TList

TList and TFPList are classes that can be used to manage collections of pointers. These pointers may conveniently point to, for example, records as in the following example -

type
  	PNote=^TNote;
  	TNote = record
		Name : ANSIString;
        IsSilly : boolean;
        NumbHeads : integer;
	end;

type
    // Note we can use TFPList (faster) or TList (and get event notification)
   TNoteList = class(TList)
   private
    	function Get(Index: integer): PNote;
    public
        destructor Destroy; override;
        function Add(ANote : PNote) : integer;
        function FindName(const Name : ANSIString) : PNote;
        property Items[Index: integer]: PNote read Get; default;
    end;


implementation

function TNoteList.Get(Index: integer): PNote;
begin
    Result := PNote(inherited get(Index));
end;

function TNoteList.Add(ANote: PNote): integer;
begin
    result := inherited Add(ANote);
end;

function TNoteList.FindName(const Name: ANSIString): PNote;
var
    P : PNote;
begin
    Result := Nil;
    for P in self do
        if P^.Name = Name then exit(P);
end;

destructor TNoteList.Destroy;
var
  I : integer;
begin
	for I := 0 to Count-1 do begin
    	dispose(Items[I]);
	end;
	inherited Destroy;
end;                                         
....

In this example, that can use either TList or TFPList, we make a record that will hold one "item of data", and, to make things easy, we also declare a type pointer to that sort of record. Its sensible to put code like this in its own unit and a practical use will almost certainly require a few extra methods to manage the data it self.

To use this example, we need to declare a variable of type TNoteList and then create and destroy it. So, perhaps in a FormCreate we would have -

NoteList := TNoteList.Create;

and in FormDestroy we might have -

NoteList.free;

To load some data we would perhaps use -

procedure TForm1.LoadData();
var
    P : PNote;     // P is a pointer to a note record, somewhere !
begin
    AddNote('Joe', False, 1);
    AddNote('Harry', True, 1);
    AddNote('David', True, 2);     // Tasmanian
    AddNote('Boris', False, 1);
end;

and to use that data we might have -

procedure TForm1.UseData();
var
    P : PNote;     // P is a pointer to a note record, somewhere !
begin
    P := NoteList.FindName('David');
    if P <> nil then
        writeln('David has ' + P^.NumbHeads.ToString + ' heads');
    for P in NoteList do
        if P^.IsSilly then
            writeln(P^.Name + ' is silly');
    if NoteList.FindName('George') = nil then
        writeln('George is not listed');
end;