   classD objD;
   
   objD.a = 100; // is correct using virtual keyword. we can access a value from one copy.
   cout<<"Using Virtual keyword: "<<endl;
   classB objB;
   objB.a =600;
   cout<<"A from class B = "<<objB.a<<endl;
   cout<<"A from class D = "<<objD.a<<endl;
   objD.b = 10;
   objD.c = 20;
   objD.d = 30;
   cout<<"B = "<<objD.b<<endl;
   cout<<"C = "<<objD.c<<endl;
   cout<<"D = "<<objD.d<<endl;
   
    
    return 0;
}
//Output:
Using Virtual keyword: 
A from class B = 600
A from class D = 100
B = 10
C = 20
D = 30