Multipath inheritance problem and solution

Multipath inheritance occurs when a derived class inherits from the same base class through more than one path. For example, Parent1 and Parent2 are both derived from Grandparent, and Child is derived from both Parent1 and Parent2. This diamond-shaped hierarchy is a form of hybrid inheritance.

        Person
        /    \
   Student  Employee
        \    /
        Intern

Problem

Intern gets two copies of Person's members: one through Student and one through Employee. Then:

• An access such as obj.name is ambiguous, and the compiler gives an error.
• Memory is wasted by the duplicate copies, and the data may become inconsistent.

Solution: virtual base class

Declare the common base as virtual in the intermediate classes. Then only one shared copy of Person is inherited.

#include <iostream>
#include <string>
using namespace std;

class Person {
public:
    string name;
};
class Student : virtual public Person { };
class Employee : virtual public Person { };
class Intern : public Student, public Employee { };

int main() {
    Intern i;
    i.name = "Hari";            // no ambiguity: one copy only
    cout << i.name << endl;
    return 0;
}

Output: Hari

Without virtual, i.name would fail to compile; it would need i.Student::name or i.Employee::name, referring to two different copies.