Delphi: Using Enumerators to filter TList<T: class> by class type?
- by afrazier
Okay, this might be confusing. What I'm trying to do is use an enumerator to only return certain items in a generic list based on class type.
Given the following hierarchy:
type
TShapeClass = class of TShape;
TShape = class(TObject)
private
FId: Integer;
public
function ToString: string; override;
property Id: Integer read FId write FId;
end;
TCircle = class(TShape)
private
FDiameter: Integer;
public
property Diameter: Integer read FDiameter write FDiameter;
end;
TSquare = class(TShape)
private
FSideLength: Integer;
public
property SideLength: Integer read FSideLength write FSideLength;
end;
TShapeList = class(TObjectList<TShape>)
end;
How can I extend TShapeList such that I can do something similar to the following:
procedure Foo;
var
ShapeList: TShapeList;
Shape: TShape;
Circle: TCircle;
Square: TSquare;
begin
// Create ShapeList and fill with TCircles and TSquares
for Circle in ShapeList<TCircle> do begin
// do something with each TCircle in ShapeList
end;
for Square in ShapeList<TSquare> do begin
// do something with each TSquare in ShapeList
end;
for Shape in ShapeList<TShape> do begin
// do something with every object in TShapeList
end;
end;
I've tried extending TShapeList using an adapted version of Primoz Gabrijelcic's bit on Parameterized Enumerators using a factory record as follows:
type
TShapeList = class(TObjectList<TShape>)
public
type
TShapeFilterEnumerator<T: TShape> = record
private
FShapeList: TShapeList;
FClass: TShapeClass;
FIndex: Integer;
function GetCurrent: T;
public
constructor Create(ShapeList: TShapeList);
function MoveNext: Boolean;
property Current: T read GetCurrent;
end;
TShapeFilterFactory<T: TShape> = record
private
FShapeList: TShapeList;
public
constructor Create(ShapeList: TShapeList);
function GetEnumerator: TShapeFilterEnumerator<T>;
end;
function FilteredEnumerator<T: TShape>: TShapeFilterFactory<T>;
end;
Then I modified Foo to be:
procedure Foo;
var
ShapeList: TShapeList;
Shape: TShape;
Circle: TCircle;
Square: TSquare;
begin
// Create ShapeList and fill with TCircles and TSquares
for Circle in ShapeList.FilteredEnumerator<TCircle> do begin
// do something with each TCircle in ShapeList
end;
for Square in ShapeList.FilteredEnumerator<TSquare> do begin
// do something with each TSquare in ShapeList
end;
for Shape in ShapeList.FilteredEnumerator<TShape> do begin
// do something with every object in TShapeList
end;
end;
However, Delphi 2010 is throwing an error when I try to compile Foo about Incompatible types: TCircle and TShape. If I comment out the TCircle loop, then I get a similar error about TSquare. If I comment the TSquare loop out as well, the code compiles and works. Well, it works in the sense that it enumerates every object since they all descend from TShape. The strange thing is that the line number that the compiler indicates is 2 lines beyond the end of my file. In my demo project, it indicated line 177, but there's only 175 lines.
Is there any way to make this work? I'd like to be able to assign to Circle directly without going through any typecasts or checking in my for loop itself.
