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Lecture 15: Object-Oriented ProgrammingCOMP 524 Programming Language Concepts Stephen OlivierMarch 23, 2009

Based on slides by A. Block, notes by N. Fisher, F. Hernandez-Campos, and D. Stotts

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Object Oriented Programming Benefits

•Reduces conceptual load

•Fault containment

•Independence between components

• Makes interface design very important

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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...

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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.

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Public Variables

•Generally, people consider it bad form to put variables in the public space.

•Why?

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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.

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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.

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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

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Inheritance

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class foo{ public int x; 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; //validbbb.y = 1; //invalidbbb.z = 1; //invalid

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Scoping

•In order to refer to a function for a particular class, use the scope resolution operator ::

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class foo{public int bar();

}

int foo::bar(){ return 0;}

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General Purpose Base Class

•Its possible to define a general purpose base class as the highest level of abstraction

• for example Object in Java

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Modifying Base Class Methods

•Its possible to redefine base methods in derived classes

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class foo{public int bar();

}...class qud : foo{public int bar(){ return 1;};

}

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Modifying Base Class Methods

•Sometimes better just to use base if possible and try to return errors

•

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class foo{public int bar();

}...class qud : foo{public int bar(){ try {return foo::bar();}...};

}

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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.

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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

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Three Issues Regarding Constructers

•Choosing a constructor

•References and Values

•Execution Order

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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

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Value and References

•Variables can refer to objects (Smalltalk, Java, Python, Ruby)

•or have values which are objects (C++)

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References

•More elegant, but can cause additional overhead.

• Space required on heap

• Indirection to access heap

• Garbage collection

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Objects as Values (C++)

•When variables are used as values in C++, a constructor can be implicitly called.

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foo b; //Calls foo::foo()foo b(10, ‘x’); //calls foo:foo(int, char)

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Costs of the Value Model

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foo a = b+c;

foo t;t = b.operator+(c);foo a = t;

foo a = b; a += c; // avoids need for t

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Execution order

•When an object employs inheritance it constructs the base classes first

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foo::foo( foo_params ) : bar (bar_params) { ...}

foo ( foo_params ){super (bar_parms) ... }

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Destructors

•Needed in OO languages with explicit heap management, e.g. C++

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class foo { char * list; foo() { list = new char[100];} ~foo() { delete list };}