Chapter 14: more about classes Static Members

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Chapter 14 more about classesStatic Members

• If a member variable is declared static, all
  objects of that class have access to a single
  instance of that variable.
• If a member function is declared static, it may
  be called before any instances of the class are
  defined and it may only use static variables.

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Member Variable is Staticint Ystatic int X
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budget.h Program

• ifndef BUDGET_H
• define BUDGET_H
• class Budget
• private
• static float corpBudget
• float divBudget
• public
• Budget ( void ) divBudget 0
• void addBudget ( float b )
• divBudget b corpBudget divBudget
  
• float getDivBudget ( void ) return divBudget
  
• float getCorpBudget ( void ) return
  corpBudget
• // Budget
• endif
• budget.cpp odd way to initialize
• float BudgetcorpBudget 250000

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Static Member Functions

• The syntax for a static member function is
• static ltreturn typegtltfunction namegt(ltparameter
  listgt)
• A classs static member variables come into
  existence before any instances of the class are
  created.
• The static member functions use only static
  variables and are callable before any instances
  of the class are created..

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A New budget.h Program

• ifndef BUDGET_H
• define BUDGET_H
• class Budget
• private
• static float corpBudget
• float divBudget
• public
• static void mainOffice ( float )
• // Budget
• endif

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budget.cpp Program

• include "budget.h"
• float BudgetcorpBudget 0
• void BudgetmainOffice ( float moffice )
• corpBudget moffice
• // BudgetmainOffice

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Using budget.h Program

• include ltiostream.hgt
• include ltiomanip.hgt
• include "budget.h"
• void main ( void )
• float amount
• cout ltlt "Enter the main office's budget request
  "
• cin gtgt amount
• BudgetmainOffice(amount)

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Friends of Classes

• A friend is a function that is not a member of a
  class, but has access to the private members of
  the class.
• The syntax of a friend function is as follows and
  is included in the .h file for the class
• friend ltreturn typegtltfunction namegt(ltparameter
  type listgt)

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auxil.h Program

• ifndef AUXIL_H
• define AUXIL_H
• class Budget // Forward declaration of Budget
  class
• class Aux
• private
• float auxBudget
• public
• Aux ( void ) auxBudget 0
• void addBudget ( float, Budget )
• float getDivBudget ( void ) return auxBudget
  
• // Aux
• endif

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budget.h Program

• ifndef BUDGET_H
• define BUDGET_H
• include "auxil.h"
• class Budget
• private
• static float corpBudget
• float divBudget
• public
• Budget ( void ) divBudget 0
• void addBudget ( float b )
• divBudget b corpBudget divBudget
• float getDivBudget ( void ) return
  divBudget
• float getCorpBudget ( void ) return
  corpBudget
• static void mainOffice ( float )
• friend void AuxaddBudget ( float, Budget
  )
• // Budget
• endif

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Friend Classes

• As mentioned before, it is possible to make an
  entire class a friend of another class.
• The Budget class could make the Aux class its
  friend with the following declaration friend
  class Aux
• Note generally a friend is bad as it violates
  the encapsulation principle.

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Memberwise Assignment

• The operator may be used to assign one object
  to another, or to initialize one object with
  another objects data.
• By default, each member of one object is copied
  to its counterpart in the other object.

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Copy Constructors

• A copy constructor is a special constructor,
  called whenever a new object is created and
  initialized with another objects data.
• Assume the class Set has a member variable as
  follows bool member which is dynamically
  allocated.
• Consider the following declarations
• Set x(5,15) // 15 possible members, 5 is a
  member
• Set y x // Copy constructor is called
• but is only a shallow copy, so when set y
  changes, set x does to

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The Default Copy Constructor

• If a class doesnt have a copy constructor, C
  automatically creates a default copy constructor.
  
• The default copy constructor performs a
  memberwise assignment.

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The this Pointer

• this is a special built-in pointer that is
  available in any member function.
• this contains the address of the object that
  called the member function. It is the name for
  ME.

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Operator Overloading

• C allows you to redefine how standard operators
  work when used with class objects.

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Issues of Operator Overloading

• You can change an operators entire meaning when
  you overload it. (But dont.)
• You cannot change the number of operands taken by
  an operator. For example, the symbol must
  always be a binary operator. Likewise, and --
  must always be unary operators.

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Operators That can be Overloaded
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• Because our unionSet has same arguments and
  return value as , we can simply make a name
  change
• Set Setoperator(Set arg2)
• Set res int min
• if(memberMaxgtarg2.memberMax)
• minarg2.memberMax
• res new Set(this)
• else
• res new Set(arg2)
• min memberMax
• for (int i0 i lt min i)
• res-gtmemberi arg2.memberi memberi
• return res

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Overloading the Prefix Operator

• Set Setoperator(void)
• bool member1 new boolmemberMax1
• for (int i0 i lt memberMax i)
• member1i memberi
• delete member
• member member1
• membermemberMaxfalse
• memberMax
• return this

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Overloading the Postfix Operator

• Set Setoperator(int x)// parameter is never
  used!
• bool member1 new boolmemberMax1
• for (int i0 i lt memberMax i)
• member1i memberi
• delete member
• member member1
• membermemberMaxfalse
• memberMax
• return this

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Overloading Relational Operators

• bool Setoperatorlt(Set arg2)// subset for us
• int min memberMax
• if (arg2.memberMax lt min)
• min arg2.memberMax
• for(int i0 i ltmini)
• if (memberi !arg2.memberi) return
  false
• if (memberMax gtmin)
• for (imin i lt memberMaxi)
• if (memberi) return false
• return true

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Overloading the Operator

• bool Setoperator(Set arg2)
• if (memberMax!arg2.memberMax) return false
• for (int i0 i lt memberMaxi)
• if (memberi!arg2.memberi) return false
• return true

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Overloading the ltlt Operator

• Note this cannot be a member function of Set as
  the first argument is not a Set
• ostream operatorltlt ( ostream strm, Set obj )
• strm ltlt Whatever I want
• strmltlt but be careful about member
  variables
• strm ltlt Since Im not a member, I cant
  access them
• // operatorltlt

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Overloading the gtgt Operator

• istream operatorgtgt ( istream strm, Set obj )
• int ele
• strm gtgt ele
• Set.addToSet(x) //or make friend or use
  input() member
• // operatorgtgt

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Overloading the Operator

• In addition to the traditional operators, C
  allows you to change the way the (subscript)
  symbols work.

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intarry.h Program

• ifndef INTARRY_H
• define INTARRY_H
• Class IntArray
• private
• int aptr
• int arraySize
• void memError ( void )
• void subError ( void )
• public
• IntArray ( int )
• IntArray ( const IntArray )
• IntArray ( void )
• int size ( void ) return arraySize
• int operator ( const int )
• // IntArray
• endif

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intarray.cpp Program

• IntArrayIntArray ( int s )
• arraySize s
• aptr new int s
• if ( aptr NULL )
• memError()
• for ( int count 0 count lt arraySize count
  )
• ( aptr count ) 0
• // IntArrayIntArray
• IntArrayIntArray ( const IntArray obj )
• arraySize obj.arraySize
• aptr new int arraySize
• if ( aptr NULL )
• memError()
• for ( count 0 count lt arraySize count )
• ( aptr count ) ( obj.aptr count )
• // IntArrayIntArray

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intarray.cpp Program (cont)

• IntArrayIntArray ( void )
• if ( arraySize gt 0 )
• delete aptr
• // IntArrayIntArray
• void IntArraymemError ( void )
• cout ltlt "ERROR Cannot allocate memory.\n"
• exit( 1 )
• IntArraymemError

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intarray.cpp Program (cont)

• void IntArraysubError ( void )
• cout ltlt "ERROR Subscript out of range.\n"
• exit( 1 )
• // IntArraysubError
• int IntArrayoperator ( const int sub )
• if ( sub lt 0 sub gt arraySize )
• subError()
• return aptrsub
• // IntArrayoperator

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Object Conversion via overloading

• Special operator functions may be written to
  convert a class object to any other type.
• FeetInchesoperator float ( void )
• float temp feet
• temp (inches / 12.0)
• return temp
• // FeetInchesoperator float

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Note

• No return type is specified in the function
  header for the previous example.
• Because it is a FeetInches-to-float conversion
  function, it will always return a float.