C++ multiple inheritance

Question

Please don't question the really odd hierarchy of workers in this code here, I have no idea why anyone would want something like this, but I decided to give myself an exercise in Multiple Inheritance, just to be sure I fully understood it. So here's the result.

using namespace std;

class Employee
{
protected:
    string name;
public:
    string getname()
    {
        return name;
    }
    void setname(string name2)
    {
        name = name2;
    }
    Employee(string name2)
    {
        name = name2;
    }
    Employee(){}
};

class Manager : public Employee
{
public:
    string getname()
    {
        return ("Manager" + name);
    }
    Manager(string name2) : Employee(name2){}
    Manager() : Employee(){}
};

class Supervisor : public Manager,public Employee
{
public:
    Supervisor(string name2) : Manager(name2) , Employee(name2){}
    Supervisor() : Manager() , Employee(){}
    string getname()
    {
        return ("Supervisor" + Employee::getname());
    }
};

Hopefully you're understanding what I'm trying to do here. I'm getting something about an "ambiguous conversion between derived class 'Supervisor' and base class 'Employee.'" So what do I do?

Answer 1

Actually, the way you have defined Supervisor class, its object will have two subjects of type Employee , each coming from it base classes. That is causing problem.

The solution is to use virtual inheritance (assuming you need multiple inheritance) as:

class Manager : public virtual Employee 

Hope you note the virtual keyword here. :-)

Answer 2

You need a virtual inheritance in this case:

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

class Employee
{
protected:
    string name;
public:
    string getname()
    {
        return name;
    }
    void setname(string name2)
    {
        name = name2;
    }
    Employee(string name2)
    {
        name = name2;
    }
    Employee(){}
};

class Manager : public virtual Employee
{
public:
    string getname()
    {
        return ("Manager" + name);
    }
    Manager(string name2) : Employee(name2){}
    Manager() : Employee(){}
};

class Supervisor : public Manager,public virtual Employee
{
public:
    Supervisor(string name2) : Manager(name2) , Employee(name2){}
    Supervisor() : Manager() , Employee(){}
    string getname()
    {
        return ("Supervisor" + Employee::getname());
    }
};

This problem is also known as Diamond inheritance problem: http://en.wikipedia.org/wiki/Diamond_problem

Answer 3

Everybody has already covered virtual inheritance, but I'd suggest reading the C++ FAQ as well.

http://www.parashift.com/c++-faq-lite/multiple-inheritance.html

Answer 4

Supervisor contains two objects of type Employee, the direct one and the one over Manager. It is ambiguous to call Employee methods on a Supervisor in consequence ( which Employee should be called?). You might want to employ virtual multiple inheritance instead.