Question

Based on an earlier post, I ve written the following code. Please excuse the verbosity of this post. I believe it s better for all parties to have the full code available to test and comment on.

program sandbox;
{$APPTYPE CONSOLE}

uses
  SysUtils,
  Generics.Collections;

type
  TDataType = class
    // Stuff common to TInt and TStr
  end;

  TInt = class(TDataType)
    FValue:  integer;
    constructor Create(Value, Low, High: integer);
  end;

  TStr = class(TDataType)
    FValue: string;
    constructor Create(Value: string; Length: integer);
  end;

  TSomeClass = class
    FIntList: TList<TInt>;
    FStrList: TList<TStr>;
    procedure AddToList<T: TDataType>(Element: T);
    constructor Create();
    procedure Free();
  end;

constructor TInt.Create(Value, Low, High: Integer);
begin
  inherited Create();
  FValue := Value;   
end;

constructor TStr.Create(Value: string; Length: Integer);
begin
  inherited Create();
  FValue := Value;
end;

procedure TSomeClass.AddToList<T>(Element: T);
begin
  if TObject(Element) is TInt then
    FIntList.Add(Element)
  else if TObject(Element) is TStr then
    FStrList.Add(Element);
end;

constructor TSomeClass.Create();
begin
  inherited;
  FIntList := TList<TInt>.Create();
  FStrList := TList<TStr>.Create();
end;

procedure TSomeClass.Free();
var
  SomeIntItem: TInt;
  SomeStrItem: TStr;
begin
  for SomeIntItem in FIntList do begin
    SomeIntItem.Free();
  end;

  for SomeStrItem in FStrList do begin
    SomeStrItem.Free;
  end;

  FIntList.Free();
  FStrList.Free();
end;

var
  Inst: TSomeClass;

begin
  try
    { TODO -oUser -cConsole Main : Insert code here }

    Inst := TSomeClass.Create;
    Inst.AddToList(TInt.Create(100, 0, 101));
    Inst.AddToList(TStr.Create( Test , 10));
    Inst.Free;

  except
    on E:Exception do
    Writeln(E.Classname,  :  , E.Message);
  end;
end.

Note that the constructors of TInt and TStr in the real world would utilize the Low, High: integer and Length: integer parameters as well. I m having an "E2089 Invalid typecast" at if TObject(Element) is TInt then and else if TObject(Element) is TStr then running Delphi 2009. Do anyone know why this happens?

Edit: Please note that TInt and TStr are just two of possibly 10-20 other types; otherwise overloading is the tool for the job. :)

Answer 1

Rethink your design. You may just use overloading instead of a generic type parameter, like this:

procedure Add (SomeString : TString); overload;
procedure Add (SomeInt : TInt); overload;

Or if you want to use polymorphism do what Gamecat suggested and just pass the base type as the parameter, using is on that parameter:

procedure Add (Element : TDataType);

Like Rob pointed out in a comment to your previous question: it s not really generic if you allow only two types and have conditionals based on the actual type. So generics might be the wrong tool here.

Hope that helps.

Answer 2

The problem is not with the generics. You add a TDataType to a list that expects TInt or TStr:

procedure TSomeClass.AddToList<T>(Element: T);
begin
  if TObject(Element) is TInt then
    FIntList.Add(TInt(Element))
  else if TObject(Element) is TStr then
    FStrList.Add(TStr(Element));
end;

Solves the problem.

But why not use:

procedure TSomeClass.AddToList(Element: TDataType);
begin
  if Element is TInt then
    FIntList.Add(TInt(Element))
  else if Element is TStr then
    FStrList.Add(TStr(Element));
end;

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