Title: ENUMERATED TYPES Part I
Question: Why use enumerated types?
Answer:
I was inspired to write this by a discussion in a Java News Group about the fact that there are no enumerated types in Java.
Delphi has simple and powerful enumerated types. Another question would be why use enumerated types, what is wrong with constants.
Heres why:
Const
acOpen := 0;
acClose := 1;
function Perform(pAction : Integer) : Boolean;
in calling the Perform method I can pass any integer thus:
Perform(acOpen); -- works
Perform(acClose); -- works
Perform(2); -- works also and could have undetermined results.
By using Enumerated types we can avoid this:
Type
TSomeAction = (taOpen, taClose);
function Perform(pSomeAction : TSomeAction) : Boolean;
now when calling the method I can only pass something of type TSomeAction:
Perform(taOpen);
Perform(taClose);
Perform(2); -- compiler error
So why use enumerated types:
Type Safety
Readability
Maintenance
Here is another example of using enumerated types:
unit Travel;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TDirection = (drNorth, drSouth, drEast, drWest);
TDirectionChangeEvent = procedure(Sender : TObject) of object;
TCompassError = class(Exception);
TCompass = class
protected
fHeading : Double;
property Heading : Double read fHeading write fHeading;
public
constructor Create (pHeading : Double = 0.0); reintroduce; virtual;
end;
TTraveller = class(TForm)
Button1: TButton;
lbHeading: TLabel;
procedure FormCreate(Sender: TObject);
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
fCompass : TCompass;
fOnDirectionChange : TDirectionChangeEvent;
procedure DoDirectionChange;
protected
procedure SetHeading (pDirection : TDirection); virtual;
procedure DisplayHeading(Sender : TObject);
public
{ Public declarations }
property OnDirectionChange : TDirectionChangeEvent read fOnDirectionChange write fOnDirectionChange;
end;
var
Traveller: TTraveller;
implementation
{$R *.DFM}
constructor TCompass.Create (pHeading : Double);
begin
fHeading := pHeading;
end;//Create
procedure TTraveller.SetHeading (pDirection : TDirection);
begin
case pDirection of
//can be used in case statements for readability and easy development
drNorth : fCompass.Heading := 0.0;
drSouth : fCompass.Heading := 180.0;
drEast : fCompass.Heading := 90.0;
drWest : fCompass.Heading := 270.0;
end;//case
DoDirectionChange;
end;//SetHeading
procedure TTraveller.DisplayHeading(Sender : TObject);
begin
lbHeading.Caption := FloatToStr(fCompass.Heading);
end;
procedure TTRaveller.DoDirectionChange;
begin
if Assigned(fOnDirectionChange) then
fOnDirectionChange(Self);
end;//DoDirectionChange
procedure TTraveller.FormCreate(Sender: TObject);
begin
fCompass := TCompass.Create;
Traveller.OnDirectionChange := DisplayHeading;
DoDirectionChange;
end;
procedure TTraveller.Button1Click(Sender: TObject);
begin
// Just so we can change the direction Randomly
SetHeading(TDirection(Round(Random(4))));
end;
end.
Another argument is its not object oriented ? Answer in part II
I have beefed up the previous example, thanks! to Pieter Valentijn for his comments.
Here it is:
unit Travel;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TDirection = (drNorth, drSouth, drEast, drWest);
TDirectionChangeEvent = procedure(Sender : TObject) of object;
TCompassError = class(Exception);
TCompass = class
protected
fHeading : Double;
(* array size is set using TDirection , thus if you add values to
TDirection such as drNorthEast. The size of your array will change
too.
*)
FAvailableHeadings : array [Low(TDirection) .. High(TDirection)] of Double;
function GetAvailableHeadings (Index : TDirection) : Double;
procedure SetAvailableHeadings (Index : TDirection ; Value : Double );
property Heading : Double read fHeading write fHeading;
public
constructor Create (pHeading : Double = 0.0); virtual;
property AvailableHeadings [index : TDirection] : Double read GetAvailableHeadings write SetAvailableHeadings;
end;
TTraveller = class(TForm)
Button1: TButton;
lbHeading: TLabel;
procedure FormCreate(Sender: TObject);
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
fCompass : TCompass;
fOnDirectionChange : TDirectionChangeEvent;
procedure DoDirectionChange;
protected
procedure SetHeading (pDirection : TDirection); virtual;
procedure ScanHorizon;
procedure DisplayHeading(Sender : TObject);
public
{ Public declarations }
property OnDirectionChange : TDirectionChangeEvent read fOnDirectionChange write fOnDirectionChange;
end;
var
Traveller: TTraveller;
implementation
{$R *.DFM}
constructor TCompass.Create (pHeading : Double);
var vDirection : TDirection;
begin
inherited Create;
fHeading := pHeading;
// Using ENUMs in for loop for better control
for vDirection := Low(TDirection) to High(TDirection) do
AvailableHeadings[vDirection] := (Ord(VDirection) * 90.0);
end;//Create
function TCompass.GetAvailableHeadings (Index : TDirection) : Double;
begin
if Index in [Low(FAvailableHeadings)..High(FAvailableHeadings)] then
Result := FAvailableHeadings[TDirection(Index)]
else
raise Exception.Create('Index out of bounds');
end;
procedure TCompass.SetAvailableHeadings (Index : TDirection ; Value : Double );
begin
if Index in [Low(FAvailableHeadings)..High(FAvailableHeadings)] then
FAvailableHeadings[Index] := Value
else
raise Exception.Create('Index out of bounds');
end;//SetAvailableHeadings
procedure TTraveller.SetHeading (pDirection : TDirection);
begin
fCompass.Heading := fCompass.AvailableHeadings[pDirection];
(*case pDirection of
drNorth : fCompass.Heading := fCompass.AvailableHeadings[drNorth];
drSouth : fCompass.Heading := fCompass.AvailableHeadings[drSouth];
drEast : fCompass.Heading := fCompass.AvailableHeadings[drEast];
drWest : fCompass.Heading := fCompass.AvailableHeadings[drWest];
end;//case *)
DoDirectionChange;
end;//SetHeading
procedure TTraveller.ScanHorizon;
var
vDirectionToLook : TDirection;
begin
// Using ENUMs in for loop for better control
for vDirectionToLook := Low(TDirection) to High(TDirection) do
//Do something such as check terrain;
end;
procedure TTraveller.DisplayHeading(Sender : TObject);
begin
lbHeading.Caption := FloatToStr(fCompass.Heading);
end;
procedure TTRaveller.DoDirectionChange;
begin
if Assigned(fOnDirectionChange) then
fOnDirectionChange(Self);
end;//DoDirectionChange
procedure TTraveller.FormCreate(Sender: TObject);
begin
fCompass := TCompass.Create;
Traveller.OnDirectionChange := DisplayHeading;
DoDirectionChange;
end;
procedure TTraveller.Button1Click(Sender: TObject);
var vMax : Integer;
begin
(* Just so we can change the direction Randomly
Random number between 0 and vMax := Ord(High(TDirection))+1;
SetHeading(TDirection(Random(vMax)));
end;
end.