Question about multi-inheritance in C++?

Question

I have the following code:

#include "stdafx.h"
#include <iostream>
#include <conio.h>
using namespace std;
#define MNAME 30
class Person {
public:
    char name[MNAME + 1];
};
class Student : public Person {
};
class Staff : public Person {
};
class Faculty : public Student, public Staff {
};

int _tmain(int argc, _TCHAR* argv[])
{
    Faculty faculty;
    cout << "Address of faculty.Person::name: " << &faculty.Person::name << endl;
    cout << "Address of faculty.Student::name: " << &faculty.Student::name << endl;
    cout << "Address of faculty.Staff::name: " << &faculty.Staff::name << endl;

    getch();
    return 0;
}

When executed, the program gives the results:

Address of faculty.Person::name: 0012FF20 // **Line 1**
Address of faculty.Student::name: 0012FF20 // **Line 2**
Address of faculty.Staff::name: 0012FF3F // **Line 3**

I don't get it. Why the address in Line 1 and Line 2 is different from Line 3, while both Student and Staff inherits name from Person?

Answer 1

When you do multiple inheritance this way you get two copies of the grandparent class. This is the classic dreaded diamond problem, where you try to do this:

        Person
       /       \
     Student  Staff
       \       /
        Faculty

but through normal inheritance you actually get this:

    Person   Person
      |        |
     Student Staff
       \       /
        Faculty

So there's really 2 Person's in an instance of Faculty, meaning youll get 2 names.

To get the diamond in the first diagram above, you want to use virtual inheritance.

class Staff : public virtual Person {
};
class Student : public virtual Person {
};

Answer 2

With regular multiple inheritance you get multiple copies of shared base classes. If you want one copy, use virtual inheritance.

Explained well in Wikipedia

class Student : public virtual Person {
};
class Staff : public virtual Person {
};

Will get you what you expected