1 Using Structures and Classes COSC 1557 C++ Programming Lecture 4.

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Using Structures and ClassesUsing Structures and Classes

COSC 1557COSC 1557

C++ ProgrammingC++ Programming

Lecture 4

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ObjectivesObjectives

• Structures

• Define a class

• Declaration and implementation

• Public and private access modifiers

• Private functions

• Scope resolution operator with class fields and functions

• Static variables

• Pointer this

StructuresStructures

• 2nd aggregate data type: struct

• Recall: aggregate meaning "grouping"– Recall array: collection of values of same type– Structure: collection of values of different types

• Treated as a single item, like arrays

• Major difference: Must first "define" struct– Prior to declaring any variables

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Structure TypesStructure Types

• Define struct globally (typically)

• No memory is allocated– Just a "placeholder" for what our struct

will "look like"

• Definition:struct CDAccountV1 Name of new struct "type"{

double balance; member namesdouble interestRate;int term;

};

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Declare Structure VariableDeclare Structure Variable

• With structure type defined, now declarevariables of this new type:CDAccountV1 account;– Just like declaring simple types– Variable account now of type CDAccountV1– It contains "member values"

• Each of the struct "parts"

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Accessing Structure MembersAccessing Structure Members

• Dot Operator to access members– account.balance– account.interestRate– account.term

• Called "member variables"– The "parts" of the structure variable– Different structs can have same name

member variables• No conflicts

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Structure Example: Structure Example: Display 6.1 Display 6.1 A Structure Definition (1 of 3)A Structure Definition (1 of 3)

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Ex4-0-1.cppEx4-0-2.cpp

Structure PitfallStructure Pitfall

• Semicolon after structure definition

– ; MUST exist:struct WeatherData{

double temperature;double windVelocity;

}; REQUIRED semicolon!

– Required since you "can" declare structurevariables in this location

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Structure AssignmentsStructure Assignments

• Given structure named CropYield

• Declare two structure variables:CropYield apples, oranges;

– Both are variables of "struct type CropYield"

– Simple assignments are legal:apples = oranges;

• Simply copies each member variable from applesinto member variables from oranges

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Structures as Function ArgumentsStructures as Function Arguments

• Passed like any simple data type– Pass-by-value– Pass-by-reference– Or combination

• Can also be returned by function– Return-type is structure type– Return statement in function definition

sends structure variable back to caller

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Initializing StructuresInitializing Structures

• Can initialize at declaration– Example:

struct Date{

int month;int day;int year;

};Date dueDate = {12, 31, 2003};

– Declaration provides initial data to all three member variables

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Creating Classes with Declaration Creating Classes with Declaration and Implementation Sectionsand Implementation Sections

• Classes provide a convenient way to group related data

• The Student class is an abstract data type (ADT)• Student is a type you define, as opposed to types

like char and int that are defined by C++• One advantage of creating the Student class is that

when you create a Student object, you automatically create all the related Student fields

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Creating Classes with Declaration Creating Classes with Declaration and Implementation Sectionsand Implementation Sections

• Another advantage is the ability to pass a Student object into a function, or receive a Student object from a function as a returned value, and automatically pass or receive all the individual fields that each Student contains

• The first step to creating a class involves determining the attributes of an object, and subsequently dealing with the object as a whole

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Encapsulating Class Encapsulating Class ComponentsComponents

• Just as the internal components of a radio are hidden, when you write a program and create a class name for a group of associated variables, you hide, or encapsulate, the individual components

• Sometimes you want to change the state of some components, but often you want to think about the entry as a whole and not concern yourself with the details

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Designing ClassesDesigning Classes

• You can think of the built-in scalar types of C++ as classes

• You do not have to define those classes; the creators of C++ have already done so

• For most object-oriented classes, then, you declare both fields and functions

• You declare a function by writing its prototype, which serves as the interface to the function

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Designing ClassesDesigning Classes

• When you declare a class with a function definition, you then create an object of that class, the object possesses more than access to each field—it also possesses access to the function

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Implementing Implementing MemberMember Functions Functions

• After you create a member function’s prototype, you still must write the actual function

• When you construct a class, you create two parts

– The first part is a declaration section, which contains the class name, variables (attributes), and function prototype

– The second part create is an implementation section, which contains the functions themselves

Dot and Scope Resolution OperatorDot and Scope Resolution Operator

• Used to specify "of what thing" they aremembers

• Dot operator:– Specifies member of particular object

• Scope resolution operator:– Specifies what class the function

definition comes from

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Student Class That Includes Student Class That Includes One Function Definition One Function Definition

and Implementationand Implementation

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Implementing Class FunctionsImplementing Class Functions

Ex4-0-3.cpp

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Data Hiding and EncapsulationData Hiding and Encapsulation

• Object-oriented programmers strive to make their classes similar to real-world objects, which usually do not provide access to their inner workings; access is available only to the interface

• One technique programmers use to provide more complete object encapsulation is to make object’s data private

• One major asset of object-oriented programming is that the information hiding can be accomplished more completely than it is with the procedures used in procedural programs

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• Traditional procedural languages do not allow data to be declared as private; object-oriented languages do

• In C++, data hiding means that you can make data members of a class inaccessible to functions that are not part of that class

• Within a C++ class, you accomplish data encapsulation by making the data private instead of public

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• When you compile the program, you receive error messages indicating that the fields idNum, lastName, and gradePointAverage are inaccessible

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Using Public Functions to Using Public Functions to Alter Private DataAlter Private Data

• You gain a major advantage when you make a data field private

• Once you create a class, including writing and debugging all of its member functions, outside functions can never modify or erroneously use the private member data of any object in the class

• When you create and test a class, and store its definition in a file, programs that use the definition can be prevented from using member data incorrectly

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• If a private member of your Student class, such as idNum, must be a four-digit number, or if you require that the idNumber always be preceded by a pound sign, functions that are not a member of your class can never change those rules (either intentionally or by accident)

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• To setLastName() function shown in Figure 5-9 is a member of the Student class

• You can determine this because the class header contains the Student class name and the scope resolution operator

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• When you use the setIdNum() function with an object like aJunior, you are assured that aJunior receives a valid idNum

• Figure 5-11 shows how the setGradePointAverage() function can be written to accept a double argument and assure that the argument is no more than 4.0 before assigning it to any Student’s gradePointAverage field

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Using a Complete ClassUsing a Complete Class

• Figure 5-12 shows the entire Student class, all its member functions, and a short program that uses the class

EX4-1.cpp

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Using Private FunctionsUsing Private Functions

• Not all function are public

• When you think of real-world objects, such as kitchen appliances, there are many public functions you control through an interface: adjusting the temperature, etc.

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Using Private FunctionsUsing Private Functions

EX4-2.cpp

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Considering Scope When Considering Scope When Defining Member FunctionsDefining Member Functions

• There are circumstances when the scope resolution operator is required with a class field name

• Whenever there is a conflict between local and class variable names, you must use the scope resolution operator to achieve the results you want

• The member variable with the same name is hidden unless you use the scope resolution operator

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Considering Scope When Considering Scope When Defining Member FunctionsDefining Member Functions

In the second function, idNum refers to only the function’s local variable named idNum

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Using Static Class MembersUsing Static Class Members

• A C++ object is an instantiation of a class that can contain both data members and methods

• When you create an object, a block of memory is set aside for the data members

• Sometimes every instantiation of a class requires the same value

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Using Static Class MembersUsing Static Class Members

• To avoid a waste, you declare the athletic fee variable as static, meaning that it is shared by all the instances

• A class variable that you declare to be static is the same for all objects that are instantiations of the class

• Static variables are sometimes called class variables because they do not belong to a specific object; they belong to the class

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Defining Static Data MembersDefining Static Data Members

• Because it uses only one memory location, a static data member is defined (given a value) in a single statement outside the class definition

• Most often this statement appears just before the class implementation section

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A Class That Contains a Static A Class That Contains a Static AthleticFee FieldAthleticFee Field

Ex4-3.cpp

•Even though each Student declared in the program has a unique ID number, all Student objects share the athletic fee, which was assigned a value just once

• A static class member exists, even when you have not instantiated any objects of the class

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Using Static FunctionsUsing Static Functions

• In the program, the athleticFee field is public, which is why you can access it directly, as in Student::athleticFee = 139.95;

• If it were private, you would have to use a public function to access the value, as with any other private variable

• You cannot use a static function to access a non-static field because static functions do not receive a pointer to the object that uses the function

• You will use a public static function to display the private static variable

Ex4-4.cpp

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Understanding the this PointerUnderstanding the this Pointer

• When you define a class, you include fields and functions

• If the class has one non-static field and one static field such as the Employee class shown in Figure 5-23, and you then declare two Employee objects, you reserve storage for two versions of the non-static field

• However, you store only one version of the static field that every object can use

• C++ does not store member functions separately for each instance of a class

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• One copy of each member function is stored, and each instance of a class uses the same function code

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Understanding the this PointerUnderstanding the this PointerEx4-5.cpp

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• Because only one copy of each function exists, when you call a function, it needs to know which objects to use

• To ensure that the function uses the correct object, you use the object’s name, such as clerk or driver, with the dot operator

• Within the displayValue() function, the address of the object is stored in a special pointer called the this pointer

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• The this pointer holds the memory address of the current object that is using the function

• That is why it is named this—it refers to “this object” as opposed to any other object

• The this pointer also is passed to member functions when they receive additional, explicitly stated arguments

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Using the this Pointer ExplicitlyUsing the this Pointer Explicitly

• Within any member function, you can prove that the this pointer exists and that it holds the address of the current object

• You do so by using the this pointer to access the object’s data fields

Ex4-6.cpp

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Using the Pointer-to-Member Using the Pointer-to-Member OperatorOperator

• The functions operate by using the C++ pointer-to-member operator, which looks like an arrow and is constructed by a programmer by using a dash followed by a right angle bracket (or greater-than sign)

• Any pointer variable that is declared to point to a class object can be used to access individual fields or functions of that class by using the parentheses and the asterisk

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Using the Pointer-to-Member Using the Pointer-to-Member OperatorOperator

Ex4-7.cpp

Structures versus ClassesStructures versus Classes

• Structures– Typically all members public– No member functions

• Classes– Typically all data members private– Interface member functions public

• Technically, same– Perceptionally, very different mechanisms

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Ex4-8.cpp

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SummarySummary

• Each class you define is an abstract data type, or a group type with its own fields and functions

• A technique programmers use to provide more complete object encapsulation is to make most objects’ data private

• You can make functions private when you want to hide their use from client programs

• You can use the scope resolution operator with class fields and functions

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SummarySummary

• When you declare a class variable to be static, only one copy of the variable is stored, no matter how many class objects you instantiate

• When you create a class, one copy of each member function is stored

• Polymorphism is the object-oriented program feature that allows the same operation to be carried out differently, depending on the object

1 Using Structures and Classes COSC 1557 C++ Programming Lecture 4.

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