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Ancestors, Descendants, and Class Hierarchies

Application developers take for granted that every control has properties named Top and Left that determine its position on the form. To them, it may not matter that all controls inherit these properties from a common ancestor, TControl. When you create a component, however, you must know which class to derive it from so that it inherits the appropriate features. And you must know everything that your control inherits, so you can take advantage of inherited features without recreating them.

The class from which you derive a component is called its immediate ancestor. Each component inherits from its immediate ancestor, and from the immediate ancestor of its immediate ancestor, and so forth. All of the classes from which a component inherits are called its ancestors; the component is a descendant of its ancestors.

Together, all the ancestor-descendant relationships in an application constitute a hierarchy of classes. Each generation in the hierarchy contains more than its ancestors, since a class inherits everything from its ancestors, then adds new properties and methods or redefines existing ones.

If you do not specify an immediate ancestor, Delphi derives your component from the default ancestor, TObject. TObject is the ultimate ancestor of all classes in the object hierarchy.

The general rule for choosing which object to derive from is simple: Pick the object that contains as much as possible of what you want to include in your new object, but which does not include anything you do not want in the new object. You can always add things to your objects, but you cannot take things out.

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