C++ Object Oriented Programming

Data and behavior together

So far a program has been variables in main and free functions that take those values. That works until related pieces scatter: a name in one variable, a score in another, a function that expects both in the right order.

Object oriented programming (OOP) groups those pieces. An object holds data and the functions that use that data. A class is the blueprint that names the type. You write the class once. Then you create as many objects of that type as you need.

Class vs object

The class is the plan. The object is a real value in memory. int is a type; int n = 3;makes a variable. Lamp is a type you define; Lamp desk; makes an object.

A tiny class

Lamp has one field, on, and one method, toggle. The method belongs to the class. You call it on an object with the dot operator. This program creates one lamp, flips it, and prints the new state.

#include <iostream>
using namespace std;

class Lamp {
 public:
  bool on;

  void toggle() {
    on = !on;
  }
};

int main() {
  Lamp desk;
  desk.on = false;
  desk.toggle();
  if (desk.on) {
    cout << "on" << endl;
  } else {
    cout << "off" << endl;
  }
  return 0;
}

The same idea without a class

You could keep a bool desk_on and a free function. The class version keeps the flag and the flip next to each other. When you add a second lamp, you do not invent a second pair of names. You create anotherLamp.

#include <iostream>
using namespace std;

int main() {
  bool desk_on = false;
  desk_on = !desk_on;
  cout << desk_on << endl;
  return 0;
}

That works for one flag. It does not scale when a lamp also needs a brightness, a room name, and three operations. The class is the type that holds those together.

What the next chapters cover

• Classes and objects — define a type, create two objects, use public members.
• Methods — functions that live on the class and run with the dot operator.
• Constructors — code that runs when an object is created, so fields start valid.
• Access specifiers — public, private, and protected.

Later chapters add encapsulation, inheritance, and polymorphism. Those are still the same idea: a class names a type, objects are values of that type.

Keep classes small

For this tutorial a class has a few fields and a few methods. Do not pack a whole program into one type. Name the class after the thing it models: Lamp, Player, Point. Next: how to write a class and create objects from it.