• The first principle of Object Oriented programming is called encapsulation
  • An object is defined in terms of data and function members
  • In fact, data abstraction principle says that an object should be defined only in terms of operations. Data members are irrelevant. That's the topic for COSC2006.
• An object in a C++ program is a variable
• Any variable is an object.
• In COSC 1046 you used objects of simple types such as int, double, char
• We also looked at an object called array which was collection of objects
• An Object has to have a type
• We dealt with simple types before, but in many cases these simple predefined types are not sufficient
• Let us consider representation of time in your program
• Time is just a singular object which happens to be collection of three different objects hour, minute and second
• We need a mechanism to define an object of the type time
• Since built-in types do not provide such a type, we need to creat our own types: user-defined types
• Our user defined type is going to be a collection of three different members/fields

hour                                                                       

minute                                                                     

second                                                                     

• The objects are not static, they can have certain functionality. For example, a variable/object of the type Time can print relevant information about itself in military format of standard format
• That's why printMilitary() and printStandard() are its member functions/operations
• You can send a message to the object of the type time to print itself in the military format

Time t;

t.printMilitary();

• Figure 6.3 shows that a variable of the type Time has two other members
• One is called setTime(int, int, int)
• t.setTime(13, 27, 6);
• The above call asks the variable t to set its data members: hour will be set to 13, minute will be set to 27 and second will be set to 6
• Quiz:
  • Why do we need user-defined types?
  • Most of the real world objects are too complex to be represented by basic types such as int, char, double, etc. It may be possible to represent them as a collection of objects of previously defined types. Example is the type Time which is defined as a collection of three integers. First one is called hour, second is called minute and the third is called second.
  • Why should user defined types have data as well as function members?
  • The objects in real world are not static, they are capable of taking actions. Member functions allow us to send a message to the object. The object will take an action based on the message. For example, t.printMilitary() asks the object t to print information about itself in the military format.