Author: Pieter Valentijn

Making properties in your objects and using the accesors to do something. 

Answer:

Property accessors are great.  They give you the power to do stuff like initialize 
an object or produce data that are dependent on the object's other properties. 

You can make properties by declaring them in your interface section and then 
pressing Ctrl+Shift+'c'  it wil make a private variable called Fsomeproperty and a 
set methode called SetSomeProperty. 

In this example i made an object that has a property that holds an other object and 
on the Read of this property i'll check whether it's not nil so i can create it on 
runtime. 

You can also check if a value is within it's bounds on the Property's Set Accessor 

1   type
2     TMySubObject = class
3   
4     end;
5   
6     TMyObject = class
7     private
8       fMySubObject: TMySubObject;
9       FASecondNum: Integer;
10      FAFirstnum: Integer;
11      function getMySubObject: TMySubObject;
12      function GetTotaalOfFirstAndSecondNum: integer;
13      procedure SetFirstnum(const Value: Integer);
14    public
15  
16      property MySubObject: TMySubObject read getMySubObject;
17      property AFirstnum: Integer read FAFirstnum write SetFirstnum;
18      property ASecondNum: Integer read FASecondNum write FAFirstnum;
19      property TotaalOfFirstAndSecondNum: integer read
20        GetTotaalOfFirstAndSecondNum;
21    end;
22  
23  implementation
24  
25  {$R *.DFM}
26  
27  { TMyObject }
28  
29  function TMyObject.getMySubObject: TMySubObject;
30  begin
31    // i check here to see if its assigned
32    if not Assigned(fMySubObject) then
33      fMySubObject := TMySubObject.Create;
34    Result := fMySubObject;
35  end;
36  
37  function TMyObject.GetTotaalOfFirstAndSecondNum: integer;
38  begin
39    Result := FAFirstnum + FASecondNum;
40  end;
41  
42  procedure TMyObject.SetFirstnum(const Value: Integer);
43  begin
44    // here on the set u can check bounds :) .
45    if (Value > 0) and (Value < 1000) then
46      FAFirstnum := Value
47    else
48      raise Exception.Create('Number out of bounds');
49  end;