Question

I am trying to create a generic list class for use with tiOPF (an Object Persistence Framework for delphi @ http://www.tiopf.com). Specifically I am trying to take an existing generic class (TtiObjectList) and make a generic version that uses TtiObject descenants.

I have limited scope for altering the base classes as they need to compile under D7 – D2009 and Free Pascal. I need to descend from TtiObjectList to keep the existing persistence mechanisms working.

// base class   type     TtiObjectList = class(TtiObject) ...   protected     function GetItems(i: integer): TtiObject; virtual;     procedure SetItems(i: integer; const AValue: TtiObject); virtual;   ...   public     function Add(const AObject : TtiObject): integer; overload; virtual;   ...   end;   

My class is defined as follows:

TtiGenericObjectList<T: TtiObject> = class(TtiObjectList)   protected     function GetItems(i:integer): T; reintroduce;     procedure SetItems(i:integer; const Value: T); reintroduce;   public     function Add(const AObject: T): integer; reintroduce;     property Items[i:integer]: T read GetItems write SetItems; default;   end;  implementation  { TtiGenericObjectList<T> }  function TtiGenericObjectList<T>.Add(const AObject: T): integer;   var obj: TtiObject;   begin     obj:= TtiObject(AObject); /// Invalid typecast   result:= inherited Add(obj);   end;    // alternate add, also fails   function TtiGenericObjectList<T>.Add(const AObject: T): integer;   begin     result:= inherited Add(AObject); /// **There is no overloaded version**  /// **of 'Add' that can be called with these arguments**   end;    function TtiGenericObjectList<T>.GetItems(i: integer): T;   begin     result:= T(inherited GetItems(i)); /// **Invalid typecast  ** end;    procedure TtiGenericObjectList<T>.SetItems(i: integer; const Value: T);   begin     inherited SetItems(i, Value);   end;   

The problem I have is that delphi is not seeing T as a TtiObject descendant. I am getting invalid typecast errors when I do something like:

function TtiGenericObjectList<T>.Add(const AObject: T): integer;   var obj: TtiObject;   begin     obj:= TtiObject(AObject); /// **Invalid typecast***   result:= inherited Add(obj);   end;   

If I don’t do a type cast, then I get overload errors instead as shown in the listing above.

Any ideas where I am going wrong?

Sean

Answer 1

It looks like the problem is not me, but the compiler :).

In the end, I have hacked around it using the following methods

class function TtiGenericObjectList<T>.GenericAsObject(const Value): TObject; begin   Result := TObject(Value); end;  class function TtiGenericObjectList<T>.ObjectAsGeneric(const Value): T; begin   Result := T(Value); end; 

used as follows

function TtiGenericObjectList<T>.Add(const AObject: T): integer; var obj: TtiObject; begin   obj:= TtiObject(GenericAsObject(AObject));   result:= inherited Add(obj);   // replaces the following which gets overload errors //   result:= inherited Add(TtiObject(AObject)); end; 

and

function TtiGenericObjectList<T>.GetItems(i: integer): T; var obj: TtiObject; begin   obj:= inherited GetItems(i);   result:= ObjectAsGeneric(obj);   // replaces the following which gets 'Invalid typecast' errors   // result:= inherited Add(AObject); end; 

I will clean these up a bit and use them till the compiler gets fixed.