Delphi Basics : Unit command

DelphiBasics

Unit
Keyword

Defines the start of a unit file - a Delphi module

keyword Unit(Unit Name;
Interface
...
Implementation
...
{Initialization
...}
{Finalization
...}
end. // Note terminating dot

Description

The Unit keyword is fundamental to Delphi, defining the start of a module.

Each Delphi module is stored in a Name.pas file, where Name must match the Unit Name.

Each unit comprises a set of interface definitions. Do not confuse these with class interfaces - the unit interface is the external interface to the module. It defines to any user of the unit what it contains. These interface definitions define private, protected, public and Published data, types and routine declarations.

Any routines defined in the interface must be implemented in the Implementation section. All function and procedure definitions must have exactly redefined arguments - see procedure and function information.

Optionally, you may have Initialization and Finalization sections. These contain statements executed when the unit is loaded and when it terminates, respectively.

If you only define an Initialization section, you may replace this keyword with Begin.

Related commands

Function		Defines a subroutine that returns a value
Implementation		Starts the implementation (code) section of a Unit
Interface		Used for Unit external definitions, and as a Class skeleton
Private		Starts the section of private data and methods in a class
Procedure		Defines a subroutine that does not return a value
Property		Defines controlled access to class fields
Protected		Starts a section of class private data accesible to sub-classes
Public		Starts an externally accessible section of a class
Published		Starts a published externally accessible section of a class

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Example code : A simple example

// 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;

// External interface to this unit module
interface

uses
  Forms, Dialogs;

type
  // Define the form class used by this unit
  TForm1 = class(TForm)
    // Define a procedure that must be implemented in the
    // implementation section below
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  // Define a form - can be referenced in the Implementation section
  Form1: TForm1;

// Implements the interface declarations above
implementation
{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  // Set and show the form title
  Form1.Caption := 'Test program';
  ShowMessage('Running = '+Form1.Caption);
end;

initialization
  // Tell the user we are starting this unit
  ShowMessage('Initialising');

end.

Show full unit code

Initialising
Running Test program