Working with Classes

1
Working with Classes
2
A method dealing with two objects

• C has a pointer which call this .
• It gives the address of an object.
• Suppose we have two objects of BankAccount class
  and we want to know which one has more balance.

3

• Solution
• Have a method that returns a reference to the
  object with larger balance.

4
Header file for BankAccount class

• ifndef BANK1_H
• define BANK1_H
• class BankAccount
• private
• double balance string name
• public
• BankAccount()
• BankAccount(double initial_balance)
• void deposit (double amount)
• void withdraw (double amount )
• double getBalance()
• const BankAccount topval( const BankAccount
  s) const
• BankAccount()
• endif

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CPP file

• includeltiostream.hgt
• include bank1.h
• BankAccount BankAccount() balance 0
• BankAccount BankAccount(double initialBalance,
  string str) balance initialBalance
• namestr

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• void BankAccount deposit(double amount
•     balance balanceamount
• void BankAccountwithdraw(double amount)
• if ( amount gt balance )
• amount0
• else
• balancebalance-amount

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• double BankAccount getBalance()
• return balance
• BankAccount BankAccount()
• coutltltgood bye

8

• const BankAccount BankAccount topval( const
  BankAccount s) const
• if (s.balance gt balance)
• return s
• else
• return this
• / const in parenthesis says that the function
  wont modify the explicitly accessed object /

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• / the const after parenthesis says that the
  function wont modify the implicitly accessed
  objects/
• The function returns a reference to one of the
  two const objects has to be constant.
• topaccount1.topval(account2)

10
Operator Overloading

• Like function overloading we can have operator
  overloading.
• We have already used operator overloading
• we use for multiplication and for pointer.
• C allows user to overload operators.

11

• Suppose we want to overload to add two
  arrays.
• for(int i0 ilt20i)
• Array3iArray1iArray2i
• // add element by element
• We can define a class that overload so we can
  have
• Array3Array1Array2

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• To overload an operator we use a special function
  call.
• operatorop(arguments-list)
• here op is a operator known for C.
• For example
• operator() // overload
• operator_at_()
• // error _at_ is not a C operator

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• ifndef MYTIME0_H_
• define MYTIME0_H_
• class Time
• private
• int hours int minutes
• public
• Time()
• Time(int h, int m)
• void AddMin(int m)
• void AddHr(int h)
• void Reset(int h , int m)
• Time Sum (const Time t) const
• void Show()
• endif

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• includeltiostreamgt
• include mytime0.h
• Time Time()
• hoursminutes0
• TimeTime(int h, int m)
• hoursh minutesm

15

• void TimeAddMin(int m)
• minutesm
• hoursminutes/60
• minutes 60
• void TimeAddHr(int h)
• hoursh

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• void TimeReset(int h, int m)
• hoursh
• minutesm
• Time TimeSum(const Time t) const
• Time sum
• sum.minutesminutest.minutes
• sum.hourshourst.hourssum.minutes/60
• sum.minutes 60
• return sum

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• void Time Show()
• coutltlthoursltlthours , ltltminutesltlt minutes

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• includeltiostreamgt
• include mytime0.h
• int main()
• Time coding(2,40)
• Time fixing(5,55)
• Time total
• totalcoding.Sum(fixing)
• total.Show()
• return 0

19

• ifndef MYTIME0_H_
• define MYTIME0_H_
• class Time
• private
• int hours int minutes
• public
• Time()
• Time(int h, int m)
• void AddMin(int m)
• void AddHr(int h)
• void Reset(int h , int m)
• Time operator(const Time t) const
• Time operator (double mult) const
• void Show()
• endif

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• includeltiostreamgt
• include mytime0.h
• Time Time()
• hoursminutes0
• TimeTime(int h, int m)
• hoursh minutesm

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• void TimeAddMin(int m)
• minutesm
• hoursminutes/60
• minutes 60
• void TimeAddHr(int h)
• hoursh

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• void TimeRese(int h, int m)
• hoursh
• minutesm
• Time Timeoperator(const Time t) const
• Time sum
• sum.minutesminutest.minutes
• sum.hourshourst.hourssum.minutes/60
• sum.minutes 60
• return sum

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• Time Timeoperator(double mult) const
• Time result
• long totalminhoursmult60minutesmult
• result.hourstotalmin/60
• result.minutes totalmin 60
• return result
• void Time Show()
• coutltlthoursltlthours , ltltminutesltlt minutes

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• includeltiostreamgt
• include mytime0.h
• int main()
• Time coding(2,40)
• Time fixing(5,55)
• Time total
• totalcodingfixing
• total.Show()
• Time morefixing(3,28)
• totalmorefixing.operator(total)
• total.Show()
• totalmorefixing 2.5
• total.Show()
• return 0

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• Time t1,t2,t3,t4
• t4t1t2t3
• t4t1.operator(t2t3)
• t4t1. operator(t2.operator(t3))

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Overloading Restriction

• Should preserve the syntax for the original
  operator.
• Time test,
• test /// invalid
• We cant create new operator symbols like
• operator ()
• We cant overload the following operators
• sizeof . ?

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Operators can be overloaded

• - /
• gt - !
• ltlt --
• () new -gt delete

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• Assignment
• () Function call
• Subscripting
• -gt class member access by pointer
• Can be overloaded only by member function.

29
Friend Function

• When a function f is a friend of a class,
• f has the same access to class as a function
  member has.
• We could write
• Time A,B
• AB2.5 // AB.operator(2.5)
• But not
• A2.5B

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Creating a Friend function

• friend Time operator( double m, const Time t)
• Place it in the class declaration.
• Time operator(double m, const Time t)
• Time result
• long totalmint.hoursm60t.minutesm
• result.hourstotalmin/60
• result.minutes totalmin 60
• return result

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• A2.5B
• Is
• Aoperator(2.5,B)
• Time operator(double m, const Time t)
• return tm

32
Overload ltlt

• We want to have
• Time t
• coutltltt instead of t.Show()
• both cout and t are objects.
• If we want to overload ltlt with a member function
  then we should write tltltcout //odd

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• Add
• friend void operatorltlt(ostream os ,const Time
  t)
• Then the body
• void operatorltlt(ostream os ,const Time t)
• osltltt.hoursltlt hours, ltltt.minutesltltminutes

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• Can we write
• coutltlt time ltlt tltlt Tuesday \n ??
• No
• Note that coutltltx return an object cout of
  class ostream
• int x,y
• coutltltxltlty
• we have
• (coutltltx) ltlty

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• ostream operatorltlt(ostream os ,const Time t)
• osltltt.hoursltlthours,ltltt.minutesltltminutes
• return os

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Type Casting for Classes

• When we write
• long count8
• double time11
• we have conversion automatically.

37

• ifndef STONEWT_H_
• define STONEWT_H_
• class Stonewt
• private
• enumlbs_per_stn14
• int stone
• double pds_left
• double pounds
• public
• Stonewt(double lbs)
• Stonewt(int stn, doub lbs)
• Stonewt()
• Stonewt()
• void show_lbs()
• void show_stn()
• endif

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• includeltiostreamgt
• include stonewt.h
• StonwtStonewt(double lbs)
• stoneint(lbs) / lbs_per_stn
• pds_leftint(lbs)lbs_per_stnlbs-int(lbs)
• poundslbs
• StonewtStonewt(int stn, double lbs)
• stonestn
• pds_leftlbs
• poundsstnlbs_per_stnlbs

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• StonewtStonewt()
• stone0
• poundpds_left0.0
• void Stonewtshow_stn()
• coutltltstoneltlt stone , ltltpds_leftltlt pounds\n
• void Stonewtshow_lbs()
• coutltltpoundsltlt pounds\n

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• now we can write
• Stonewt mycat
• mycat19.6
• this conversion automatically happen and is
• implicit conversion.
• // it can lead to unexpected conversion
• In order to make a conversion explicit we can
  write
• explicit Stonewt(double lbs)

41

• Stonewt mycat
• mycat19.6 // is not valid
• mycatStonewt(19.6)
• Can we convert an object to double ?
• Yes, but not using constructors. We can use
• conversion function.

42

• ifndef STONEWT_H_
• define STONEWT_H_
• class Stonewt
• private
• enumlbs_per_stn14
• int stone
• double pds_left
• double pounds
• public
• Stonewt(double lbs)
• Stonewt(int stn, double lbs)
• Stonewt() Stonewt()
• void show_lbs()
• void show_stn()
• operator int() const
• operator double() const

43

• Stonewtoperator int() const
• return int (pounds0.5)
• Stonewtoperator double() const
• return pounds

44

• We can write
• Stonewt poppins(9,2.8)
• double p_wt poppins //implicit
• coutltltp_wtltltendl
• coutltltint( poppins)