Classes 2

1
Classes (2)

• CSC 2110 - Data Structures Abstraction
• Spring/Summer 2001
• Wayne State University
• Instructor Anne-Marie Bosneag

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Content

• Records vs. classes
• Inheritance
• Composition
• Static dynamic binding
• Virtual functions
• Class invariants
• Exceptions
• Namespace

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Comparison of struct and class

• Members in structs are public by default, which
  provides more flexible and convenient access
• Members in classes are private by default, which
  requires public member functions to access.
  Better information hiding and security

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Example struct

• struct StudentType
• char name50
• int age
• char gender //M or F
• StudentType student1
• strcpy(student1.name, John R. Smith)
• student1.age30
• student1.genderM
• cout ltlt student1.name
• cout ltlt student1.age
• cout ltlt student1.gender

• void out_put(StudentType student)
• cout ltlt Name is ltlt student.name ltltendl
• cout ltlt Gender is ltlt student.gender ltlt
  endl
• cout ltlt Age is ltlt student.age ltltendl
• //the name may cause ambiguity
• //if there is another struct that
• //needs the output function
• void output(PersonType person)
• output(student1)

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Example class

• StudentType student1
• strcpy(student1.name, John R. Smith) //not
  allowed
• void StudentType set_name(char str50)
• strcpy(name, str)
• student1.set_name(John R. Smith)
• cout ltlt student1.age//not allowed
• int StudentTypeget_age()
• return age
• cout ltlt student1.get_age()

• class StudentType
• public
• void set_name(char str50)
• void set_age(int num)
• void set_gender(char g)
• char get_name()
• int get_age()
• int get_gender()
• void out_put()
• private
• char name50
• int age
• char gender

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Example class

• void StudentTypeoutput()
• cout ltlt Name is ltlt student.name ltltendl
• //the above statement //constains an error
• cout ltlt Gender is ltlt gender ltlt endl
• cout ltlt Age is ltlt
• age ltltendl
• student1.output()

• Notes
• in classes, all data and related operations are
  encapsulated together
• in structs, data and functions are two separate
  part
• therefore classes are easier to reuse
• classes are more suitable in developing large
  programs

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Two Programing Paradigms
Structural (Procedural) Object-Oriented
PROGRAM PROGRAM
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Object-Oriented Programming Language Features

• 1. Data abstraction
• 2. Inheritance of properties
• 3. Dynamic binding of operations to objects

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Object oriented terms
OOP Terms C Equivalents
Object Class object or class instance Inst
ance variable Private data member Method Publi
c member function Message passing Function call
( to a public member function )
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What is an object?
OBJECT
set of methods (public member functions) interna
l state (values of private data members)
Operations Data
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Inheritance hierarchy among vehicles
Every car is a wheeled vehicle.
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Inheritance

• is a mechanism by which one class acquires
  (inherits) the properties (both data and
  operations) of another class.
• The class being inherited from is the Base Class
  (Parent).
• The class that inherits is the Derived Class
  (Child).
• The derived class is then specialized by adding
  properties specific to it.

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Example

• // SPECIFICATION FILE ( time.h )
• class TimeType
• public
• void Set ( int hours , int minutes ,
  int seconds )
• void Increment ( )
• void Write ( ) const
• Time ( int initHrs, int initMins, int
  initSecs )
• Time ( )
• private
• int hrs
• int mins
• int secs

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Class interface diagram
Time class
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Using inheritance to add features

• // SPECIFICATION FILE ( exttime.h)
• include time.h
• enum ZoneType EST, CST, MST, PST, EDT, CDT,
  MDT, PDT
• class ExtTime public Time // Time is
  the base class
• public
• void Set ( int hours, int minutes,
  int seconds ,
• 
  ZoneType timeZone )
• void Write ( ) const
• ExtTime ( int initHrs , int initMins ,
  int initSecs ,
• ZoneType initZone ) //
  constructor
• ExtTime ( ) // default constructor
• private
• ZoneType zone // added data member

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class ExtTime public Time

• says class Time is a public base class of the
  derived class ExtTime.
• As a result, all public members of Time (except
  constructors) are also public members of ExtTime.
• In this example, new constructors are provided,
  new data member zone is added, and member
  functions Set and Write are overridden.

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Class Interface Diagram
ExtTime class
Private data hrs mins secs
Privatedata zone
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Client Code Using ExtTime

• include exttime.h
• .
• .
• ExtTime thisTime ( 8, 35, 0, PST )
• ExtTime thatTime // default
  constructor called
• thatTime.Write( ) // outputs
  000000 EST
• cout ltlt endl
• thatTime.Set (16, 49, 23, CDT)
• thatTime.Write( ) // outputs
  164923 CDT
• cout ltlt endl
• thisTime.Increment ( )
• thisTime.Increment ( )
• thisTime.Write ( ) // outputs
  083502 PST
• cout ltlt endl

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Constructor Rules for Derived Classes

• At run time, the base class constructor is
  implicitly called first, before the body of the
  derived classs constructor executes.
• If the base class constructor requires
  parameters, they must be passed by the derived
  classs constructor.

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Implementation of ExtTime Default Constructor

• ExtTime ExtTime ( )
• // Default Constructor
• // Postcondition
• // hrs 0 mins 0 secs
  0
• // (via implicit call to base class
  default constructor)
• // zone EST
• zone EST

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Implementation of Another ExtTime Class
Constructor

• ExtTime ExtTime ( / in / int
  initHrs,
• / in / int
  initMins,
• / in / int
  initSecs,
• / in /
  ZoneType initZone )
• Time (initHrs, initMins, initSecs)
  // constructor initializer
• // Precondition 0 lt initHrs lt 23 0
  lt initMins lt 59
• // 0 lt initSecs lt 59 initZone is
  assigned
• // Postcondition
• // zone initZone Time set by base class
  constructor
• zone initZone

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Implementation of ExtTime Set function

• void ExtTime Set ( / in / int
  hours,
• / in / int
  minutes,
• / in / int
  seconds,
• / in /
  ZoneType time Zone )
• // Precondition 0 lt hours lt 23 0 lt
  minutes lt 59
• // 0 lt seconds lt 59 timeZone is
  assigned
• // Postcondition
• // zone timeZone Time set by base class
  function
• Time Set (hours, minutes,
  seconds)
• zone timeZone

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Implementation ExtTime Write function

• void ExtTime Write ( ) const
• // Postcondition
• // Time has been output in form HHMMSS ZZZ
• // where ZZZ is the time zone
  abbreviation
• static char zoneString84
• EST, CST, MST, PST, EDT, CDT, MDT,
  PDT
• Time Write ( )
• cout ltlt ltlt zoneString zone

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Composition (containment)

• is a mechanism by which the internal data (the
  state) of one class includes an object of another
  class.

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Example

• A TimeCard object has a Time object
• include time.h
• class TimeCard
• public
• void Punch ( / in / int hours,
  / in / int minutes,
• / in /
  int seconds )
• void Print ( ) const
• TimeCard ( / in / long idNum, /
  in / int initHrs,
• / in / int initMins,
  / in / int initSecs )
• TimeCard ( )
• private
• long id
• Time timeStamp

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Class Interface Diagram
TimeCard class
Private data id timeStamp
Print . .
Private data hrs mins secs
Write . . .
TimeCard
TimeCard
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Implementation of TimeCard Class Constructor

• TimeCard TimeCard ( / in / long idNum,
• / in / int initHrs,
• / in / int initMins,
• / in /
  int initSecs )
• timeStamp (initHrs, initMins, initSecs)
  // constructor initializer
• // Precondition 0 lt initHrs lt 23 0
  lt initMins lt 59
• // 0 lt initSecs lt 59 initNum is
  assigned
• // Postcondition
• // id idNum timeStamp set by its
  constructor
• id idNum

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Order in which constructors are executed

• Given a class X,
• If X is a derived class its base class
  constructor is executed first.
• Next, constructors for member objects (if any)
  are executed (using their own default
  constructors if none is specified).
• Finally, the body of Xs constructor is executed.
  

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In C . . .

• When the type of a formal parameter is a
• parent class, the actual parameter can be
• the same type as the formal parameter,
• or,
• any descendant class type.

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Static Binding

• is the compile-time determination of which
  function to call for a particular object based on
  the type of the formal parameter.
• When pass-by-value is used, static binding occurs.

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Static Binding Is Based on Formal Parameter Type

• void Print ( / in / Time someTime )
• cout ltlt Time is
• someTime.Write ( )
• cout ltlt endl
• CLIENT CODE OUTPUT
• Time startTime ( 8, 30, 0 ) Time is
  083000
• ExtTime endTime (10, 45, 0, CST) Time is
  104500
• Print ( startTime )
• Print ( endTime )

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Dynamic Binding

• is the run-time determination of which function
  to call for a particular object of a descendant
  class based on the type of the actual parameter.
• declaring a member function to be virtual
  instructs the compiler to generate code that
  guarantees dynamic binding.

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Virtual Member Function

• // SPECIFICATION FILE ( time.h )
• class TimeType
• public
• . . .
• virtual void Write ( ) const
  // for dynamic binding
• . . .
• private
• int hrs
• int mins
• int secs

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Dynamic binding requires pass-by-reference

• void Print ( / in / Time someTime )
• cout ltlt Time is
• someTime.Write ( )
• cout ltlt endl
• CLIENT CODE OUTPUT
• Time startTime ( 8, 30, 0 ) Time
  is 083000
• ExtTime endTime (10, 45, 0, CST) Time
  is 104500CST
• Print ( startTime )
• Print ( endTime )

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Using virtual functions in C

• Dynamic binding requires pass-by-reference when
  passing a class object to a function.
• In the declaration for a virtual function, the
  word virtual appears only in the base class.
• If a base class declares a virtual function, it
  must implement that function, even if the body is
  empty.
• A derived class is not required to re-implement a
  virtual function. If it does not, the base class
  version is used.

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Class invariants

• the relationship that should always hold for the
  class (denoted CLASSINV by convention)
• ex. for the container class, numunits gt 0
• all member functions (except the constructor)
  assume the invariant is true at invocation
• it is the constructors responsibility to
  initialize the invariant to be true
• all member functions (including the constructor)
  must ensure the invariant is still true at return

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Exceptions

• An exception is an unusual situation that occurs
  when the program is running.
• Exception Management
• Define the error condition
• Enclose code containing possible error (try).
• Alert the system if error occurs (throw).
• Handle error if it is thrown (catch).

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try, catch, and throw

• Try
• // code that contains a possible error
• throw string(An error has occurred in
  function )
• Catch (string message)
• stdcout ltlt message ltlt stdendl
• return 1

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Namespace

• namespace mySpace
• // All variables and functions within this
• // block must be accessed using scope
• // resolution operator ().
• Purpose Avoid namespace pollution.

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3 Ways to Access Members within a Namespace

• Qualify each reference
• mySpacename with every reference.
• Using declaration
• using mySpacename
• All future references to name refer to
  mySpacename.
• Using directive
• using namespace mySpace
• All members of mySpace can be referenced without
  qualification.

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Rules for Use of Namespace std (within text)

• Qualify names in prototypes and/or function
  definitions.
• If name used more than once in a function block,
  use a using declaration.
• If more than one name is used from a namespace,
  use a using directive.