(Homework Help on C++ OOPS) Dynamic Binding and Overriding

Question

Following is the question from my assignment :

Create a class telephone_directory (name, occupation, phone, address) and an virtual function search (), that searches the object contents in a linear fashion. Extend the class in a specific class called doctor (qualification, clinic, visiting time) and override the virtual function such that it searches using binary search. Also create another class lawyer (qualification, civil/criminal, cases attended, contact_no, office_address) that extends the telephone_directory such that it wont override the virtual function. Search for a doctor and a lawyer by taking the option from the user.

Following is my attempt so far :

/*18. Create a class telephone_directory (name, occupation, phone, address) and an
virtual function search (), that searches the object contents in a linear fashion. Extend
the class in a specific class called doctor (qualification, clinic, visiting time) and
override the virtual function such that it searches using binary search. Also create
another class lawyer (qualification, civil/criminal, cases attended, contact_no,
office_address) that extends the telephone_directory such that it wont override the
virtual function. Search for a doctor and a lawyer by taking the option from the user.
(m)*/

#include<iostream>
#include<string>
#include<algorithm>
#include<stdlib.h>
#include<conio.h>
using namespace std;
static int size;

class telephone
{
public:
    string name, address;
    int phone;
    telephone();
    virtual void search(telephone *a);
    void display();
};

class doctor : public telephone
{
public:
    string qualification, visit, clinic, occupation;
    doctor();
    void search(doctor *a);
    void display();
};

class lawyer : public telephone
{
public:
    string qualification, type, cases, office, occupation;
    void display();
    void search(lawyer *a);
    lawyer();
};

bool sortByName(const telephone &lhs, const telephone &rhs) { return lhs.name < rhs.name; }

void telephone :: display()
{
    cout << "\nName : " << name << "\nAddress : " << address << "\nPhone : " << phone << endl;
}

telephone :: telephone()
{
    string n[13] = {"Bill","Roy","Drake","Robbie","Elvis","John","Sara","Kate","Lauren","Julia","Nigella","Gordon","Ellen"};
    string add[5] = {"Delhi", "Chennai", "Kolkata", "Mumbai", "Vellore"};
    name = n[rand() % 13];
    address = add[rand() % 5];
    phone = 1000 + rand () % 5000;
}

void telephone :: search(telephone *a)
{
    string n, add;
    int tel;
    int i = 0, j, com, flag;
            cout << "\nEnter name\n";
            cin >> n;
            flag = 0;
            for(j = 0; j < size; j++)
            {
                com = n.compare(a[j].name);
                if(com == 0)
                {
                    flag = 1;
                    a[j].display();
                }
            }
            if(flag == 0)
            {
                cout << "\nNo matches were found\n";
            }

}

doctor :: doctor()
{
    occupation = "Doctor";
    string q[4] = {"Intern", "Resident", "Attending", "Fellow"};
    string v[4] = {"4 to 5", "5 to 6", "6 to 7", "7 to 8"};
    string c[4] = {"General", "Free", "Charity", "Specialist"};
    visit = v[rand() % 4];
    clinic = c[rand() % 4];
    qualification = q[rand() % 4];
}

void doctor :: display()
{
    cout << "\nName : " << name << "\nOccupation : " << occupation << "\nAddress : " << address << "\nPhone : " << phone << "\nQualification : " << qualification << "\nClinic : " << clinic << "\nVisiting Hours : " << visit << endl;
}

void doctor :: search(doctor *a)
{
    string n;
    int i = 0, j, com, flag, first, last, middle;
    cout << "\nEnter name\n";
    cin >> n;
    flag = 0;
    first = 0;
    last = size - 1;
    middle = (first + last) / 2;
    sort(a, a + size, sortByName);

    while ( first <= last )
    {
        middle = (first + last) / 2;
        if(a[middle].name < n)
            first = middle + 1;
        else if ( a[middle].name > n)
            last = middle - 1;
        else
        {
            flag = 1;
            a[middle].display();
            break;
        }
    }
    if(flag == 0)
    {
        cout << "\nNo matches were found\n";
    }
}

lawyer :: lawyer()
{
    string q[5] = {"Associate", "Senior Associate", "Junior Partner", "Senior Partner", "Manager"};
    string t[2] = {"Civil", "Criminal"};
    string c[5] = {"40", "70", "90", "Fresh", "No experience"};
    string o[3] = {"Vellore", "Chennai", "Delhi"};
    qualification = q[rand() % 5];
    type = t[rand() % 2];
    cases = c[rand() % 5];
    office = o[rand() % 3];
    occupation = "Lawyer";
}

void lawyer :: display()
{
    cout << "\nName : " << name << "\nOccupation : " << occupation << "\nAddress : " << address << "\nPhone : " << phone << "\nQualification : " << qualification << "\nType : " << type << "\nCases Attended : " << cases << "\nOffice Address : " << office << endl;
}

void lawyer :: search(lawyer *a)
{
    string n;
    int i = 0, j, com, flag, first, last, middle;
    cout << "\nEnter name\n";
    cin >> n;
    flag = 0;
    first = 0;
    last = size - 1;
    middle = (first + last) / 2;
    sort(a, a + size, sortByName);

    while ( first <= last )
    {
        middle = (first + last) / 2;
        if(a[middle].name < n)
            first = middle + 1;
        else if ( a[middle].name > n)
            last = middle - 1;
        else
        {
            flag = 1;
            a[middle].display();
            break;
        }
    }
    if(flag == 0)
    {
        cout << "\nNo matches were found\n";
    }
}

main()
{
    int i = 0, k;
    doctor *doc;
    lawyer *law;
    cout << "\nEnter the size\n";
    cin >> size;
    doc = new doctor [size];
    law = new lawyer [size];
    do
    {
        cout << "\n1.) List the doctors\n2.) List the lawyers\n3.) Search for a doctor by name\n4.) Search for a lawyer by name\n5.) Exit\n";
        cin >> i;
        switch(i)
        {
            case 1:
                cout << "\nDOCTORS LIST\n\n";
                for(i = 0; i < size; i++)
                    doc[i].display();
                break;
            case 2:
                cout << "\nLAWYERS LIST\n\n";
                for(i = 0; i < size; i++)
                    law[i].display();
                break;
            case 3:
                doc[0].search(doc);
                break;
            case 4:
                law[0].search(law);
                break;
            case 5:
                break;
        }
    } while(i != 5);
}

Here I am not able to do the part "such that it won't override the virtual function". Please tell me how will I be able to apply this in my program.

Answer 1

Your base class, the telephone class, does not have a virtual search function. Your declaration is

void search(telephone *a);

However, it should be

virtual void search(telephone *a);

This tells the compiler that when you derive another class from this one, you want to override the telephone's search function with the new one, which is re-implemented (with binary search, in your case) under the derived class.

So...

1) add virtual before the declaration in your telephone class

2) create the standard search function implementation

3) create another implementation (binary search) for the search() class in the doctor class

Done!

Answer 2

There are couple of issues:

1. You have to make the search method virtual in Telephone class

            virtual void search(telephone* a);
   then implement it to search in linear fashion.

2. Override this method in doctor

            virtual void search(telephone* a) override; // note the parameter type, it is telephone*
   then implement it to search in binary.

3. Don't override the method in lawyer class, so that it uses the default implementation of telephone class.

The power of overriding/dynamic binding comes from the fact that, at runtime, the correct search() method will be called based on the object type. If it is telephone* or lawyer*, the default implementation will be called (as laywer does not override it). If it is doctor*, the implementation in doctor class will be called.