Which lists could serve as temporary lists? - delphi

Question

When working with lists of items where the lists just serve as a temporary container - which list types would you recommend me to use?
I
don't want to destroy the list manually
would like to use a built-in list type (no frameworks, libraries, ...)
want generics
Something which would make this possible without causing leaks:
function GetListWithItems: ISomeList;
begin
Result := TSomeList.Create;
// add items to list
end;
var
Item: TSomeType;
begin
for Item in GetListWithItems do
begin
// do something
end;
end;
What options do I have? This is about Delphi 2009 but for the sake of knowledge please also mention if there is something new in this regard in 2010+.

Answer 1

An (somehow ugly) workaround for this is to create an 'autodestroy' interface along with the list. It must have the same scope so that when the interface is released, your list is destroyed too.
type
IAutoDestroyObject = interface
end;
TAutoDestroyObject = class(TInterfacedObject, IAutoDestroyObject)
strict private
FValue: TObject;
public
constructor Create(obj: TObject);
destructor Destroy; override;
end;
constructor TAutoDestroyObject.Create(obj: TObject);
begin
inherited Create;
FValue := obj;
end;
destructor TAutoDestroyObject.Destroy;
begin
FreeAndNil(FValue);
inherited;
end;
function CreateAutoDestroyObject(obj: TObject): IAutoDestroyObject;
begin
Result := TAutoDestroyObject.Create(obj);
end;
FList := TObjectList.Create;
FListAutoDestroy := CreateAutoDestroyObject(FList);
Your usage example gets more complicated, too.
type
TSomeListWrap = record
List: TSomeList;
AutoDestroy: IAutoDestroyObject;
end;
function GetListWithItems: TSomeListWrap;
begin
Result.List := TSomeList.Create;
Result.AutoDestroy := CreateAutoDestroyObject(Result.List);
// add items to list
end;
var
Item: TSomeItem;
begin
for Item in GetListWithItems.List do
begin
// do something
end;
end;

Answer 2

Inspired by Barry Kelly's blog post here you could implement smart pointers for your purpose like this :
unit Unit80;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, Generics.Collections;
type
TMyList =class( TList<Integer>)
public
destructor Destroy; override;
end;
TLifetimeWatcher = class(TInterfacedObject)
private
FWhenDone: TProc;
public
constructor Create(const AWhenDone: TProc);
destructor Destroy; override;
end;
TSmartPointer<T: class> = record
strict private
FValue: T;
FLifetime: IInterface;
public
constructor Create(const AValue: T); overload;
class operator Implicit(const AValue: T): TSmartPointer<T>;
property Value: T read FValue;
end;
TForm80 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
function getList : TSmartPointer<TMyList>;
{ Public declarations }
end;
var
Form80: TForm80;
implementation
{$R *.dfm}
{ TLifetimeWatcher }
constructor TLifetimeWatcher.Create(const AWhenDone: TProc);
begin
FWhenDone := AWhenDone;
end;
destructor TLifetimeWatcher.Destroy;
begin
if Assigned(FWhenDone) then
FWhenDone;
inherited;
end;
{ TSmartPointer<T> }
constructor TSmartPointer<T>.Create(const AValue: T);
begin
FValue := AValue;
FLifetime := TLifetimeWatcher.Create(procedure
begin
AValue.Free;
end);
end;
class operator TSmartPointer<T>.Implicit(const AValue: T): TSmartPointer<T>;
begin
Result := TSmartPointer<T>.Create(AValue);
end;
procedure TForm80.Button1Click(Sender: TObject);
var i: Integer;
begin
for I in getList.Value do
Memo1.Lines.Add(IntToStr(i));
end;
{ TMyList }
destructor TMyList.Destroy;
begin
ShowMessage('Kaputt');
inherited;
end;
function TForm80.getList: TSmartPointer<TMyList>;
var
x: TSmartPointer<TMyList>;
begin
x := TMyList.Create;
Result := x;
with Result.Value do
begin
Add(1);
Add(2);
Add(3);
end;
end;
end.
Look at getList and Button1click to see its usage.

Answer 3

To fully support what you're after the language would need to support 2 things:
Garbage collector. That's the only thing that gives you the freedom to USE something without bothering with freeing it. I'd welcome a change in Delphi that gave us even partial support for this.
The possibility to define local, initialized variables. Again, I'd really love to see something along those lines.
Meanwhile, the closest you can get is to use Interfaces in place of garbage collection (because interfaces are reference-counted, once they go out of scope they'll be released). As for initialized local variables, you could use a trick similar to what I'm describing here: Declaring block level variables for branches in delphi
And for the sake of fun, here's a Console application that demonstrates the use of "fake" local variables and Interfaces to obtain temporary lists that are readily initialized will be automatically freed:
program Project1;
{$APPTYPE CONSOLE}
uses
SysUtils, Generics.Collections;
type
ITemporaryLocalVar<T:constructor> = interface
function GetL:T;
property L:T read GetL;
end;
TTemporaryLocalVar<T:constructor> = class(TInterfacedObject, ITemporaryLocalVar<T>)
public
FL: T;
constructor Create;
destructor Destroy;override;
function GetL:T;
end;
TTempUse = class
public
class function L<T:constructor>: ITemporaryLocalVar<T>;
end;
{ TTemporaryLocalVar<T> }
constructor TTemporaryLocalVar<T>.Create;
begin
FL := T.Create;
end;
destructor TTemporaryLocalVar<T>.Destroy;
begin
TObject(FL).Free;
inherited;
end;
function TTemporaryLocalVar<T>.GetL: T;
begin
Result := FL;
end;
{ TTempUse }
class function TTempUse.L<T>: ITemporaryLocalVar<T>;
begin
Result := TTemporaryLocalVar<T>.Create;
end;
var i:Integer;
begin
try
with TTempUse.L<TList<Integer>> do
begin
L.Add(1);
L.Add(2);
L.Add(3);
for i in L do
WriteLn(i);
end;
ReadLn;
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.

Answer 4

The standard list classes, like TList, TObjectList, TInterfaceList, etc, do not implement automated lifecycles, so you have to free them manually when you are done using them. If you want a list class that is accessible via an interface, you have to implement that yourself, eg:
type
IListIntf = interface
...
end;
TListImpl = class(TInterfacedObject, IListIntf)
private
FList: TList;
...
public
constructor Create; override;
destructor Destroy; override;
...
end;
constructor TListImpl.Create;
begin
inherited;
FList := TList.Create;
end;
destructor TListImpl.Destroy;
begin
FList.Free;
inherited;
end;
function GetListWithItems: IListIntf;
begin
Result := TListImpl.Create;
// add items to list
end;

Answer 5

Another option is to implement a generic IEnumerable adapter (as one of the ways to satisfy the for .. in compiler requirement) and rely on reference counting of the interface. I don't know if the following works in Delphi 2009, it seems to work in Delphi XE:
program Project1;
{$APPTYPE CONSOLE}
uses
SysUtils, Classes,
Generics.Collections;
type
// IEnumerator adapter for TEnumerator
TInterfacedEnumerator<T> = class(TInterfacedObject, IEnumerator<T>)
private
FEnumerator: TEnumerator<T>;
public
constructor Create(AEnumerator: TEnumerator<T>);
destructor Destroy; override;
function IEnumerator<T>.GetCurrent = GetCurrent2;
{ IEnumerator }
function GetCurrent: TObject;
function MoveNext: Boolean;
procedure Reset;
{ IEnumerator<T> }
function GetCurrent2: T;
end;
// procedure used to fill the list
TListInitProc<T> = reference to procedure(List: TList<T>);
// IEnumerable adapter for TEnumerable
TInterfacedEnumerable<T> = class(TInterfacedObject, IEnumerable<T>)
private
FEnumerable: TEnumerable<T>;
public
constructor Create(AEnumerable: TEnumerable<T>);
destructor Destroy; override;
class function Construct(InitProc: TListInitProc<T>): IEnumerable<T>;
function IEnumerable<T>.GetEnumerator = GetEnumerator2;
{ IEnumerable }
function GetEnumerator: IEnumerator; overload;
{ IEnumerable<T> }
function GetEnumerator2: IEnumerator<T>; overload;
end;
{ TInterfacedEnumerator<T> }
constructor TInterfacedEnumerator<T>.Create(AEnumerator: TEnumerator<T>);
begin
inherited Create;
FEnumerator := AEnumerator;
end;
destructor TInterfacedEnumerator<T>.Destroy;
begin
FEnumerator.Free;
inherited Destroy;
end;
function TInterfacedEnumerator<T>.GetCurrent: TObject;
begin
Result := TObject(GetCurrent2);
end;
function TInterfacedEnumerator<T>.GetCurrent2: T;
begin
Result := FEnumerator.Current;
end;
function TInterfacedEnumerator<T>.MoveNext: Boolean;
begin
Result := FEnumerator.MoveNext;
end;
procedure TInterfacedEnumerator<T>.Reset;
begin
// ?
end;
{ TInterfacedEnumerable<T> }
class function TInterfacedEnumerable<T>.Construct(InitProc: TListInitProc<T>): IEnumerable<T>;
var
List: TList<T>;
begin
List := TList<T>.Create;
try
if Assigned(InitProc) then
InitProc(List);
Result := Create(List);
except
List.Free;
raise;
end;
end;
constructor TInterfacedEnumerable<T>.Create(AEnumerable: TEnumerable<T>);
begin
inherited Create;
FEnumerable := AEnumerable;
end;
destructor TInterfacedEnumerable<T>.Destroy;
begin
FEnumerable.Free;
inherited Destroy;
end;
function TInterfacedEnumerable<T>.GetEnumerator: IEnumerator;
begin
Result := GetEnumerator2;
end;
function TInterfacedEnumerable<T>.GetEnumerator2: IEnumerator<T>;
begin
Result := TInterfacedEnumerator<T>.Create(FEnumerable.GetEnumerator);
end;
type
TSomeType = record
X, Y: Integer;
end;
function GetList(InitProc: TListInitProc<TSomeType>): IEnumerable<TSomeType>;
begin
Result := TInterfacedEnumerable<TSomeType>.Construct(InitProc);
end;
procedure MyInitList(List: TList<TSomeType>);
var
NewItem: TSomeType;
I: Integer;
begin
for I := 0 to 9 do
begin
NewItem.X := I;
NewItem.Y := 9 - I;
List.Add(NewItem);
end;
end;
procedure Main;
var
Item: TSomeType;
begin
for Item in GetList(MyInitList) do // you could also use an anonymous procedure here
Writeln(Format('X = %d, Y = %d', [Item.X, Item.Y]));
Readln;
end;
begin
try
ReportMemoryLeaksOnShutdown := True;
Main;
except
on E: Exception do
begin
ExitCode := 1;
Writeln(Format('[%s] %s', [E.ClassName, E.Message]));
end;
end;
end.

Answer 6

No, not 'out of the box' in Delphi.
I know that you don't need a library but you may be interessed by the principle of TDynArray.

Answer 7

In Jedi Code Library, exist the Guard function that already implements what
Gabr's code does.