Object-Oriented Programming, Classes

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1 Object-Oriented Programming, Classes Data Processing Course, I. Hrivnacova, IPN Orsay Object-Oriented vs. Procedural Programming Classes and Instances Class Definition Using A Class Data Encapsulation Header and Implementation files 1

Procedural Programming Chua Hock-Chuan: Programming Notes Traditional procedural-oriented programming language: C, Fortran, Cobol, or Pascal The programs are made up of functions.

2 Procedural Programming Chua Hock-Chuan: Programming Notes Traditional procedural-oriented programming language: C, Fortran, Cobol, or Pascal The programs are made up of functions. Functions are often not reusable. The procedural languages are not suitable of high-level abstraction for solving real life problems. The data structures and algorithms of the software entities are separated. The traditional procedural language forces you to think in terms of the structure of the computer (e.g. memory bits and bytes, array, decision, loop) 2

Object-Oriented Programming (OOP) The basic unit of OOP is a class A class is an implementation of an abstract data type: Chua Hock-Chuan: Programming Notes It describes both the attributes (data)

3 Object-Oriented Programming (OOP) The basic unit of OOP is a class A class is an implementation of an abstract data type: Chua Hock-Chuan: Programming Notes It describes both the attributes (data) of an object and its operations (methods) The OOP languages let you think in the problem space, and use software objects to represent and abstract entities of the problem space to solve the problem. 3

OOP Example As an example: a computer soccer games Modeling accordingly to the "real things": Player: Chua Hock-Chuan: Programming Notes Attributes: name, number, location in the field, and etc.

4 OOP Example As an example: a computer soccer games Modeling accordingly to the "real things": Player: Chua Hock-Chuan: Programming Notes Attributes: name, number, location in the field, and etc. Operations: run, jump, kick-the-ball, and etc. Ball, Field, Referee, Audience, Weather Some of these classes (Ball, Audience) can be reused in another application, e.g., computer basketball game 4

5 OOP Benefits Ease in software design as you could think in the problem space rather than the machine's bits and bytes. You are dealing with high-level concepts and abstractions. Ease in design leads to more productive software development. Ease in software maintenance: object-oriented software are easier to understand, therefore easier to test, debug, and maintain. Reusable software: you don't need to keep re-inventing the wheels and re-write the same functions for different situations The fastest and safest way of developing a new application is to reuse existing codes - fully tested and proven codes. 5

6 Classes A class is a definition of objects of the same kind. An instance is a realization of a particular item of a class. In other words, a class is a blueprint, template, or prototype that defines and describes the static attributes and dynamic behaviors common to all objects of the same kind. In other words, an instance is an instantiation of a class. All the instances of a class have similar properties, as described in the class definition. For example, you can define a class called "Student" and create three instances of the class "Student" for "Peter", "Paul" and "Pauline". The term "object" usually refers to instance. But it is often used quite loosely, which may refer to a class or an instance. 6

7 A Class A class can be visualized as a three-compartment box, as illustrated: Classname (or identifier): identifies the class. Data Members (or variables, attributes, states, fields): Member Functions (or methods, behaviors, operations): Chua Hock-Chuan: Programming Notes contain the static attributes of the class. contain the dynamic operations of the class. In other words, a class encapsulates the static attributes (data) and dynamic behaviors (operations that operate on the data) in a box. Class Members: The data members and member functions are collectively called class members. 7

8 Example Of Classes, Instances The class and instances diagrams are drawn according to the Unified Modeling Language (UML) notations. Chua Hock-Chuan: Programming Notes 8

9 Class Definition class Car { int yearofmanufacture; int mileage; int licenseplate; int getyearofmanufacture(); void drive(float m); float getmileage(); }; void Car(); void ~Car(); A car can be an be described using properties such as mileage, license plate and year of manufacture. These attributes indicate the object, its state The car object is described also with the operations, its behaviour A car can be driven from A to B, we can query mileage or year of manufacture. 9

10 Using A Class int main() { Car car; } create car and initialize it car.drive(77); drive 77 miles float mileage = car.getmileage(); query mileage cout << Car mileage is: << mileage << endl; print mileage on the screen 10

11 A Car as a Class class Car { int yearofmanufacture; int mileage; int licenseplate; int getyearofmanufacture(); void drive(float m); float getmileage(); }; void Car(); void ~Car(); attributes operations Special functions which can be used to influence the behaviour of the car during construction and destruction: The year of manufacture, milage and license plate will be initialized in Car() 11

12 Data Encapsulation To prevent any application programmer from doing anything they want with an object, access to an object is restricted via a well defined interface An application programmer can only call the functions for which the designer of the class granted public access C++: keyword public:, private: 12

13 Data Encapsulation class Car { private: int yearofmanufacture; int mileage; int licenseplate; public: int getyearofmanufacture(); void drive(float m); float getmileage(); }; The members declared after private are protected from being accessed by application programers For the application developer only the public interface is of interest. void Car(); void ~Car(); 13

14 Separating Header and Implementation For better software engineering the class declaration and implementation are usually kept be kept in 2 separate files: Declaration is a header file ".h" Implementation in a ".cxx". This is known as separating the public interface (header declaration) and the implementation. Interface is defined by the designer, implementation can be supplied by others. Only the header files are exposed to the users, the implementation can be provided in an object file ".o" (or in a library). The source code needs not to be given to the users. 14

15 Functions and Modules getarea.h // Function Prototype double getarea(double radius); getarea.cxx Header File - extension.h testfunction.cxx #include getarea.h // Function Definition double getarea(double radius) { return radius*radius*pi; } #include getarea.h // 'testfunction program int main() { double area1 = getarea(radius1); //... } Source Files - extension.cxx 15

16 Classes and Modules Car.h Header File - extension.h // Class Car Definition //... Car.cxx // Class Car Implementation //... testcar.cxx #include Car.h // 'testcar program int main() { //... } Source Files - extension.cxx 16

17 Compiling & Linking The header file (.h) is included by both the source file with the class implementation and by the application source file Both source files (.cxx) are converted into object files (.o) by the C++ compiler The object files are then linked to the executable program ( testcircle) by a linker compiler Car.cxx Car.o linker include testcar Car.h include testcar.cxx testcar.o linker compiler 17

Classes - 2. Data Processing Course, I. Hrivnacova, IPN Orsay

Classes - 2. Data Processing Course, I. Hrivnacova, IPN Orsay Classes - 2 Data Processing Course, I. Hrivnacova, IPN Orsay OOP, Classes Reminder Requirements for a Class Class Development Constructor Access Control Modifiers Getters, Setters Keyword this const Member