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Description
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The Default directive
is used in two separate, and quite different ways
with properties to establish default
processing.
Version
1
When you have one or more properties that used an
index as a parameter (see the example and
Index for further details), you
can specify the Default
directive. It allows you to use a more compact way
of using the property.
Instead of :
myValue :=
MyObject.GetItem(23);
We can use :
myValue :=
MyObject[23];
This can make for more readable and compact code,
but can be confusing for newcomers to Delphi - they
will see MyObject is not an array and find it
difficult to find the mechanism. Especially if the
class is big.
Version
2
Has a much more specialised use. It stores the
supplied default ordinal property value in the run
time information for the class. Its use is beyond
the scope of Smart Pascal.
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Related commands
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Private
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Starts the section of private data and
methods in a class
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Property
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Defines controlled access to class fields
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Protected
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Starts a section of class private data
accesible to sub-classes
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Public
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Starts an externally accessible section of a
class
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Published
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Starts a published externally accessible
section of a class
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Example code : A simple
example
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// Full Unit
code.
//
-----------------------------------------------------------
// You must store
this code in a unit called Unit1 with a
form
// called Form1 that
has an OnCreate event called
FormCreate.
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants,
Classes, Graphics, Controls, Forms,
Dialogs;
type
// Class
with Indexed properties
TRectangle = class
private
fCoords: array[0..3] of
Longint;
function GetCoord(Index:
Integer): Longint;
procedure SetCoord(Index:
Integer; Value: Longint);
public
property Left :
Longint Index 0 read GetCoord write SetCoord;
property
Top : Longint Index 1 read
GetCoord write SetCoord;
property Right :
Longint Index 2 read GetCoord write SetCoord;
property Bottom : Longint
Index 3 read GetCoord write SetCoord;
property Coords[Index:
Integer] : Longint
read GetCoord write SetCoord; Default;
end;
// The
form class itself
TForm1 = class(TForm)
procedure
FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
// TRectangle
property 'Getter' routine
function TRectangle.GetCoord(Index:
Integer): Longint;
begin
// Only
allow valid index values
if (Index >= 0) and (Index <=
3)
then Result := fCoords[Index]
else Result := -1;
end;
// TRectangle
property 'Setter' routine
procedure TRectangle.SetCoord(Index, Value:
Integer);
begin
// Only
allow valid index values
if (Index >= 0) and (Index <=
3)
then fCoords[Index] := Value;
end;
// Main line
code
procedure TForm1.FormCreate(Sender:
TObject);
var
myRect : TRectangle;
begin
// Create
my little rectangle
myRect := TRectangle.Create;
// And
set the corner coordinates
myRect.Left :=
22;
// Left using direct
method
myRect.Top := 33;
myRect.SetCoord(2,44);
// Right using
indexed method
myRect.SetCoord(3,55);
// Get
the first two coords using the get method
name
WriteLn('myRect coord 0 =
'+intToStr(myRect.GetCoord(0)));
WriteLn('myRect coord 0 =
'+intToStr(myRect.GetCoord(1)));
// Now
use the more compact version to get the remaining
coords
// This
is only possible when we have a 'default' indexed
property
WriteLn('myRect coord 1 =
'+intToStr(myRect[2]));
WriteLn('myRect coord 1 =
'+intToStr(myRect[3]));
end;
end.
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myRect coord 0 = 22
myRect coord 1 = 33
myRect coord 2 = 44
myRect coord 3 = 55
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