The Delphi Language, Part 2 - Object-Oriented Programming
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Volumes have been written on the subject of object-oriented programming (OOP). Often, OOP seems more like a religion than a programming methodology, spawning arguments about its merits (or lack thereof) that are passionate and spirited enough to make the Crusades look like a slight disagreement. We're not orthodox OOPists, and we're not going to get involved in the relative merits of OOP; we just want to give you the lowdown on a fundamental principle on which Delphi's language is based.
OOP is a programming paradigm that uses discrete objects—containing both data and code—as application building blocks. Although the OOP paradigm doesn't necessarily lend itself to easier-to-write code, the result of using OOP traditionally has been easy-to-maintain code. Having objects' data and code together simplifies the process of hunting down bugs, fixing them with minimal effect on other objects, and improving your program one part at a time. Traditionally, an OOP language contains implementations of at least three OOP concepts:
Encapsulation—Deals with combining related data fields and hiding the implementation details. The advantages of encapsulation include modularity and isolation of code from other code.
Inheritance—The capability to create new objects that maintain the properties and behavior of ancestor objects. This concept enables you to create object hierarchies such as VCL—first creating generic objects and then creating more specific descendants of those objects that have more narrow functionality.
The advantage of inheritance is the sharing of common code. Figure 5.1 presents an example of inheritance—how one root object, fruit, is the ancestor object of all fruits, including the melon. The melon is ancestor of all melons, including the watermelon. You get the picture.
Figure 5.1 An illustration of inheritance.
Note that the .NET CLR does not support the concept of C++-style multiple inheritance, and neither does the Delphi language.
You should understand the following three terms before you continue to explore the concept of objects:
Field—Also called field definitions or instance variables, fields are data variables contained within objects. A field in an object is just like a field in a Delphi record. In C#, fields sometimes are referred to as data members.
Method—The name for procedures and functions belonging to an object. Methods are called member functions in C#.
Property—An entity that acts as an accessor to the data and code contained within an object. Properties work similar to fields with attached logic to insulate the end user from the implementation details of an object.
Note - It's generally considered bad OOP style to access an object's fields directly because the implementation details of the object might change. Instead, use accessor properties, which allow a standard object interface without becoming embroiled in the details of how the objects are implemented. Properties are explained in the "Properties" section later in this chapter.
 This chapter is from Delphi for .NET Developer's Guide, by Xavier Pacheco (Sams, 2004, ISBN: 0-672-32443-1). Check it out at your favorite bookstore today.
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