Delphi Parameters

Object Pascal allows parameters of the same type to be listed together, separated by commas, and followed with a single data type (more params of different data types can follow, after a semi-colon).

Delphi also supports default parameters (a form of overloading).

By Reference or Value (and by constant): The default for parameters is by value. For by reference, add var in front of the parameter. Object Pascal also offers constant parameters where you add const in front of the parameter. A constant parameter is like a local constant or read-only parameter the compiler can optimize. You cannot assign a value to a constant parameter, nor can you pass one as a var parameter to another routine. (But when you pass an object reference as a constant parameter, you can still modify the object's properties.)

function Add(a, b: integer) : integer; 
begin
  Result := a + b;
end;
  
procedure ReplaceTime(var pDT: TDateTime; const pNewDT: TDateTime);
begin
end;

Working Example

Assuming a button on a form, here's the supporting code for a quick demo:

unit ParametersUnit;

  
interface

  
uses
    Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
    Dialogs, StdCtrls;

  
type
    TForm1 = class(TForm)
      Button1: TButton;
      procedure Button1Click(Sender: TObject);
    private
      { Private declarations }
    public
      { Public declarations }
    end;

  
var
    Form1: TForm1;

  
implementation

  
{$R *.dfm}

  
function Add(a, b : integer) : integer;
begin
    Result := a + b;
end;
  

function AddByRef(var a, b : integer) : integer;
begin
    Result := a + b;
end;
  

function AddByConst(const a, b : integer) : integer;
begin
  Result := a + b;
end;

  
procedure TForm1.Button1Click(Sender: TObject);
var
  x, y: Integer;
begin
    x := 4;
    y := 3;

  
    ShowMessage('by val pass literals 2+2=' + IntToStr(Add(2,2)));
    ShowMessage('by val pass vars 4+3=' + IntToStr(Add(x,y)));

  
    //With by reference, you cannot pass literals!
    //Does not work...ShowMessage('by ref 4+3=' + IntToStr(AddByRef(2,2));
    ShowMessage('by ref pass vars 4+3=' + IntToStr(AddByRef(x,y)));

    
  //With by constant, the compiler optimizes parameters.
    ShowMessage('by const pass literals 2+2=' + IntToStr(AddByConst(2,2)));
    ShowMessage('by const pass vars 4+3=' + IntToStr(AddByConst(x,y)));
end;

  
end.

Default Paremeters

Delphi introduced default parameters with Delphi 4. Default parameters is a form of overloading that allows you to specify a default for a parameter which is used when the routine is called without that particular parameter.

For example, the following function demonstrates default parameter overloading. It allows you to add up to four numbers:

function Add(a, b: integer; c:integer=0; d:integer=0): Integer;
begin
  Result := a+b+c+d;
end;

To test our new function, add the following code to a button click event:

ShowMessage('2+2=' + IntToStr(Add(2,2)));
ShowMessage('2+2+3=' + IntToStr(Add(2,2,3)));
ShowMessage('2+2+3+4=' + IntToStr(Add(2,2,3,4)));