Delphi: how to use TObjectList<T>?

Question

I need to understand how to use the generic Delphi 2009 TObjectList. My non-TObjectList attempt looked like

TSomeClass = class(TObject)
private
  FList1: Array of TList1;
  FList2: Array of TList2;
public
  procedure FillArray(var List: Array of TList1; Source: TSource); Overload;
  procedure FillArray(var List: Array of TList2; Source: TSource); Overload;
end;

Here, TList1 and TList2 inherits the same constructor constructor TParent.Create(Key: string; Value: string);. However, due to different specialization (e.g. different private fields), they will not be of the same type. So I have to write two nearly identical fill methods:

procedure TSomeClass.FillArray(var List: Array of TList1; Source: TSource);
begin
  for i := 0 to Source.List1.Count - 1 do begin
    SetLength(List, Length(List) + 1);
    List[i] := TList1.Create(Source.List1[i].Key, Source.List1[i].Value);
  end;
end;

with FillArray(List: Array of TList2; Source: TSource); being identical, except for the replacement of TList1 with TList2 throughout. As far as I understand, this could be neatly circumvented by using TObjectList and a single fill method; yet, I don't have a clue how to go about this. Do anyone have some good pointers on this? Thanks!

Answer 1

You wouldn't be able to condense that down by using a generic list, since a generic's type is part of the class definition. So a TObjectList<TMyClass1> is different from (and incompatible with) a TObjectList<TMyClass2>. The main benefit of using generic lists over normal TList/TObjectList is improved type safety, with less casts and cleaner code.

Also, if you're using key/value pairs, are you putting them into a list and then retrieving them by searching for a key and returning the associated value? If so, take a look at TDictionary in Generics.Collections. It's a generic key/value hash table that will greatly simplify this process for you.

Answer 2

The Official Embarcadero documentation Wiki on the Generics.Collections.TObjectList contains a simple code example of the TObjectList in action.

I'm not certain exactly what the question is driving at but to address the broad use of a TObjectList, the example initialisation code for a TObjectList might look like this:

var
  List: TObjectList<TNewObject>;
  Obj: TNewObject;
begin
  { Create a new List. }
  List := TObjectList<TNewObject>.Create();

  { Add some items to the List. }
  List.Add(TNewObject.Create('One'));
  List.Add(TNewObject.Create('Two'));

  { Add a new item, but keep the reference. }
  Obj := TNewObject.Create('Three');
  List.Add(Obj);

The example code should give you an idea of what the TObjectList can do but If I've understood the question correctly it seems that you would like to be able to add more than one class type to a single instance of the TObjectList? A TObjectList can only be initiated with a single type so it might be better if you initiated the TObjectList with a Interface or Abstract class that is shared by all of the classes you wish to add to it.

One important difference when using a TObjectList compared to creating your own is the existance of the OwnsObjects property which tells the TObjectList whether it owns the objects you add to it and therefore consequently whether it should manage freeing them itself.

Answer 3

Something like this?

TSomeClass = class
private
  FList1: TArray<TList1>;
  FList2: TArray<TList2>;
public
  procedure FillArray<T>(var List: TArray<T>; Source: TSource);
end;

procedure TSomeClass.FillArray<T>(var List: TArray<T>; Source: TSource);
begin
  for i := 0 to Source.List1.Count - 1 do begin
    SetLength(List, Length(List) + 1);
    List[i] := T.Create(Source.List1[i].Key, Source.List1[i].Value);
  end;
end;

This, or something like it should do what you want, afaict.