Latest revision as of 06:09, 17 January 2011

Classes and Abstraction

Knowledge goals

• To understand the difference between specification and implementation of an abstract data type.
• To understand the concepts of encapsulation and abstraction.
• To understand how control and data abstraction facilitate modifiability and reuse.
• To understand the basic class design principles.
• To understand how encapsulation and information hiding are enforced by the C++ compiler.

To be able to

• Declare a C++ class type.
• Declare class objects, given the declaration of a class type.
• Write client code that invokes class member functions.
• Implement class member functions.
• Organize the code for a C++ class into two files: the specification (.h) file and the implementation file.
• Write a C++ class constructor.

ADT - Abstract Data Type

An Abstract Data Type is a constructed datatype with its functions. Often gathered in a Class. With Object Oriented Programming it is important to be consequent about the abstraction layers. That will make it easy to understand even very huge and complicated programming projects.

Why ADT

The main purpose of ADT is to hide the implementation details, and allow a user of the implementation to abstract from the inner details of the implementation. The black-box theory. Changes in the implementation will not affect the use of the ADT.

ADT Specification

The Type specification below of TimeOfDay hides the implementation details and lists the operations that can be performed on the TimeOfDay objects. (Objects are the TimeOfDay instances)

TYPE

TimeOfDay

DOMAIN

Each TimeOfDay value is a time of day in the form of hours, minutes, and seconds.

OPERATIONS

Create a time object.

Print (or write) the time.

Return an object containing the time, incremented by one second.

Compare two times for equality.

Determine if one time is before another time.

Remove timeobject

Buzzwords of Object Oriented Programming

Constructor

An operation that initializes a new instance (variable) of an ADT.

Transformer

An operation that changes the value of the ADT; also known as a mutator.

Observer

An operation that allows us to observe the state of an instance of an ADT without changing it; also known as an accessor.

Destructor

An operation that cleans up the state of an ADT instance just prior to releasing its storage for reuse.

Iterator

An operation that allows us to process—one at a time—all the components in an instance of an ADT.

Class

Class

A Data Type that is used to represent a ADT - Abstract Data Type.

Object

An instantiated example of a class

Syntax

Example

• TimeOfDay Example

class TimeOfDay
{
public:
  TimeOfDay() ;
  // Constructor: Creates a zero TimeOfDay object
  TimeOfDay(int hours, int minutes, int seconds) ;
  // Constructor: Creates a TimeOfDay object with the given time
  Void Increment() const;
  // Transformer: Increment TimeOfDay by one second
  TimeOfDay getTime() const;
  // Observer: returns the TimeOfDay object to cout
  bool Equal(TimeOfDay otherTime) const;
  // Observer: Returns true if this TimeOfDay object equals otherTime
  bool LessThan(TimeOfDay otherTime) const;
  // Observer: Returns true if thi s TimeOfDay object is earlier than otherTime
private:
  int hours;
  int minutes;
  int seconds;
} ;