Question

Summarization:

TList.IndexOf (TList defined in the unit Classes.pas) iterates linearly through the contained items, and compares the reference. TList.IndexOf (TList defined in the unit Generics.Collections.pas) also iterates linearly through the contained items, but uses a comparer to compare whether the items are equal.

Both TList.Sort and TList.Sort can use a comparer.

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For an instance of the TForceList type defined in the following unit, I could use

instance.Sort(@ForceCompare);

to QuickSort it using its Value field as sorting criteria. However, when I call

instance.IndexOf(AnotherInstance)

I want to use its ElementZ field as comparing criteria, i.e., the ForceEqual function. I am wondering how can I achieve this?

PS: If the generics collection is used, I guess I could use

TList<TForce>.Create(TComparer<TForce>.Construct(ForceEqual));

The unit:

    unit uChemParserCommonForce;

    interface

    uses
      uMathVector3D,
      Contnrs;

    type

      TForce = class;
      TForceList = class;

      TForce = class
      private
        FElementZ: Integer;
        FValue: TVector3D;
      public
        property ElementZ: Integer read FElementZ;
        property Value: TVector3D read FValue;
        constructor Create(aElementZ: Integer; aX, aY, aZ: Double);
        function ToString(): string; {$IF DEFINED(FPC) OR DEFINED(VER210)} override; {$IFEND}
      end;

      // Mastering Delphi 6 - Chapter 5 -
      TForceList = class(TObjectList)
      protected
        procedure SetObject(Index: Integer; Item: TForce);
        function GetObject(Index: Integer): TForce;
      public
        function Add(Obj: TForce): Integer;
        procedure Insert(Index: Integer; Obj: TForce);
        property Objects[Index: Integer]: TForce read GetObject
          write SetObject; default;
      end;

    function ForceCompare(Item1, Item2: Pointer): Integer;
    function ForceEqual(Item1, Item2: Pointer): Boolean;

    implementation

    uses
      Math, SysUtils;

    function ForceCompare(Item1, Item2: Pointer): Integer;
    begin
      // Ascendent
      //  Result := CompareValue(TForce(Item1).Value.Len, TForce(Item2).Value.Len);
      // Descendent
      Result := CompareValue(TForce(Item2).Value.Len, TForce(Item1).Value.Len);
    end;

    function ForceEqual(Item1, Item2: Pointer): Boolean;
    begin
      Result := TForce(Item1).ElementZ = TForce(Item2).ElementZ;
    end;

    constructor TForce.Create(aElementZ: Integer; aX, aY, aZ: Double);
    begin
      FElementZ := aElementZ;
      FValue := TVector3D.Create(aX, aY, aZ);
    end;

    function TForce.ToString: string;
    begin
      Result := IntToStr(FElementZ) + ' X: ' + FloatToStr(FValue.X) + ' Y: ' +
        FloatToStr(FValue.Y) + ' Z: ' + FloatToStr(FValue.Z);
    end;

    { TForceList }

    function TForceList.Add(Obj: TForce): Integer;
    begin
      Result := inherited Add(Obj);
    end;

    procedure TForceList.SetObject(Index: Integer; Item: TForce);
    begin
      inherited SetItem(Index, Item);
    end;

    function TForceList.GetObject(Index: Integer): TForce;
    begin
      Result := inherited GetItem(Index) as TForce;
    end;

    procedure TForceList.Insert(Index: Integer; Obj: TForce);
    begin
      inherited Insert(Index, Obj);
    end;

    end.

Answer 1

The non-generic TObjectList uses TList.IndexOf, which simply iterates through the internal array and compares pointers.

Likewise, the generic TObjectList<T> uses TList<T>.IndexOf, which uses an IComparer. TList<T>.Sort uses TArray.Sort<T> passing in whatever IComparer was assigned at the list’s creation.

The comparer is private and only assigned in the list constructor so I don’t see an easy way to override this behavior.

Update

TList<T> provides and overloaded Sort that accepts a comparer as an argument, without modifying the private comparer. So you can sort using one comparer and indexof can use a different one.