Object-Oriented Programming

Object-Oriented Programming Gimme, gimme yer homework! What is it? Construction and Destruction Inheritance Models Ad-hoc Polymorphism G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 1 of 20 What is OOP? Support for objects, whatever those are Encapsulation Inheritance Polymorphism Various dimensions of capabilities Often tied to programming environment G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 2 of 20 Programming Environment Interpreter/Compiler trade-off Additional or required tools, libraries, etc. Minimal: C++ Many compilers and even some interpreters Very few required libraries No required IDE Maximal: C# C# without .NET is crippled C# development is in context of entire .NET environment G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 3 of 20 So What Is an Object? Intuitive sense: just look around you Traditional programmer sense: Data and functionality An approach arising from Turing/von Neumann architectures These separated memory/tape from actions/state machine Realistically, we never have one without the other What must it have? Identity Capabilities Class? G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 4 of 20 Objects Need Identity Is it value-based? 3 is the same number no matter where you get it Is the same true for objects? In general, no Polymorphic definition of equality can make things slippery Is it intrinsic? For most OOPLs, there is Explicit: C++ Implicit: Java, Eiffel, Lisp/CLOS some intrinsic identity: address G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 5 of 20 Capabilities Message Has a name Is sent somewhere What target(s) do depend on target(s), not sender Method Has a name Is used to respond to a message Is tied to the identity of the object Are instance variables different? First, let’s consider encapsulation G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 6 of 20 Encapsulation Object as a “black box” You can push the buttons You can’t peek inside A form of abstraction Removal of extraneous details How is task accomplished? Not relevant Real world: “How” often resources, etc) is important (e.g., run time, shared Modules provide encapsulation, but not identity CLU, Modula-2 early examples of strong encapsulation Creating value-specific copies of modules provides identity 7 of 20 G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 Goals of Encapsulation Simplify programming: Only learn buttons on box Increase reusability Encapsulation to produce generic chunks of capabilites Reuse whenever capability needed Problem: Genericity/utility trade-off Increase maintainability Isolate problems and solutions into smaller chunks of code Complex patterns can distribute capabilities G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 8 of 20 Encapsulation as a Design Pattern in C Provide an opaque pointer struct Stack; typedef struct Stack *StackPtr; Provide a list of functions taking that ptr as param void StackPush(StackPtr *, void *); void *StackPop(StackPtr *); Struct definition at other end of ptr is unavailable List of functions is list of buttons Struct definition is the contents of the black box G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 9 of 20 Instance Variables as Methods Self: Names of slots Pairs of get/set slots look like an instance variable Get/set slots could actually do more, e.g., instrumenting Most languages provide state for an object that an external entity can change Strictly speaking, breaks encapsulation External entity knows something about how object works Changes to the object could break functionality of external objects G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 10 of 20 Examples C++: class/struct members Methods can (almost) duplicate pure abstraction C++ requires function call operator () for methods, not for instance variables Java: class members Again, methods (almost) duplicate pure abstraction Smalltalk, Self: external entity can’t tell In Self, encapsulation is stronger: object itself can’t tell In Self, object sends message to itself to access “variable” Sender (self) can’t tell anything about response G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 11 of 20 Class Categories or sets of objects If object is in set/class, it is an instance of the class Declaration of what capabilities object supports Class is therefore an encapsulation mechanism Most languages: Class declaration specifies instance variables (i.e., mutable data) Every instance of a class has the listed kinds of state and supports the listed messages/methods Can have ramifications for recompile decisions G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 12 of 20 Class as a Pattern in Self BallClass bounce roll new: WithMaterial: prototypeBall BallClass new: WithMaterial: parent* newSize newMaterial prototype ball copy size: newSize material: newMaterial prototypeBall size material parent* a ball instance size material parent* G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 13 of 20 Nature of Capabilities Encapsulation Inheritance Ad-hoc polymorphism G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 14 of 20 Encapsulation Access control Public: Anyone can use it Class private: Only instances of my class can use it (C++, Java) Instance private: Only this instance can use it (Eiffel) Protected: I can use it and my subtypes can use it Loopholes C++ ugliness: friend, casts Eiffel: export to specific classes G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 15 of 20 Names Prefixes based on class name C++: classname::foo Java: com.nellardo.Dingo.boo Self: messages Replacement names for inconveniently named objects Eiffel: import allows renaming Often tied to a (separate) module mechanism Do objects provide encapsulation or just modules? More later G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 16 of 20 Inheritance An object shares information with another Most OOPLs: instances share with class, class shares with other class(es) For classes, a subclass inherits from a superclass All instance vars and methods for superclass are available in subclass In general, subclass may add inst vars and may add or replace methods Issue: Multiplicity Issue: Mutability G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 17 of 20 Multiplicity in Inheritance One and only one superclass? Or more? One leads to forests or trees Forest: No common base class Tree: A single common base class Many can cause ambiguity What is shared and what is duplicated? Many langs use one but allow mixins or interfaces Mix-in: add a “flavor” Interface: All virtual methods G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 18 of 20 Polymorphism Parametric polymorphism vs ad-hoc polymorphism Parametric: Polymorphic over a family of types Ad-hoc: A single type can be extended arbitrarily G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 19 of 20 Construction and Destruction Construction Primitive types have useful default values Objects may, but the compiler can’t necessarily deduce them E.g., color with opacity Garbage collection System reclaims memory when it can no longer be accessed Destruction Required for manual memory management Since you release mem yourself, must explicitly release any mem object allocated G22.2110-001: Programming Languages Object-Oriented Programming May 28, 2002 20 of 20