General Info: Computer Language Constants

Delphi Constants

In Delphi, you define constants similar to how you define variables but use the Const keyword instead of the Var keyword. Declare global constants in a unit's interface section and unit constants (scope limited to unit) in the implementation section. Declare local constants above the begin..end block.

Const 

  kFeetToMeter: Double = 3.2808;

  kMeterToFeet: Double = .3048;

  kName: String = "Mike";

//Local constants:

procedure SomeProcedure;

const

  kPI: Double=3.1459;

begin

end;

Writable Typed Constants

Delphi also supports writable typed constants which are not constants. They are initialized global variables and are a hold-over from earlier versions of Delphi and Turbo Pascal. They are turned off by default in later versions of Delphi so to use them you have to use the compiler directive {$J+} to enable then {$J-} to disable.

Writable Typed Constants Example:

{$J+}
const
   clicks : Integer = 1; //not a true constant
{$J-}
begin
  Form2.Caption := IntToStr(clicks) ;
  clicks := clicks + 1;
end;

Although the Delphi help says not to use writable typed constants, you may find them useful when you wish to use static local variables. You'll have to decide if you wish to use them. The benefit #3 below has over the suggested #1 is that #3 is local and you therefore don't  have to worry about duplicate constant/variable names; otherwise, they both work well.

//1. Global variable (suggested technique).
//var
//  ButtonClicks: Integer = 0;
procedure TForm2.Button5Click(Sender: TObject);
//2. Does not work (local variable is destroyed with each click 

//   so the compiler doesn't even allow this to compile).
//var
//  ButtonClicks: Integer = 0;  //Does not compile!

//3. Writable typed constant is global
//   (same as more correct #3 above, but weird).
{$J+}
const
  ButtonClicks: Integer = 0;
{$J-}
begin
  If ButtonClicks >= 5 then
    ShowMessage('Stop clicking the button.')
  Else

  begin
    ButtonClicks := ButtonClicks + 1;

    Form2.Caption := IntToStr(ButtonClicks);

  end;
end;