Lecture 15: Object-Oriented Programming

Transcription

1 Lecture 15: Object-Oriented Programming COMP 524 Programming Language Concepts Stephen Olivier March 23, 2009 Based on slides by A. Block, notes by N. Fisher, F. Hernandez-Campos, and D. Stotts

2 Object Oriented Programming Benefits Reduces conceptual load Fault containment Independence between components Makes interface design very important 2

3 Brief History Simula (60 s) : Objects, classes, etc. Smalltalk (70 s) : OO as we know it C++ (80 s) : OO meets C, first widespread OO language Java (90 s) : C/C++ syntax but OO-centric design and more... 3

4 Class-as-type A class can be viewed as a class-as-type, when it contains multiple variables and/or functions to manipulate these variables. 4

5 Public Variables Generally, people consider it bad form to put variables in the public space. Why? 5

6 Class-as-manager A class can be viewed as a class-as-manager when it is not directly used to contain variables but rather to manage other objects. 6

7 Public, Private, and Protected (in C++) Public fields are available to everyone Private fields are available only to the class Protected are available only to descendants. 7

8 Deriving Classes Classes can be derived from other class, and inherit their functions. This is a KEY component of object-oriented programming. Also opens possibility to have subtype polymorphism 8

9 class foo{ public int x; Inheritance private int y; protected int z; } class bar : foo { public int xy() {return x+y;} //invalid public int xz() {return x+z;} //valid }... bar bbb; bbb.x = 1; //valid bbb.y = 1; //invalid bbb.z = 1; //invalid 9

10 Scoping In order to refer to a function for a particular class, use the scope resolution operator :: class foo{ public int bar(); } int foo::bar(){ return 0;} 10

11 General Purpose Base Class Its possible to define a general purpose base class as the highest level of abstraction for example Object in Java 11

12 Modifying Base Class Methods Its possible to redefine base methods in derived classes class foo{ public int bar(); }... class qud : foo{ public int bar(){ return 1;}; } 12

13 Modifying Base Class Methods Sometimes better just to use base if possible and try to return errors class foo{ public int bar(); }... class qud : foo{ public int bar(){ try {return foo::bar();}...}; } 13

14 Protection Rules (in C++) Any class can limit visibility of its member A derived class can restrict the visibility of a base class, but never increase it A derived class that limits visibility of members of a base class as protected or private can restore the visibility of individual members by inserting a using declaration in protected or public portion of the derived class. 14

15 Java vs C++ Under Java derived classes cannot change the visibility from the base class But members can be redefined. Protected has a slightly different meaning. Under Java, protected means that it is visible by everything in the package and derived classes Without protected, it is visible by everything in the package, but not derived classes in other packages 15

16 Three Issues Regarding Constructers Choosing a constructor References and Values Execution Order 16

17 Choosing Two common ways: 1. Named constructors (Smalltalk, Eiffel) 2. Overloading (C++, Java, C#) Some languages (Ada95, Modula-3) don t initialize objects at elaboration time 17

18 Value and References Variables can refer to objects (Smalltalk, Java, Python, Ruby) or have values which are objects (C++) 18

19 References More elegant, but can cause additional overhead. Space required on heap Indirection to access heap Garbage collection 19

20 Objects as Values (C++) When variables are used as values in C++, a constructor can be implicitly called. foo b; //Calls foo::foo() foo b(10, x ); //calls foo:foo(int, char) 20

21 Costs of the Value Model foo a = b+c; foo t; t = b.operator+(c); foo a = t; foo a = b; a += c; // avoids need for t 21

22 Execution order When an object employs inheritance it constructs the base classes first foo::foo( foo_params ) : bar (bar_params) {...} foo ( foo_params ){ super (bar_parms)... } 22

23 Destructors Needed in OO languages with explicit heap management, e.g. C++ class foo { char * list; foo() { list = new char[100];} ~foo() { delete list }; } 23