A study of C++ object-oriented features

Save the following C++ program into a file classes.cpp

#include <iostream>
using namespace std;

class A {
  int x;
  A() { x = 0;}
  void increment() { x++; }
  virtual void decrement() { x--; }
  void print() {cout << "x = " << x << endl;}

class B : public A {
  void increment() { x = x + 2; }
  virtual void decrement() { x = x - 2; }

int main() {
  A a;       // a is an object of class A
  A * ptr;   // ptr is a pointer to an object of class A
  ptr = &a;

  ptr -> increment();
  ptr -> print();
  ptr -> decrement();
  ptr -> print();

Compile the program by typing g++ classes.cpp, run it by typing ./a.out. Make sure that the program compiles and runs.

Question 1. In main add an object b of class B. Set the pointer ptr to point to b. Call increment() and decrement() on the object pointed to by ptr, print the object after each call. Which method gets called in each case? Why?

Question 2. Add a protected variable y to B and a constructor to initialize it to 0. Add a print() method in B so that both x and y are printed. Which print method is called on the object B referenced by ptr? If needed, change method declarations so that the print method of B is called for the B object. What did you need to change and why?

Question 3. Assuming that a is a variable of type A, b is an object of type B, and ptr is the pointer of type A, what would you expect to be printed by the following statement:

  a = b;
  ptr = &a;
  ptr -> print();

What gets printed? Why? Please explain the difference in behavior between this example and Question 2.

Question 4. What happens if you have an object b of class B and you call the print method like this: b.A::print()?

Question 5. Write a function that takes an object of class A and prints it (don't forget to add the function prototype at the top of the file). Can you pass an object of a class B to this function?

Question 6. Write a function that takes an object of class A and returns it. Can you pass an object of a class B to this function? If yes, can you assign the result to a variable of a class A? Of a class B?

Question 7. What happens if you change the declaration

class B : public A
class B : A

CSci 4651 course.

The views and opinions expressed in this page are strictly those of the page author. The contents of this page have not been reviewed or approved by the University of Minnesota.