Kate Gregory with material from Deitel and Deitel

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Week 4

• Questions from Last Week
• Hand in Lab 2
• Classes

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Data Abstraction

• An extension of the concept of a structure
• Gathers together a number of related variables
• e.g. Bank Account
• Owner
• Balance
• transactions

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Defining your own Types

• Built-in types have allowed operations
• Cant multiply strings
• Cant use on a float
• Cant pass an integer to strlen()
• In the same vein, you choose the operations for
  your own types
• Bank account can have deposit and withdraw

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An Object is a Clump

• Information, Variables, Data, State, Attributes
• Behaviour, Methods, Functions, Operations,
  Services
• All belong together

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Defining a Class

• //BankAccount.h
• class BankAccount
• float Balance
• void Deposit(float amount)
• bool Withdraw (float amount)
• Dont forget that final semi colon!

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Inside and Outside
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Defining a Class

• //BankAccount.h
• class BankAccount
• private
• float Balance
• public
• void Deposit(float amount)
• bool Withdraw (float amount)

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Creating Objects

• An object is an instance of a class
• Class is BankAccount object is my account,
  another object is someone elses
• include BankAccount.h
• // ...
• BankAccount KateChequing
• BankAccount BillSaving

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Private means no access

• All code can access public functions
• KateChequing.Deposit(50)
• But not private variables
• KateChequing.Balance 100

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Implementing a Class

• //BankAccount.cpp
• include BankAccount.h
• void BankAccountDeposit(float amount)
• Balance amount
• is called Scope Resolution Operator

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Encapsulation

• Holding all the rules together
• Not forcing users of a class to know what the
  inside means
• Retaining the freedom to change the inside of a
  class
• Variable name
• Type (int, float)
• Units or other meaning
• Information hiding design information

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Gets and Sets

• class Truck
• private
• float currentweight
• public
• float getcurrentweight()
• void setcurrentweight(float w)

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Constructors

• Sometimes your only motivation for a set is to
  give something an initial value
• A constructor can be used to initialize the
  member variables of an object as it is created
• Name is always the name of the class
• Never a return type
• Parameters vary

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Default Constructor

• No parameters
• Runs when instances are created without
  parameters
• class BankAccount
• // ...
• BankAccount()

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Default Constructor

• //BankAccount.cpp
• include BankAccount.h
• BankAccountBankAccount()
• Balance 0

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Overloaded Functions

• In C, two functions cannot have the same name
• In C, they can, as long as something else
  differs
• Class
• Parameters
• Return type or placeholder names dont matter

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Overloaded Constructors

• class BankAccount
• // ...
• BankAccount()
• BankAccount(float openingbalance)
• BankAccount(float a) // not allowed
• BankAccount(char name)

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Passing Parameters to Constructors

• BankAccount KateChequing(50.0)
• Uses the constructor that takes a float
• BankAccount BillSaving(Bill)
• Uses the constructor that takes a char
• BankAccount KateChequing(1000)
• Uses the constructor that takes a float, and
  converts the int to float first

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Dont Do This

• BankAccount KateChequing()
• This is declaring a function called KateChequing!
• BankAccount GetAccount(int x)

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Initializer Syntax

• //BankAccount.cpp
• include BankAccount.h
• BankAccountBankAccount()
• Balance(0)
• Slightly more efficient and readable
• Becomes important once inheritance enters the
  picture
• Also useful when a member variable is actually an
  object

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Destructors

• Opposite of constructors
• Name is plus name of class
• Joke based on boolean syntax
• Never take arguments
• No return type
• Rarely explicitly called

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Leaving Scope

• // ... Program fragment
• int i
• BankAccount Kate(1000)
• Destructor runs as Kate leaves scope

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Coding Destructors

• class BankAccount
• // ...
• BankAccount()
• //BankAccount.cpp
• include BankAccount.h
• BankAccountBankAccount()
• cout ltlt There goes your money

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Reuse Thoughts

• Objects should be self contained
• Constructors should set good default values
• If there is no default value for something, every
  constructor should demand it, so that it cannot
  remain uninitialized
• You should add all the methods others are likely
  to use
• Dont add gets and sets without thinking
• The object should handle its own error checking
• Avoid side-effects such as error messages

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Inline functions

• Dramatically improve performance
• Compiler actually expands the code like a macro
• You retain the protection of encapsulation at no
  performance cost

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Inline Gets and Sets

• class Truck
• private
• float currentweight
• public
• float getcurrentweight()
• return currentweight
• void setcurrentweight(float w)
• currentweight w

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Error checking

• class Truck
• private
• float currentweight
• public
• float getcurrentweight()
• return currentweight
• void setcurrentweight(float w)
• if (w gt 0) currentweight w

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Private Functions

• Serve some useful purpose
• You dont want the whole system to be able to
  call them
• You arent willing to commit they will always
  exist
• Can only be called from inside the object itself

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For Next class

• Read chapter 7
• Get ready for Lab 3 to be handed out