Languages Focus: Comments

Commenting code generally has three purposes: to document your code, for psuedo coding prior to coding, and to embed compiler directives. Most languages support both a single line comment and a multiple line comment. Some languages also use comments to give instructions to the compiler or interpreter.

//This is a single line comment.

 
{

Multiple line

comment.

}

 
(*
This too is a
multiple line comment.
*)

 
{$IFDEF TEMPOUT}
  //...code here
{$ENDIF}

Compiler Directives

In the $IFDEF example above, the TEMPOUT identifier is not defined, the $IFDEF evaluates to false and skips the code during compile. The code is never compiled.
 

Common Examples:

The following are some commonly used compiler directives you should be familiar with:
 

{$APPTYPE CONSOLE}
The $APPTYPE compiler directive is used in the project source file (.DPR) to tell the compiler to create a Win32 console application or a GUI application. When left out, the default is {$APPTYPE GUI}.

The following is a simple console application:

program Hello;

{$APPTYPE CONSOLE}

uses
  SysUtils;

begin
  WriteLn('Hello, World!')
end.

{$R *.res} and {$R *.DFM}
The $R*.res compiler directive is frequently found in .DPR source files and the $R *.dfm is found in form source units. The $R compiler directive specifies the name of a resource file to be included. The * indicates to include the base name of the file, no extension, it does not mean to include all files (it is not a wild card character).

{$E cpl}

The $E directive sets the extension of the executable.The following is the project source code from a basic Control Panel Applet. Notice the extension is changed to .cpl. Also note the header starts with library instead of with program. There are three types of headings: program, library, and package.

library Project3;

uses

  CtlPanel,

  AppletModule1 in 'AppletModule1.pas' {AppletModule1AppletModule: TAppletModule};

exports CPlApplet;

{$R *.RES}

{$E cpl}

begin

  Application.Initialize;

  Application.CreateForm(TAppletModule1AppletModule, AppletModule1AppletModule);

  Application.Run;

end.

Custom Compiler Directives

In addition to using the many built-in compiler directives, you can define your own. Here is a simple example.

//Switch on debug mode.

{$Define MYDEBUGMODE}

 
{$IfDef MYDEBUGMODE}
  ShowMessage('In debug mode.');
{$Else}
  ShowMessage('Out of debug mode.');
{$EndIf}

 
//Switch off debug mode.
{$UnDef MYDEBUGMODE}

 
{$IfDef MYDEBUGMODE}
  ShowMessage('In debug mode.');
{$Else}
  ShowMessage('Out of debug mode.');
{$EndIf}

Notice the $IFDEF above does NOT use a Then. If you put one, it will work because Delphi for Win32 sees the Then as a comment. However, Delphi Prism does not and you will get an error.