Delphi - Declaring Methods

Subroutines and Functions associated with classes are called Methods.

Polymorphism

One of the reasons to use objects is polymorphism. This is indicated with the virtual and override directives.

virtual indicates that this method may be overridden.
override indicates that this class will provide a new implementation of the method.

This is used by defining variables of the base class type and assigning them to objects of one of the derived types. This way you can treat many different types of objects in a similar manner, but the actual method is based on the specific type of object. (Examples explain this much better.)

type

  TFigure = class
    procedure Draw; virtual;
  end;
  TRectangle = class(TFigure)
    procedure Draw; override;
  end;
  TEllipse = class(TFigure)
    procedure Draw; override;
  end;

If the key word override is missing, then mehtod calls to the base class are not polymorphic and the base class method is called instead. In order to suppress compiler warnings, use the reintroduce directive if this is what you actually want.

virtual and dynamic perform the same function, but have different implementations.

abstract virtual methods define only the name, parameters, and return type of a method. One of the derived classes MUST define an implementation. You can not create objects until all the abstract methods of all the ancessor classes have implementations.

type

  TFigure = class
    procedure Draw; virtual; abstract;
  end;

Overload

When 2 (or more) methods have the same name but different parameters, they are said to be Overloaded.

  function Add(a, b: Integer): Integer; overload;
  function Add(a, b: Real   ): Real   ; overload;

The actual function called depends on the types of the parameters.

Default Parameters

Default parameters allow you to specify a method which can be called with a subset of the available parameters. For instance,

  function  Mid_mc(string, start: integer; length: integer = -1):string;

  xx := mid_mc('abcdefg', 3);    // xx = 'defg' - reads to end of string
  xx := mid_mc('abcdefg', 3, 2); // xx = 'de'

There is no need to repeat the default values when the function details are specified.

In general, use overload for different types of parameters, default parameters for a different number of parameters.

(Search for default parameters in the Delphi 5 help.)

Named Parameters

Basically, Delphi 5 does not support Named Parameters.

I did find the following example, but that was all (see Named Parameters). Apparently, this syntax can only be used with Automation objects.

Word.FileSaveAs(Password := 'secret', Name := 'test.doc');

Author: Robert Clemenzi - clemenzi@cpcug.org