Review of Key OO Characteristics
Objects separate interface from implementation, "what" is wanted (on the outside) from "how" it is accomplished (on the inside). This idea of hiding the data inside a package with a fixed set of allowed manipulations is called encapsulation. Why? If the object always selects how to perform a requested manipulation, you guarantee that the procedure and the data it operates on always match.
A message describes an operation that can be performed on an object. The code that describes how to perform an operation on a specific object type is called a method. From the outside, objects receive messages. On the inside, these messages are mapped to methods that perform appropriate actions for the specific kind of object. An object’s interface is the set of messages to which it can respond. For example, several object types may have a "dump" method to cause the object to display its state. Each kind of object will need a unique method to accomplish this. So the message-method idea separates the interface from the implementation. The idea that different kinds of objects might invoke different methods to respond to the same message is called polymorphism. Methods can be bound to messages as soon as the type of the object receiving the message is known., or this mapping can wait until run-time.
Some languages (notably C++) make a syntactic distinction between early and late binding of methods to messages (virtual functions are bound late, while all others are bound early – at link time). Smalltalk makes no such distinction. The C++ approach has potentially dangerous consequences when you manipulate an object through a pointer to its parent class. If a method of the superclass is virtual, you get late binding to the appropriate function for the object’s class. On the other hand, if that method is not virtual, you get early binding to the parent class’s definition even if the child class has overridden it. Smalltalk adheres rigorously to the idea that the object itself has sole ownership of the mapping from methods to messages.
A specific object described by a particular
class is called an instance of the class.
(Example: Dog is a class; Poodle is a subclass of Dog; FiFi is an object,
an instance of Poodle).
A class is a kind of object that describes the behaviors (methods)
of its instances, and whose methods provide for creation, initialization,
and destruction of an instance. All instances of a particular class use
the same method to respond to a given message. Classes may define other
members that are shared by all instances of the class. These are called
class variables. (In C++, these would be static members.)
This mechanism – decorating symbol names with extra characters that uniquely identify their class and prototype – is the traditional C++ method for operator overloading, method overloading, and namespace separation. C provides a much smaller set of namespaces than C++ requires, so this strategy allowed C++ to be implemented as a preprocessor for C compilers originally.
Rigorous type checking
Because of the function prototype requirement and name mangling, a C++ compiler can provide strict type checking for method invocations.
Automatic lifetime control
Guarantees constructor call upon creation and destructor call upon deletion
Multiple Storage classes (same as C)
Automatic, static, dynamically allocated
Typed dynamic memory management
Default constructor, destructor, copy constructor, and assignment operator
Explicit early and late binding support
And more…
Run-time type identification
This is a Big Deal in C++, but it’s relatively trivial in an interpreted language to know the type of a reference at run-time.
Metaclasses
Classes are objects, too (Smalltalk, Java?)
Garbage collection
Java and smalltalk both manage memory for you, freeing objects when they go out of scope or are no longer referenced anywhere.
Single Inheritance only
Java, Smalltalk 80
Everything is an object
Smalltalk
Operator overloading
Smalltalk makes no distinction between operators and other kinds of messages. The message syntax is flexible enough that you can define operators in the same way as any other kind of message.
Visibility control
Smalltalk 80 does not appear to provide options for visibility control (based on my quick survey). Instance variables are always private, methods are always public as far as I can tell.
Pointers
Not in Smalltalk, Java: Both languages deal with objects through implicit references. It is still possible to create data structures, but the language hides much of the memory management work.
No Casting
As far as I can tell, smalltalk has no equivalent of a C/C++ cast.
Late binding
Smalltalk makes no syntactic distinction between late and early bound methods (unlike C++ "virtual" methods)
Messages and methods: mapping table (first cut: use macros like MFC to create a mapping between messages and methods). Alternative: message map can be built at run-time (hash, tree, linked list) – method resolution may be slower.
Inheritance: aggregate the parent struct (recursively) at the beginning of the derived one, link the child method table to the parent and search recursively to resolve messages to methods
Pros and cons:
+ flexible and minimal manual steps required
+ relatively simple – no need to write a preprocessor!
- all methods are late bound, run-time penalty
defeats compile time type checking, may require a single prototype for all methods
Strategy 2: manual name mangling
This is how the Samek
article handles encapsulation
Strategy 3: preprocessor
This is how C++ started out.
References