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Object-Oriented Programming Paradigm and Design Trade-Offs

Learn about object-oriented programming paradigm and its design trade-offs. Explore examples in languages like Smalltalk, Java, and C++. Develop predicate classes for logic programming.

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Object-Oriented Programming Paradigm and Design Trade-Offs

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  1. Object-Oriented Programming • A design method as well as a programming paradigm • For example, CRC cards, noun-verb parsing of requirements • Hinges on inheritance-based polymorphism • Classes define behaviors and structures of objects • Subclasses may refine or extend base classes • Extensions external to classes may be supported (e.g., C#) • Distinct objects (class instances) interact • The sets of objects (and behaviors) may vary dynamically • OO paradigm introduces several key ideas • Independent objects with separate state • Encapsulate all but the most necessary details (e.g., public/private/protected access in C++) • Abstraction, polymorphism, substitution, extensibility

  2. Design Trade-Offs in OO Languages • Classes must declare all information compiler needs • E.g., sizes of by-value member variables, public interfaces • E.g., definitions of any methods the compiler should inline • Inlining copies method’s code to each place it’s called • Saves performance overhead of a function call each time • But requires more space for duplicate copies of instructions • How classes interact with the type system • Can exclude classes from type checking (not very safe) • Can use classes as type constructors (as in C++) • Can express entire type system as classes (as in Smalltalk) • Controlled violations of encapsulation may be allowed • E.g., C++ friend declaration to support symmetric operators

  3. OO Language Example: Smalltalk • A purely object-oriented language • Classes, constants, etc. are all objects • Powerful but somewhat elaborate execution model • Send a message to a receiver, which dispatches a method • Useful for event handling, simulation, similar applications • Methods read or modify objects’ state • I.e., accessor and mutator methods • Method calls are performed using reference semantics • Passes an alias to an object rather than copying its value • An object may refer to itself in a method • Objects are constructed via the class object • Send the new message to the class type

  4. OO Language Example: Java • An almost purely object-oriented language • E.g., integers are not objects, but can be wrapped with them • Much more common execution model • Call a method directly • Preserves flexibility but with a more understandable style • Methods again read or modify objects’ state • I.e., accessor and mutator methods • Method calls again use reference semantics • Passes an alias to an object rather than copying its value • But, Java may bind a call statically if possible for efficiency • Classes declare object constructor methods • Default constructors, constructor chaining are supported

  5. OO Language Example: C++ • Classes declared, objects instantiated (similar to Java) • Except, can instantiate an object directly on the stack as in EvenIntegerPredicate eip; // default ctor • Also can instantiate heap object via new (returns ptr) as in TruePredicate * tpp = new TruePredicate; • Object destructors managed differently for these cases • Return from a function calls local stack object destructors automatically • The program itself (or “smart pointer” it declares) must call delete directly (delete tpp; or shared_ptr<TruePredicate> tppsp (tpp);) • C++11 smart pointers implement reference counting and other object lifetime management semantics • C++ allows object methods to be defined (in .cpp files) separately from their class declarations (in .h files) • Also can define within declaration, which inlines the method • Class methods are passed implicit this parameter (arity is +1)

  6. Today’s Studio Exercises • We’ll code up predicate classes for logic programming • Developing a class hierarchy with abstract/concrete methods • Please see slides for next studio (and/or read ahead in the text book for more about abstract vs. concrete classes) • E.g., C++ methods are polymorphic only if declared virtual • Today’s exercises are all in C++ • Logic programs fit well within object-oriented features • As always, please ask us for help as needed • When done, e-mail your answers with subject line “Object-Oriented Programming Studio I” to the cse425@seas.wustl.edu course e-mail account

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