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 is 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 about some 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 -
To load some data we would perhaps use -
procedure AddNote(N: string; Silly: boolean; NHeads: integer); var aPRec : PNote; // Not a record, just a pointer to one. begin new(aPRec); aPRec^.Name := N; aPRec^.IsSilly := Silly; aPRec^.NumbHeads := NHeads; NoteList.Add(aPRec); end; procedure TForm1.LoadData(); 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;