Question

I have a number of Typed TLists which I am having problems getting to sort

Normally, for an untyped TList, I would have a function such as:

function SortByJob(Item1: Pointer; Item2: Pointer): Integer;
var
  p1, p2: JobPointer;
begin
   p1 := JobPointer(Item1);
   p2 := JobPointer(Item2);
   if p1.job > p2.job then
      Result := 1
   else
      if p1.job = p2.job then
         Result := 0
      else
         Result := -1
end;

Which would be called by the list

JobList.Sort(SortByJob)

However I have decided in my current application that we want to lock the TLists to certain pointer types, so in the above example we would have the JobList declared as:

JobList: array[0..4] of TList<JobsPointer>;

Now when I call

JobList[0].Sort(SortByJob)

I get a “not enough parameters” error.

Any ideas?

I have compared that if I use the Sort function above on an untyped “standard” TList then it will compile correctly…

Answer 1

A generic list is sorted using a instance of IComparer. Here is an example that sorts a list of integers:

uses Generics.Collections, Generics.Defaults;

procedure TForm1.FormCreate(Sender: TObject);
var
  L : TList<integer>;
begin
  L := TList<integer>.Create;
  L.Add(2);
  L.Add(1);

  L.Sort(TComparer<integer>.Construct(
    function (const L, R: integer): integer
    begin
      Result := L - R;
    end
  )) ;

  L.Free;
end;