Title: Garbage Collector for Delphi Objects and Components
Question: Garbage Collector for Delphi Objects and Components.
Create objects and let them be freed automatically for you without using try..finally.
Advanced techniques for handling several exceptions.
Answer:
Garbage Collector For Delphi Objects and Components
One of the fundamental questions in object oriented programming is how
the memory management of objects should be done. Different languages
take different approaches. C++ calls the constructor/destructor of
stack allocated objects automaticaly, but for heap allocated objects
you have to do it manually and there is no try..finally statement. In
Java you create the objects when you need them and the garabage collector
takes care of the memory cleanup, but there are no destructors, so you
can not explictly say you don't need an object anymore and there is little
control over the process of garbage collection.
Delphi provides three ways of object management :
1. Create/destroy the objects using try..finally.
2. Use TComponent descendants - create a component and
let its owner free it.
3. Interfaces - when the reference count for an interface
becomes 0 the object which implements it is destroyed.
Interfaces are great for new development - start using
them ! ;) - but sometimes they can be an overhead because there are
two declarations of the same thing. Also, most of the base VCL classes
- TList, TStream, etc. - are not components or interface enabled -
so you have to create/destroy them explicitly.
THE OBJECT SAFE
The Delphi help says you shouldn't mix the TComponent
owner approach with the interface memory management, but as always
the forbidden fruit is the sweetest ;). As you'll see it is really
useful to have a TComponent descendant which implements an interface
and at the same time IS reference counted so when it goes out of scope
it frees itself and all the components it owns. We'll extend it so it
keeps a list of TObjects and frees them too.
Lets name the interface IObjectSafe and the reference counted
TComponent descendent which implements it - TObjectSafe.
Here is the source code for SafeUnit.pas :
unit SafeUnit;interfaceuses Classes;type IObjectSafe = interface function Safe : TComponent; function New (out aReference {: Pointer}; const aObject : TObject) : IObjectSafe; procedure Guard (const aObject : TObject); procedure Dispose (var aReference {: Pointer}); end; IExceptionSafe = interface procedure SaveException; end;function ObjectSafe : IObjectSafe; overload;function ObjectSafe (out aObjectSafe : IObjectSafe) : IObjectSafe; overload;function ExceptionSafe : IExceptionSafe;function IsAs (out aReference {: Pointer}; const aObject : TObject; const aClass : TClass) : Boolean;implementationuses Windows, SysUtils;type TExceptionSafe = class (TInterfacedObject, IExceptionSafe) private FMessages : String; public destructor Destroy; override; procedure SaveException; end; TInterfacedComponent = class (TComponent) private FRefCount : Integer; protected function _AddRef : Integer; stdcall; function _Release : Integer; stdcall; public procedure BeforeDestruction; override; end; TAddObjectMethod = procedure (const aObject : TObject) of object; TObjectSafe = class (TInterfacedComponent, IObjectSafe) private FObjects : array of TObject; FEmptySlots : array of Integer; AddObject : TAddObjectMethod; procedure AddObjectAtEndOfList (const aObject : TObject); procedure AddObjectInEmptySlot (const aObject : TObject); procedure RemoveObject (const aObject : TObject); public constructor Create (aOwner : TComponent); override; destructor Destroy; override; function Safe : TComponent; function New (out aReference; const aObject : TObject) : IObjectSafe; procedure Guard (const aObject : TObject); procedure Dispose (var aReference) ; end;function TInterfacedComponent._AddRef : Integer;begin Result := InterlockedIncrement (FRefCount);end;function TInterfacedComponent._Release : Integer;begin Result := InterlockedDecrement (FRefCount); if Result = 0 then Destroy;end;procedure TInterfacedComponent.BeforeDestruction;begin if FRefCount 0 then raise Exception.Create (ClassName + ' not freed correctly');end;{ TObjectSafe }constructor TObjectSafe.Create (aOwner : TComponent);begin inherited Create (aOwner); AddObject := AddObjectAtEndOfList;end;destructor TObjectSafe.Destroy; var aIndex : Integer; aComponent : TComponent;begin with ExceptionSafe do begin for aIndex := High (FObjects) downto Low (FObjects) do try FObjects [aIndex].Free; except SaveException; end; for aIndex := Pred (ComponentCount) downto 0 do try aComponent := Components [aIndex]; try RemoveComponent (aComponent); finally aComponent.Free; end; except SaveException; end; try inherited Destroy; except SaveException; end; end;end;function TObjectSafe.Safe : TComponent;begin Result := Self;end;procedure TObjectSafe.AddObjectAtEndOfList (const aObject : TObject);begin SetLength (FObjects, Succ (Length (FObjects))); FObjects [High (FObjects)] := aObject;end;procedure TObjectSafe.AddObjectInEmptySlot (const aObject : TObject);begin FObjects [FEmptySlots [High (FEmptySlots)]] := aObject; SetLength (FEmptySlots, High (FEmptySlots)); if Length (FEmptySlots) = 0 then AddObject := AddObjectAtEndOfList;end;procedure TObjectSafe.RemoveObject (const aObject : TObject); var aIndex : Integer;begin for aIndex := High (FObjects) downto Low (FObjects) do begin if FObjects [aIndex] = aObject then begin FObjects [aIndex] := Nil; SetLength (FEmptySlots, Succ (Length (FEmptySlots))); FEmptySlots [High (FEmptySlots)] := aIndex; AddObject := AddObjectInEmptySlot; Exit; end; end;end;procedure TObjectSafe.Dispose (var aReference);begin try try if TObject (aReference) is TComponent then RemoveComponent (TComponent (TObject (aReference))) else RemoveObject (TObject (aReference)); finally TObject (aReference).Free; end; finally TObject (aReference) := Nil; end;end;procedure TObjectSafe.Guard (const aObject : TObject);begin try if aObject is TComponent then begin if TComponent (aObject).Owner Self then InsertComponent (TComponent (aObject)); end else AddObject (aObject); except aObject.Free; raise; end;end;function TObjectSafe.New (out aReference; const aObject : TObject) : IObjectSafe;begin try Guard (aObject); TObject (aReference) := aObject; except TObject (aReference) := Nil; raise; end; Result := Self;end;{ TExceptionSafe }destructor TExceptionSafe.Destroy;begin try if Length (FMessages) 0 then raise Exception.Create (FMessages); finally try inherited Destroy; except end; end;end;procedure TExceptionSafe.SaveException;begin try if (ExceptObject Nil) and (ExceptObject is Exception) then FMessages := FMessages + Exception (ExceptObject).Message + #13#10; except end; end;function ExceptionSafe : IExceptionSafe;begin Result := TExceptionSafe.Create;end;function ObjectSafe : IObjectSafe;begin Result := TObjectSafe.Create (Nil);end;function ObjectSafe (out aObjectSafe : IObjectSafe) : IObjectSafe; overload;begin Result := ObjectSafe; aObjectSafe := Result;end;function IsAs (out aReference {: Pointer}; const aObject : TObject; const aClass : TClass) : Boolean;begin Result := (aObject Nil) and (aObject is aClass); if Result then TObject (aReference) := aObject;end;end.
How do you use a safe ? It's pretty simple :
procedure TestTheSafe; var aMyObject : TMyObject; aMyComponent : TMyComponent;begin with ObjectSafe do begin New (aMyObject, TMyObject.Create); // or // aMyObject := TMyObject.Create; Guard (aMyObject); aMyComponent := TMyComponent.Create (Safe); end;end;
Notice the use of the 'with' statement - you can use a safe without
having to declare a local variable for it. When you create a component just
pass the 'safe' component as the owner to the constructor. When the
execution of the code reaches the 'end' of the 'with' statement the reference
count of IObjectSafe will hit 0, the destructor of TObjectSafe will be called
and all the components and objects it owns will be freed. So now you have
the best of both worlds - you can create an object when you need it, be sure
it will be automaticaly destroyed and know exactly when it will happen.
The 'New'/'Dispose' methods of IObjectSafe use the 'untyped'
pointer type to return a reference to an object - this will cause
exception if you mismatch the types of the reference and the actual
object created (there won't be a memory leak though), but it is flexible
and shorter to type. If you want to play it safe use the 'Guard' function
instead.
You can also create one IObjectSafe in the constructor
of a complex object which uses a lot of internal objects so you don't
need to explicitly free them in the destructor.
Take a look at the implementation of the AddObject 'procedure'
inside TObjectSafe. This is a method pointer technique you can use
when you need to do one operation most of the time - add an object
at the end of the array - and some other operation rarely - put an object
into an empty slot - and you don't want to check each time which one
of them to perform.
THE EXCEPTION SAFE
Another useful safe used in the implementation of TObjectSafe is IExceptionSafe.
Many times you need to perform an action over many objects but sometimes
you can get an exception. The usual practice is to write something
like :
for aIndex := 1 to 10 dotry // do something which might raise an exceptionexceptend;
and ignore the exceptions, but it's better to remember the exception
messages and show them later.
That's what IExceptionSafe is used for. It has only one procedure
'SaveException' without parameters - it uses the system function 'ExceptObject'
to get a pointer to the current exception. Create a new ExceptionSafe
interface at the start of the block where you want to remember the exceptions
and when the execution reaches the end of the 'with' statement the destructor
of TExcetionSafe checks if there were any exceptions remembered and raises
a new exception with all of the exception messages :
with ExceptionSafe dotry for aIndex := 1 to 10 do try // do something which might raise an exception except SaveException; end; for aIndex := 10 to 20 do try // do something which might raise an exception except SaveException; end; // do something which might raise an exceptionexcept SaveException;end;
THE 'IsAs' OPERATOR
Often you need to check the type of some object and typecast it to a
reference using the 'is' and 'as' operators, like this :
if aSomeObject is TMyObject thenbegin aMyObject := aSomeObject as TMyObject; // do something with aMyObjectend;
With the 'IsAs' function you can accomplish all this in just one line
:
if IsAs (aMyObject, aSomeObject, TMyObject) thenbegin // do something with aMyObjectend;
As you can see 'untyped' pointer types can be quite handy.
CONCLUSION
By using the presented techniques you can greatly simplify the memory
management of Delphi objects/components and make your programs safer.
Suggestions and comments are welcomed -- just
write me!
The source code is available at
CodeCentral