Why is no output displayed? - delphi

Question

I started learning classes and objects programming today. There is code in the handbook that I must copy to run and save. I need to create a class(TLine) and use that class for instantiating an object.
Problem : No output is displayed in my RichEdit component. I copied the code exactly from the book to delphi, but no output is displayed.
How the output should look: "**********"
My class:
unit Lines_U;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
Type
TLine = Class
Public
fSize : integer;
fPattern : char;
public
Constructor Create;
Procedure Draw(Var line: string);
end;
implementation
{ TLine }
Constructor TLine.Create;
begin
fSize := 10;
fPattern := '*';
end;
Procedure TLine.Draw(Var line: string);
Var
loop : integer;
begin
for loop := 1 to fSize do
begin
line := line + fPattern;
end;
end;
end.
Code for instantiating the Object of the TLine Class:
unit UseLine_U;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Lines_U, StdCtrls, ComCtrls;
type
TForm1 = class(TForm)
redOut: TRichEdit;
Procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
line : TLine;
implementation
{$R *.dfm}
Procedure TForm1.FormCreate(Sender: TObject);
Var tempLine : string;
begin
line := TLine.Create;
line.Draw(tempLine);
redOut.Lines.Add(tempLine);
end;
end.

Answer 1

The reason your code is not running is that your event handler Form1.FormCreate is not linked to the OnCreate event. Restore the link in the object inspector.
About event handlers
Never write event handlers (all those procedures starting with On...) manually. Always use the Object inspector to create them.
If you double click on an event, Delphi will create a code template for you that you can fill with data.
Make sure your event handlers are filled in the object inspector. If not they will not work (as you've seen).
If you want to remove an event handler do not remove it in the object inspector, but reduce the code inside the event handling procedure back to the empty template.
Delphi will see that it is empty and remove it on the next compile.
About your code
Other than the missing link there is nothing wrong with your code. It runs just fine.
There are a few style issues though, these have no bearing on the operation, but are important none the less.
Here's how I would rewrite your code.
unit Lines_U;
interface
//only import units that you actually use.
type //please type reserved words in all lowercase, this is Pascal not VB.
TLine = class
private //make data members private.
fSize : integer;
fPattern : char;
public
constructor Create;
procedure Draw(var line: string);
property Size: integer read fSize write fSize; //Use properties to expose data members.
property Pattern: char read fPattern write fPattern;
end;
implementation
{ TLine }
constructor TLine.Create;
begin
inherited; //make the inherited call in your constructor explicit.
fSize := 10;
fPattern := '*';
end;
procedure TLine.Draw(var line: string);
//var
//loop : integer; //use consistent indentation
begin
//Changing a string ten times in a row is inefficient.
//try to do your changes all at once.
//for loop := 1 to fSize do begin
// line := line + fPattern;
//end;
Line:= Line + StringOfChar(fPattern, fSize);
end;
end.
Your form:
unit UseLine_U;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ComCtrls, Lines_U;
//put your own unit last, to prevent name clashes with built in classes and functions.
type
TForm1 = class(TForm)
//note that the {nothing} line is really **published**.
//And data members should be private
//Line : TLine; //Line should be private.
RedOut: TRichEdit;
procedure FormCreate(Sender: TObject);
private
//Prefix all private data with `F` for Field.
FLine: TLine; //Line should be a item in the form, not a global var.
public
property Line: TLine read FLine; //read only access to line.
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
var
tempLine : string;
i: integer;
begin
//tempLine:= ''; //local variables should be initialized.
//However strings are always initialized to '', because they are managed types.
//everything else will contain random data unless you fill it!
FLine := TLine.Create;
Line.Draw(tempLine);
i:= 0; //init i, otherwise it will be random!
while i < 5 do begin //always use `begin-end` in loops, never a naked `do`
RedOut.Lines.Add(tempLine);
i:= i + 1;
end; {while} //I like to annotate my loop `end`s, but that's just me.
FreeAndNil(FLine); //Dispose of TLine when you're done with it.
end;
end.
I can think of other things, but I don't want to overload you.