C ++派生类

Question

Having some trouble figuring out how to go about an assignment I have. The assignment is dealing with package objects We were told to have a base class that called CPackages , and two classes derived from it called COverNightPackage and CTwoDayPackage . That was all fine, I could do that.

The problem I am having is that the over night package MAY or MAY NOT have a courier. If a courier is required, details such as company name, address etc. needs to be recorded. If a package needs a courier, when show details of all packages is called it will have to say if one is needed for that package. There will also be another menu option to display all the details of the couriers used.

What is the best way to add couriers? Make a courier class? I've been stuck on this for a couple days now, haven't done anything like it before.

Answer 1

Yes you do need a courier class. Now to tell your class if courier is required or not you have two ways. You can create a constructor in your class which takes Courier class object as null.Some thing like

#include <iostream>
using namespace std;

class {
    public:
    Courier();
};
Courier::Courier(){

};

class COverNightPackage {
    public:
    COverNightPackage(Courier* obj);
};

COverNightPackage::COverNightPackage(Courier* obj){
        if(obj == NULL){
            cout<<"Courier is null"<<"\n";
        }else{
            cout<<"Courier is not null"<"\n"; \\ Store package info here
        }

};
int main() {
    Courier* o2 = new Courier();
    COverNightPackage* o1 = new COverNightPackage(NULL);
    COverNightPackage* o3 = new COverNightPackage(o2);
    return 0;
}

Now you can store its value as per your convenience. While showing the value you can check if courier class is null or not.

Second way could be to keep a Courier class object inside your class and provide a setter to help the user to set value at runtime. Hope it will help you :-)

Answer 2

Indeed, you will need a Courier class, and you should store a pointer to this Courier class within your COverNightPackage class. You just want to be sure that this pointer is set to NULL when there is no Courier . To do this, you should create two constructors for your COverNightPackage . Use initializers for each constructor to be sure that the pointer to the Courier class is either initialized to NULL or pointing to a Courier class.

class Courier;

class COverNightPackage: public CPackages {
public:
  COverNightPackage()
   : m_courier(NULL)
  { }

  COverNightPackage(Courier* courier)
   : m_courier(courier)
  { }

private:
  Courier* m_courier;
}

Alternatively, you can accomplish this using a single constructor with a default argument of NULL

class Courier;

class COverNightPackage: public CPackages {
public:
  COverNightPackage(Courier* courier = NULL)
   : m_courier(courier) { }

private:
  Courier* m_courier;
}

If the constructor is called with no argument, the Courier pointer will be initialized to NULL