Lecture 28 - Nov 16, 2023

Summary

In this lecture, we continue our discussion on inheritance, data protection within a derived classes and pointers to derived classes.

Last lecture

Inheritance.

Today

Inheritance continuation.

We inherit the attributes from Person.

We do not inherit constructors.

If we create a new setName(), it will overwrite the previous one.

In Person.h:

#include <iostream>

using namespace std;

class Person {
 private:
  string name;
  int age;

 public:
  Person() {
    name = "";
    age = 0;
  }

  Person(string n, int a) {
    name = n;
    age = a;
  }

  void setName(string n) { name = n; }

  void print() {
    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
  }
};

In Student.h:

#include <iostream>

using namespace std;

class Student : public Person {
 private:
  int ID;

  // age and name are innaccessible

  // we don't inherit constructors of Person

 public:
  Student() { ID = 0; }

  // setName is inherited
  
  void setNameID(string n, int d = 0) {
    Person::setName(n);
    ID = d;
  }

  // cancel/overwrite the print function inherited from Person
  // we write our own
  void print() {
    // I can't access age and name
    Person::print();
    cout << "ID: " << ID << endl;
  }

  ~Student() { cout << "Student destructor" << endl; }
};

In main.cpp:

#include "Person.h"
#include "Student.h"

using namespace std;

int main() {
  Person p("Joe", 23);
  Student s;  // 2 constructors are called
  // 1st is Person() default constructor
  // 2nd is Student() default constructor
  p.setName("Joseph");
  p.print();  // print name and age

  s.setNameID("Ryan");  // setNameID is inherited from Person
  s.print();  // we invoke print on Student object, hence we call print of
              // Student, not of person

  s.setNameID("Marina", 125);
  s.print();

  return 0;
}

Inheritance versus membership

Inheritance

Inheritance represents an “is-a” relationship.

Example: Student is a Person, as Student inherits from Person.

class Person {
  ...

};

class Student : public Person {
  ...

};

Student s;
s.SetName();

// s is Student
// s is Person

Membership

Membership represents a “has-a” relationship.

Example: Student has a member Person p.

class Student {
  private:
    Person p;
};

Student s;

// s is Student
// s is NOT person

Access different constructors

What if I want to call a different Person constructor when I create a Student object?

class Student : public Person {
  private:
    int ID;

  public:
    Student() { ID = 0; }
    // or
    // Student() : ID(0);

    // call the 2nd constructor in Person
    // between fun() and {, the space allows an initializer's list
    Student(string n, int a, int d) : Person(n,a) {
      ID = d;
    }
};

int main() {
  // 1. Person(n,a)
  // 2. Student(n,a,d)
  Student s("Armaan", 20, 354);
}

Data protection

protecteddata and function members are inherited and accessible to derived classes but not to all classes (somewhere between public and private).

class Person {
  protected:
    int age;
    string name;
};

class Student : public Person {
  private:
    int ID;
  public:
    Student(string n, int a, int d) {
      Person::name = n;
      Person::age = a;
      ID = d;
    }
};

Important. We do not inherit

1. Constructors: You can call them.
2. Copy constructors
3. Operator equal
4. Destructors

2-4: You can create derived’s own version of them.

int main() {
  Student s;
  Person p;

  p = s; // => p.operator=(s)
         // Person& operator=(Person& rhs) { ... }

  s = p; // => s.operator=(p) - LHS is student, RHS is person => ERROR
}

Very important. student is a Person and Student. person is a Person only.

Student* s1;
Person* p1;

p1 = &s;

p1 pointer can be used to access Person members which also exist in Student s.

p1->setName("Selim"); // calls setName of Person
p1->print(); // calls print of Person
// p1->setNameID(); // error, because setNameID is not a member of Person

s1 pointer cannot access all Student members as not all of them exist in Person (base class).

// s1 = &p; // error