[fpc-pascal] PASCAL programming for Novice

[Joao Morais]

Francisco Reyes wrote:
> Michael Van Canneyt writes:
> 
>> Both Turbo Pascal's Object Pascal and Delphi's object pascal are 
>> supported, depending on which mode you compile in.
> 
> If I have a class like:
> program myclass;
> type
>   THelloWorld = class
>     procedure Put;
>   end;
> var
>   HelloWorld: THelloWorld;
> procedure THelloWorld.Put;
> begin
>   WriteLn('Hello, World!');
> end;
> begin
>   HelloWorld := THelloWorld.Create;
>   HelloWorld.Put;
>   HelloWorld.Free;
> end.
> 
> 
> How do I use that class in another program?

You can:

uses
   UnitWithHello;

...

var
   VHello: THelloWorld;
begin
   VHello := THelloWorld.Create;
   try
     VHello.Put;
   finally
     VHello.Free;
   end;


or you can:

unit UnitWithHello;

type
   THelloWorldClass = class of HelloWorld;
   THelloWorld = class
     procedure Put; virtual; abstract;
   end;

...

unit AnotherUnit;

uses
   UnitWithHello;

...

procedure DoHello(AHelloClass: THelloWorldClass);
var
   VHello: THelloWorld;
begin
   VHello := AHelloClass.Create;
   try
     VHello.Put;
   finally
     VHello.Free;
   end;


or you can:

uses
   UnitWithHello;

type
   THelloWrapper = class
   private
     FHello: THelloWorld;
     function GetHello: THelloWorld;
   protected
     function InternalHelloClass: THelloWorldClass; virtual; abstract;
     property Hello: THelloWorld read GetHello;
   public
     destructor Destroy; override;
   end;

...

destructor THelloWrapper.Destroy;
begin
   FHello.Free;
   inherited;
end;

function THelloWrapper.GetHello: THelloWorld;
begin
   if not Assigned(FHello) then
     FHello := InternalHelloClass.Create;
   Result := FHello;
end;


or you can:

uses
   UnitWithHello;

type
   THelloWrapper = class
   private
     FHello: THelloWorld;
     FHelloClass: THelloWorldClass;
     function GetHello: THelloWorld;
  protected
     property Hello: THelloWorld read GetHello;
   public
     constructor Create(AHelloClass: THelloWorldClass);
     destructor Destroy; override;
   end;

...

constructor THelloWrapper.Create(AHelloClass; THelloWorldClass);
begin
   Assert(Assigned(AHelloClass), 'error');
   inherited Create;
   FHelloClass := AHelloClass;
end;

destructor THelloWrapper.Destroy;
begin
   FHello.Free;
   inherited;
end;

function THelloWrapper.GetHello: THelloWorld;
begin
   if not Assigned(FHello) then
     FHello := FHelloClass.Create;
   Result := FHello;
end;


And so on.

--
Joao Morais