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Warm-Up ProblemWarm-Up Problem

Just like with primitive data types (int, double, etc.), we can create arrays of objects.

ex: bankAccount employees[100];Problem: It’s payday. Put $500 in

each employee’s account.for(int i = 0; i < 99; i++){ employees[i].deposit(500);}

(i < 99 should be i <= 99 or i < 100, but I'm leaving that be as a place for an alertness point in the review of this problem in future lectures. )

Classes:Classes: Member Functions and Member Functions and

Starting ImplementationStarting Implementation

Edited for CMPSC 122Penn State University

Prepared by Doug Hogan

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OverviewOverview

Review and Follow UpPreview of ImplementationClassification of Class

FunctionsClass interface and Functions

Review of the formConstructorsModifiersAccessors

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Review of StringsReview of Strings Last time, we finished with a problem:

string returnedExpression(string inputString) // PRE: inputString is a line of valid C++ code

// (<=80 chars) containing the "return"// keyword and ending with a semicolon

// POST: FCTVAL == the expression that follows // the return keyword, not including the// semicolon

Recall the string member functions: stringObject.length() stringObject.find(string) stringObject.substr(startLocation, length)

How do we use these functions to construct a solution?

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SolutionSolution string returnedExpression(string inputString)

// PRE: inputString is a line of valid C++ code (<=80 chars) containing // the "return" keyword and ending with a semicolon

// POST: FCTVAL == the expression that follows the return keyword, // not including the semicolon

First, find where "return" is located int returnLoc = inputString.find("return");

Next, find how long the string is int exprLength = inputString.length();

Find how long the expression is: exprLength -= 1; // don’t want semicolon exprLength -= returnLoc; // or stuff before "return" exprLength -= 7; // or "return "

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SolutionSolution string returnedExpression(string inputString)

// PRE: inputString is a line of valid C++ code (<=80 chars) containing // the "return" keyword and ending with a semicolon

// POST: FCTVAL == the expression that follows the return keyword, // not including the semicolon

Finally, construct and return the expression as a substring:return inputString.substr(returnLoc+7, exprLength);

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Object Oriented TermsObject Oriented Terms

Class Object Member Encapsulation Information Hiding Message Abstraction

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Review: Library Book Review: Library Book ProblemProblem

Your ideas?member data?member functions?

We’ll work with this set of member data:string author;string title;int year; string isbn;

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A Preview of A Preview of ImplementationImplementation

Implementation of class member functions is similar to implementing nonmember functionsFunction headers vs. function prototypes

The main difference: Member functions need to know what

class they belong to (their scope)Scope resolution operator (::)

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Scope ResolutionScope Resolution

The class name and the scope resolution operator (::) go directly before the function namee.g. void bankAccount::withdraw(int amount)

NOT bankAccount::void withdraw(int amount)

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Implementation ExampleImplementation Example

Implementation of the bankAccount constructor:

bankAccount::bankAccount()// POST: default bankAccount object constructed with // name == “?” and balance == 0{ name = "?"; balance = 0;}

private member data

scope resolution

name

balance

default_acct

0

“?”

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More Implementation More Implementation ExamplesExamples

void bankAccount::deposit(int amount)// PRE: amount in dollars and amount > $0// POST: amount has been added on to balance{ balance = balance + amount;}

Problem: Implement withdraw. void bankAccount::withdraw(int amount)

// PRE: amount in dollars and amount > $0// POST: amount has been subtracted from balance{ balance = balance - amount;}

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Class InterfaceClass Interface Defines the WHAT, not the HOW

General form: class className{ public: // member function declarations

private: // member data declarations};

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Three Types of FunctionsThree Types of Functions

Class member functions are classified into three categories:constructors

create objects (allocate memory, set initial state)

modifierschange the state of objects

accessorsmake information about the state of the

object available outside the class

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ExerciseExercise

Classify the functions of bankAccountconstructors

bankAccount();modifiers

void withdraw(int amount);void deposit(int amount);

accessorsdouble getBalance();

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Continuing Bank Account…Continuing Bank Account…

Work with a partner to think of one more constructor, modifier, and accessor that would be good additions to the bankAccount class.

Your ideas??

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Onward…Onward…

Now we'll look at each of the three kinds of member functions in more detail.

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ConstructorsConstructors

Goal:construct objects of the classallocate memory

Four important observations…namereturn typeoverloadingcalling

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1. Constructors: Names1. Constructors: Names

Constructors MUST have the same name as the class itself.

That's how you'll make instances of the class (objects).

Example: bankAccount class

constructor: bankAccount(); rectangle class

constructor: rectangle();

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2. Constructors – Return 2. Constructors – Return TypeType

They don't have a return type. It's simply omitted.

Ex: bankAccount(); NOT voidNOT double, int, etc.

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3. Constructors - 3. Constructors - OverloadingOverloading

Constructors can be overloadedCan have several constructors

same namedifferent lists of parameters

This ability allows you to create a default constructor

no parametersinitializer constructors

parameters specifying initial state of an object

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3. Constructors - 3. Constructors - OverloadingOverloading

Example default constructor: bankAccount();

// POST: A default bankAccount object is // created with name set to a blank and // and balance set to $0.00

Example initializer constructors: bankAccount(string initName, double initBalance); // PRE: initName has been assigned a value // && initBalance >= 0.00 and initBalance is in // dollars // POST: A bankAccount object is created with // name set to initName// and balance set to initBalance

bankAccount(string initName); bankAccount(double initBalance);

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4. Constructors – Calling 4. Constructors – Calling (Client)(Client)

Not called directly, i.e. no dot notation

Default constructor call:bankAccount myAcct; no parentheses

Initializer constructor call:bankAccount myAcct("Homer", 100);

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ProblemProblem

Given the libraryBook class… string author;string title;int year; string isbn;

Define a default constructor.Define two initializer constructors.

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ProblemProblem

Define a default constructor.libraryBook();

Define two initializer constructors. libraryBook(string initTitle, string initAuthor, string initISBN, int initYear);

libraryBook(string initTitle);

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Another exercise…Another exercise… Given the constructors we defined

libraryBook(); libraryBook(string initTitle, string initAuthor, string initISBN, int initYear);

libraryBook(string initTitle);

Construct a default libraryBook object. libraryBook book1;

Construct a libraryBook object with the initial title Algorithms Unlocked. libraryBook book2("Algorithms Unlocked");

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ModifiersModifiers

The functions that do most of the work.

Note: Objects have three characteristics: namestateset of operations

Modifiers define the set of operations.

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ModifiersModifiers Allow the client to make changes to the

private variables. Declarations look like the ones for

nonmember functions. Often, but not always, they have a void

return type.

“Set” functions or setters Modifiers that just "set" the value of a private

variable from a parameter without doing any calculations

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Modifiers - ExamplesModifiers - Examples void withdraw(double amount); // PRE: amount >= 0.00 and amount // is in dollars // POST: amount is deducted from // balance

A set function: void resetAccount(string newName, double newBalance); // PRE: newName has been assigned a value // && newBalance >= 0.00 and newBalance is // in dollars // POST: This account object is reset with // name set to newName and balance// set to newBalance

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AccessorsAccessors Allow the client to see what values the

private variables have. Don't allow the client to make changes. Return type is that of the variable being

"accessed."

"Get" functions or getters Accessors that just "get" the value of a private

variable without doing any calculations

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AccessorsAccessors

Should be declared const They don't change the state of any

variables.  Forces the issue of not making changes

Example:double getBalance() const; // POST: FCTVAL == current // balance of this account // in dollars

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More complicated accessorsMore complicated accessors

Some calculation based on the data as long as it doesn’t change the member data e.g. balance after interest w/o actually

crediting it A data member converted to different

units e.g. Fahrenheit version of Celsius temp.

Part of a data member e.g. the cents part of a dollar amount

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ExerciseExercise

Declare an accessor for the libraryBook type.string getTitle() const;

Write the function header for the accessor. string libraryBook::getTitle() const

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Pre- and PostconditionsPre- and Postconditions

A few new things to notePreconditions

What must be true for the method to behave as intended

Anything about the state of the object?Should another method have been called

first?May need to look at private data members

individually

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Pre- and PostconditionsPre- and Postconditions

PostconditionsWhat is the state of the object after this

method has been called? What is returned or displayed?

What private data members have changed? How?

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SummarySummary Implementation

Scope resolution operator (::) and class name directly before function name

Remove semicolons Interface & functions

Constructors – create instances, allocate memory same name as class no return type can have multiple with same name not called with dot notation

Modifiers – change state of private variables, define operations

Accessors – allows client to see state of private variables Pre- and postconditions

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Preview of What’s to Preview of What’s to Come…Come…

Implementation Tips for implementing each kind of function

Client end More on working with objects Test drivers

More Examples Advanced Issues

For next time: work on the blue worksheet and a short lab exercise