Delphi in a Nutshell

Name

Virtual Directive

Syntax

Method declaration; virtual;

Description

You can declare a virtual method in a base class by using the virtual directive or the dynamic directive. The semantics of both directives is the same. The only difference is the implementation of the method and how it is called. For details, see Chapter 3.

Derived classes must use the override directive to override the method.

Tips and Tricks

• You should almost always use the virtual directive instead of dynamic. In most cases, virtual methods are faster and take up less memory than dynamic methods.

• The virtual directive must follow the reintroduce and overload directives and precede the calling convention and abstract directives (if the declaration uses any of these directives).

Example

type
  WholeNumber = 0..MaxInt;
  TIterator = function(Item: Pointer): Boolean of object;
  // Abstract base class for a variety of collection classes.
  // Derived classes must override several methods to implement
  // the detailed behavior.
  TCollection = class
  protected
    constructor Create; virtual;
    function Add(Item: Pointer): WholeNumber; virtual; abstract;
    function GetCount: WholeNumber; virtual; abstract;
    function Delete(Index: WholeNumber): Pointer; virtual; abstract;
  public
    procedure Remove(Item: Pointer); virtual;
    procedure ForEach(Iterator: TIterator); virtual; end; // Abstract class for several different binary trees. // Implements the binary-tree methods, but leaves some of the // details to derived classes. TBinaryTree = class(TCollection) ...