TList
TList or TFPList
TList is a class that can be used to manage collections of pointers. It introduces methods and properties to store the pointers, search in the list of pointers, sort them. It manages its memory by itself, no intervention for that is needed (but, of course, it does not manage the memory of what ever your pointers point to). It has an event notification mechanism which allows to notify of list changes. This slows down some of TList mechanisms, and if no notification is used, TFPList may be used instead.
Both TList and TFPList are declared in classes and that usually already in your uses clause.
The 'managed pointers' may conveniently point to, for example, records, just about an record you care to define.
Here is a very basic unit that will manage a set of notes anout friends of mine -
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, 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;
See also
- Data_Structures,_Containers,_Collections
- An AVL Tree will be faster.