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?

"Hopefully you're understanding what I'm trying to do here." You might be better off explaining what you are trying to do. — thiton
– thiton, Commented May 4, 2012 at 19:21
@Justin : The more pertinent question, since Manager inherits from Employee, is why does Supervisor inherit from Employee instead of just Manager? — ildjarn
– ildjarn, Commented May 4, 2012 at 19:23
@Justin : It makes it so that Supervisor is-a Manager, which is important for any realistic dynamic polymorphism usage. — ildjarn
– ildjarn, Commented May 4, 2012 at 19:25

Sarfaraz Nawaz · Accepted Answer · 2012-05-04 19:23:28Z

6

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. :-)

answered May 4, 2012 at 19:23

Sarfaraz Nawaz

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1 Comment

Grimm The Opiner Over a year ago

You should of course only use virtual inheritance if you can't refactor the diamond out. And here the inheritance of 'Employee' by Supervisor is utterly superfluous.

Stephen Newell · Accepted Answer · 2012-05-04 20:06:08Z

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

answered May 4, 2012 at 20:06

Stephen Newell

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Comments

AlexTheo · Accepted Answer · 2012-05-06 08:56:16Z

3

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

edited May 6, 2012 at 8:56

answered May 4, 2012 at 19:26

AlexTheo

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1 Comment

Whovian Over a year ago

Thanks. Took me a little while, cos I didn't realize supervisor also had to inherit virtually from Employee at first!

thiton · Accepted Answer · 2012-05-04 19:23:05Z

1

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.

answered May 4, 2012 at 19:23

thiton

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