Monday, Jan 27, 2003Kate Gregory with material from Deitel and Deitel Week 4 Questions from Last Week Hand in Lab 2 Classes

Monday, Jan 27, 2003 Kate Gregorywith material from Deitel and Deitel

Week 4

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


Date Week Topic ChapterHand Out Due Back Test

6-Jan-03 1Administrivia / Overview / Intro to C++ / Control Structures 1

13-Jan-03 2 Functions / Arrays, Pointers, Strings 2,3,4,5Lab 1 / Lab 2

20-Jan-03 3 Classes, Data Abstraction 6 Lab 1 5%27-Jan-03 4 More on Classes 7 Lab 3 Lab 2 5%3-Feb-03 5 No Lecture

10-Feb-03 6 Operator Overloading 8 Lab 4 Lab 3 5%17-Feb-03 Reading Break24-Feb-03 7 Inheritance 9 Lab 4 5% Midterm 25%3-Mar-03 8 Virtual Functions and Polymorphism 10 Lab 5

10-Mar-03 9 Stream IO 11 Lab 6 Lab 5 5%17-Mar-03 10 Templates 12,13 Lab 7 Lab 6 5%24-Mar-03 11 Exceptions31-Mar-03 12 File IO 14 Lab 7 5%

??? Exam Final 40%


Data Abstraction

• An extension of the concept of a structure• Gathers together a number of related

variables• e.g. Bank Account:

– Owner– Balance– transactions


Defining your own Types

• Built-in types have allowed operations– Can’t multiply strings– Can’t use ++ on a float– Can’t pass an integer to strlen()

• In the same vein, you choose the operations for your own types– Bank account can have deposit and withdraw


An Object is a Clump

• Information, Variables, Data, State, Attributes

• Behaviour, Methods, Functions, Operations, Services

• All belong together


Defining a Class

//BankAccount.hclass BankAccount{ float Balance; void Deposit(float amount); bool Withdraw (float amount);};• Don’t forget that final semi colon!


Inside and Outside

PrivateA ttributes

andm ethods

Public A ttributesand M ethods

System

M yO bject


Defining a Class

//BankAccount.h class BankAccount{private: float Balance;public: void Deposit(float amount); bool Withdraw (float amount);};


Creating Objects

• An object is an instance of a class• Class is BankAccount: object is my

account, another object is someone else’s#include BankAccount.h// ...BankAccount KateChequing;BankAccount BillSaving;


Private means no access

• All code can access public functions

KateChequing.Deposit(50);

• But not private variables

KateChequing.Balance = 100;


Implementing a Class

//BankAccount.cpp #include “BankAccount.h”void BankAccount::Deposit(float amount)

{ Balance += amount;}

• :: is called Scope Resolution Operator


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


Gets and Sets

class Truck{private: float currentweight;public: float getcurrentweight(); void setcurrentweight(float w);};


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


Default Constructor

• No parameters• Runs when instances are created without

parametersclass BankAccount{// ... BankAccount(); };


Default Constructor

//BankAccount.cpp #include BankAccount.hBankAccount::BankAccount(){ Balance = 0;}


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 don’t matter


Overloaded Constructors

class BankAccount{// ... BankAccount(); BankAccount(float openingbalance); BankAccount(float a); // not allowed BankAccount(char* name); };


Passing Parameters to Constructors

BankAccount KateChequing(50.0);• Uses the constructor that takes a floatBankAccount 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


Don’t Do This

BankAccount KateChequing();• This is declaring a function called

KateChequing!

BankAccount GetAccount(int x);


Initializer Syntax

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


Destructors

• Opposite of constructors• Name is ~ plus name of class

– Joke based on boolean syntax• Never take arguments• No return type• Rarely explicitly called


Leaving Scope

// ... Program fragment{ int i; BankAccount Kate(1000);}

• Destructor runs as Kate leaves scope


Coding Destructors

class BankAccount{// ... ~BankAccount(); };

//BankAccount.cpp #include BankAccount.hBankAccount::~BankAccount(){ cout << “There goes your money”;}


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

• Don’t add gets and sets without thinking• The object should handle its own error checking

– Avoid side-effects such as error messages


Inline functions

• Dramatically improve performance• Compiler actually expands the code like a

macro• You retain the protection of encapsulation

at no performance cost


Inline Gets and Sets

class Truck{private: float currentweight;public: float getcurrentweight() { return currentweight; } void setcurrentweight(float w) { currentweight = w;}};


Error checking

class Truck{private: float currentweight;public: float getcurrentweight() { return currentweight; } void setcurrentweight(float w) { if (w > 0) currentweight = w;}};


Private Functions

• Serve some useful purpose• You don’t want the whole system to be able

to call them• You aren’t willing to commit they will

always exist• Can only be called from inside the object

itself


For Next class

• Read chapter 7

• Get ready for Lab 3 to be handed out

Documents

Monday, Jan 27, 2003Kate Gregory with material from Deitel and Deitel Week 4 Questions from Last Week Hand in Lab 2 Classes