Derived class inside parent class array, possible?

xemous · 10-16-2006, 11:38 PM

Can you derive a class, say fulltime_employee from an employee class and add the fulltime_employee object to an employee theEmployees = new employee[10]; array and call the new fulltime_employee methods?

nadroj · 10-17-2006, 12:06 AM

are you referring to polymorphism?

ie, is the function that you want called in fulltime_employee overriding a base function in employee? or is it a function that isnt in employee?

if the former, then this is called polymorphism. if its the later then id assume youd have to cast the variable to a fulltime_employee object then you can use the methods.

demon_vox · 10-17-2006, 09:37 AM

Hi,
I havent used C++ in a while, but you can do that... its just that you really dont want to do that

As nadroj said, if you want to invoke methods from fulltime_employee (which arent part of the employee interface), then you'll have to cast your object. First, if you have an array, you dont know which type of object you have, so you cant safely cast them all, because it could break in runtime. Second, I guess you can ask for the class of an object (I dont know how to do it in C++ but I guess it can be done) but then your code is kinda ugly. The rule of the thumb says that if you have to cast some objects of your array, then there is something that doesnt fit quite right. Perhaps you are think more like a structured program

If you have to cast them all, just define your array as a fulltime_employee and be happy.

The bottom line is that while casting is allowed an sometimes necesary, it is also something you want to avoid as much as you can since it skips types-checking and it will break your program on runtime which is harder to test and debug.

Hope this is useful.
Cheers!

xhi · 10-17-2006, 11:35 AM

Quote:

Originally Posted by xemous

Can you derive a class, say fulltime_employee from an employee class and add the fulltime_employee object to an employee theEmployees = new employee[10]; array and call the new fulltime_employee methods?

to do this without casting the Employee class would need to be abstract. which is the solution that i would probably go with.. here is an example that is completely untested..

Code:

class Employee
{
    public:
       virtual void work()=0;
};
class Cook : public Employee
{
    public:
       virtual void work() { cout << "Im cooking" << endl; }
};
class Janitor : public Employee
{
    public:
       virtual void work() { cout << "Im cleaning" << endl; }
};
class EmployeeList
{
    void add(Employee* emp)
    {
        emps.push_back(emp);
    }
    void doWork()
    { 
        for(int i=0; i<emps.size(); ++i)
            emps[i]->work();
    }
    
    vector<Employee*> emps;
};
int main()
{
   EmployeeList el;
   el.add(new Cook());
   el.add(new Janitor());
   el.doWork();
   return 0;
}