1

Classes and Data Abstraction

Chapter 15

2

What a Class ! !

Specificationand

implementation

Privateand

public elements

Declaring classesdata andfunctions

What is aconstructor?

Classes wehave already

seen

3

Abstract Data Types

Defn => A type whose properties (domain and operations) are specified independent of any particular implementation

Domain => what are possible values for the type

Operations => what things can be done to/with the values in the domain

4

Data Type

set of values(domain)

allowable operationson those values

FOR EXAMPLE, data type int has

domain

-32768 . . . 32767

operations

+, -, *, /, %, >>, <<

5

Example of Abstract Data Type

Type:complex numbers

Domain:numbers of the form a + bi

Operations– add, subtract– multiply– divide

(4 + 2i) + (6 - 3i)(4 + 2i) + (6 - 3i)

(7 - 2i) * (5 + 4)(7 - 2i) * (5 + 4)

i

i

5

)23(

i

i

5

)23(

6

Categories of Operations

Constructor => create new instance (variable) of Abstract Data Type (ADT)

Transformer => builds a new value of the ADT, given one or more previous values of the type

complex c1, c2;// constructor initializes a + bi

complex c1, c2;// constructor initializes a + bi

complex c1, c2;c1.assign (3, -4);// stores new values in c1

complex c1, c2;c1.assign (3, -4);// stores new values in c1

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Categories of Operations

Observer => Allows a look at the state of an instance of an ADT without changing it.

Iterator => Enables processing (one at a time) of all the components of an instance of an ADT

complex c1, c2;cout << c1.real_part();

complex c1, c2;cout << c1.real_part();

complex c1, c2;c1.double();

complex c1, c2;c1.double();

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C++ Classes

Data and Operations bound into a single unit– Like a struct which includes functions

to manipulate the data

class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;

class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;

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Classes, Objects, Members

Class => a structured type in a programming language– used to represent an abstract data type

Class Member => Component of a class– functions– data

class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;

class Complex{ public : void assign (float a, float b); void print_complex (); float real_part (); . . . private: float a, b; } ;

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Classes, Objects, Members

Class Object <=> class instance– variable of a class type

Client => Software that declares and manipulates objects of a particular class:

complex c1, c2;cout << c1.real_part();

complex c1, c2;cout << c1.real_part();

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Built In Class Operations

Programmer defined classes can be like built in types

Declare as many objects of a class as you like

Pass as parameters to functions

Return as function values

void do_whatever (Complex z);void do_whatever (Complex z);

Complex new_value ( … );Complex new_value ( … );

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Built In Class Operations

Arrays of class objects

Can be automatic or staticAssign operator and dot . operator

both work

complex c1, c2;Complex c_list [20];

complex c1, c2;Complex c_list [20];

complex c1, c2;. . .c1 = c2;cout << c1.real_part();

complex c1, c2;. . .c1 = c2;cout << c1.real_part();

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Class Operations

Recall that built in types (int, float, etc.) have some operators + - * / == <= <<

Classes do NOT come with these available by default– if you need them, you must program

them

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Class Scope

Name of a class member is local to the class.

Same identifier declared outside the class will be unrelated;

complex c1, c2;int a, b, c;. . .c1 = c2;cout << c1.real_part();// but NOT // cout << real_part

complex c1, c2;int a, b, c;. . .c1 = c2;cout << c1.real_part();// but NOT // cout << real_part

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Information Hiding

Class object has an "invisible wall"– called the abstraction barrier

Protects private data and functions– client code cannot access private

elements– client can access only public members

Think of a class as a "black box"– it will act on the data– but you need not

know how it works

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Information Hiding

Class implementation details are hidden from the client’s view. This is called information hiding.

Public functions of a class provide the interface between the client code and the class objects.

clientcode

specification

abstraction barrierimplementation

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Information Hiding

EncapsulationHiding of implementation detailsKeeps client/user of the ADT from…

– depending on details– incorrectly manipulating the details

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Specification File

File which describes the behavior of the data type– does not reference the implementation

details

This is the .h file with the prototypes, the declarations

Both the client and implementation file will have#include <xxxx.h> ExampleExample

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Implementation File

Contains all the function definitions– includes function heading– includes body of function

Similar to function definitions below main ( )– except is in different file

Function headings in this file must match prototypes in .h file

ExampleExample

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2 Separate Files Generally Used for class Type

// SPECIFICATION FILE ( timetype .h ) // Specifies the data and function members. class TimeType { public: . . .

private: . . . } ;

// IMPLEMENTATION FILE ( timetype.cpp ) // Implements the TimeType member functions. . . .

21Familiar Class Instances and Function Members

The member selection operator ( . ) selects either data members or function members.

Header files iostream and fstream declare the istream, ostream,and ifstream, ofstream I/O classes.

Both cin and cout are class objects Both get and ignore are function members.

cin.get (someChar) ;cin.ignore (100, ‘\n’) ;cin.get (someChar) ;cin.ignore (100, ‘\n’) ;

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Familiar Class Instances and Function MembersDeclare myInfile as an instance of

class ifstream.

Invoke function member open.

ifstream myInfile ;myInfile.open ( “A:\\mydata.dat” ) ;

ifstream myInfile ;myInfile.open ( “A:\\mydata.dat” ) ;

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Class constructors

Guarantee initialization of class object

Constructor function has same name as the class

Can have different versions of constructor

Complex::Complex ( ) { a = 0; b = 0;Complex::Complex (int real_part, int Imag_part) { a = real_part; b = imag_part; }

Complex::Complex ( ) { a = 0; b = 0;Complex::Complex (int real_part, int Imag_part) { a = real_part; b = imag_part; }

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Invoking a Constructor

Invoked automatically whenever a class object is created (instantiated)

Example:

Which version gets used, depends on the number of parameters

Complex c1, c2;Complex c3 (5.3, -6);Complex c1, c2;Complex c3 (5.3, -6);

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Guidelines for Constructors

A constructor canNOT return a value– no return value type is declared

Multiple constructors are allowed– compiler chooses appropriate one– according to number & data types of

parameters

Parameters passed to a constructor – place actual parameter list after name of

class object being declared

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Guidelines for Constructors

If class object declared WITHOUT a parameter list – results depend on what constructors

are provided– even if NONE provided, there is a default

constructor which allocates memory for private data elements

When array of class objects declared, default constructor is invoked

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Testing and Debugging Hints

Don't forget semicolon ; at end of class type declaration

Function declaration in .h file (specification) end with semicolons

Function definitions in .cpp file (implementation) do NOT end with ;

In implementation, don't forget to prefix function name with – name of class– double colon ::

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Testing and Debugging Hints

Only built in operations (we know about) are– the dot . for member selection– the assignment =

Functions which inspect (but do not modify) should be const member functions