Richard Taylor - Easing into OOP

by Mike Hanson

Published 1997-09-01    Printer-friendly version

Richard Taylor gave an extremely well-organized and educational presentation entitled "Easing into OOPs". For those audience members who were new to OOP, they could not have attended a more informative session to get them ramped-up.

With Clarion 4 moving so much into the OOP world, many developers will be dragged kicked and screaming into this new environment. Once they realize the benefits and recognize that it's actually not that hard, they should be a little more comfortable with it. Richard's session was the perfect head start for this expedition. This was by far the most concise and understandable presentation of this material that I've ever seen.

Using non-OOP methods (i.e.: CW 2.0 applications), each procedure contains both the code and data to perform its job. With OOP, all procedures share the same basic code, and only the data and code variations are unique to each procedure. This results in a significant savings over non-OOP programming.

The terms that Richard covered were:

• Class - This structure is used to define an "object" in CW.
• Property - This is object data in a Class. As was mentioned above, each object gets its own copy of the data.
• Method - This is a procedure or function associated with the Class. All objects derived from this class will share the same code.
• Encapsulation - This merely implies that the complexity of the classes is hidden within the class definitions. Additionally, properties and methods may be "PRIVATE" to the Class (not accessible outside the Class) or "PROTECTED" (accessible only within the Class itself, and in any other Classes derived from the original Class).
• Instantiation - This occurs when an actual instance of object is created at run-time. At that time, the object's data is allocated and initialized. If a Class is defined without the TYPE attribute, it will automatically be treated as both a Type and an Instance. New objects can be instantiated by referring to this new Class Type.
• Reference - This is a variable that refers to an actual instance of a particular type of variable (e.g.: reference to a QUEUE). The actual data space must be created with the NEW command, and destroyed with the DISPOSE command.
• Constructors and Destructors - If procedures called "Construct" and "Destruct" are declared as class members, the Construct procedure will be called when the object is created, and the Destruct procedure will be called when it is disposed. These procedures, which cannot take parameters or return a value, could be used for creating and destroying Reference variables, and other housekeeping chores.
• Inheritance - This occurs when a new class is defined by deriving an existing class. It will automatically acquire all attributes and methods of its parent class. When both the parent and child classes have their own constructors and destructors, the constructors are called in order from the parent down to the child, while the destructors are called in reverse order.
• Composition - This is a technique that can be used to simulate multiple inheritance (where a single new class is created by deriving from two or more existing classes). Essentially, a Reference variable is defined within the new Class for each additional parent Class beyond the first. It's not quite as good as multiple-inheritance, but it does avoid the problem of "disambiguation".
• Polymorphism - A single method name can have multiple definitions with different parameter lists. This allows the object's methods to be called depending on the type of task to be performed. Polymorphism also related to "Virtual Methods".
• Overridden Method - When a Method is declared in a derived child Class that also exists in its parent class, the child's Method will be called instead of the parent's. If the child's Method is adding only partial functionality to the parent Method, then it must manually call the parent's method itself.
• Virtual Method - This is a special type of overridden method. With a normal override, methods in the parent class will still call the parent's method, while methods in the child class (and elsewhere) will call the child's method. A virtual method is called by everyone (including the parent's methods). Richard's description probably said it best: "Inheritance allows Derived Class methods to "call down" to Parent Class methods. Virtual Methods are the opposite: they allow Parent Class methods to "call up" to execute the Derived Class methods."
• Late Binding - This is a technique that is required when using Virtual Methods. The parent class does not know which of its children's virtual methods will be called until it is actually needed. It simply knows that there might be a different method that must be called at run-time. This is substantially different from regular compilation, where all call references are resolved at link time.
• Local Derived Methods - This is a new scoping feature added to CW Beta 2. Normally local data and routines can be accessed only within the procedure in which they are declared. Now they can also be accessed from the methods defined in derived classes that are local to the original procedure.

This was a huge amount of material cover in a single hour, and Richard did an admirable job. The best tribute that we can pay to him now is to read his equally impressive documentation for C4. For those of you who are still intimidated by OOP, give it a try - you may be surprised and fall head over heels in love with it. 

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