BorlandTalk.com Forum Index BorlandTalk.com
Borland discussion newsgroups
 
Archives   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Incompatible types ISerializableObject and TObject...

 
Post new topic   Reply to topic    BorlandTalk.com Forum Index -> Delphi OO design
View previous topic :: View next topic  
Author Message
Paul Nicholls
Guest
PostPosted: Thu Jul 10, 2003 12:30 am    Post subject: Incompatible types ISerializableObject and TObject... Reply with quote


Hi all, I have defined this type

TObjectClass = class of TObject;

I have an array

var
RegisteredClasses: array of record
ClassID: AnsiString;
RegisteredClass: TObjectClass;
end;

and created these two routines

procedure RegisterClassByClassID(ClassID: AnsiString; const RegisteredClass:
TObjectClass);
var
i: Integer;
ClassIDFound: Boolean;
begin
ClassIDFound := False;
for i := 0 to High(RegisteredClasses) do
if(RegisteredClasses[i].ClassID = ClassID)then
begin
ClassIDFound := True;
Break;
end;
if(ClassIDFound)then
raise Exception.Create('RegisterSerializableClass: Class with ClassID
"'+ClassID+'" already registered.')
else
begin
SetLength(RegisteredClasses,High(RegisteredClasses)+2);
RegisteredClasses[High(RegisteredClasses)].ClassID := ClassID;
RegisteredClasses[High(RegisteredClasses)].RegisteredClass :=
RegisteredClass;
end;
end;

function CreateNewObjectFromClassID(ClassID: AnsiString): TObject;
var
i: Integer;
begin
Result := nil;
for i := 0 to High(RegisteredClasses) do
if(RegisteredClasses[i].ClassID = ClassID)then
begin
Result := RegisteredClasses[i].RegisteredClass.Create;
Break;
end;
end;

These are used like so:

RegisterObjectByClassID('SomeClassID',TMyClass);

I can then get back any registered object using the class id as the key eg.
(returns nil if not found). (works nicely)

NewObject := CreateNewObjectFromClassID('SomeClassID');

This work very nicely for 'normal' classes, but I have a problem when I
store classes that 'inherit' interfaces, eg.

Lets say I have an interface

ISerializableObject = interface
['{A8CE6CD8-2246-488E-A292-65F7C56F6360}']
procedure UnSerialize(var s: AnsiString);
function Serialize: AnsiString;
function ToString: AnsiString;
end;

and a class using this

TSerializableClass = class(TInterfacedObject,ISerializableObject)
private
{...}
end;

I do this

var
SerializableObject: ISerializableObject;

begin
RegisterClassByClassID('TestID',TSerializableObject);
SerializableObject := CreateNewObjectFromClassID('TestID');

Even though TSerializableClass is stored in the RegisteredClasses array and
inherits the interface, the
compiler complains with

Incompatible types: 'ISerializableObject' and 'TObject'

Is there anyway I can get around this?

Thanks in advance,
Paul Nicholls (Delphi 5/6 Professional)
"The simple things in life are often a pest..." - Chris Valentine
[email]paul-nicholls (AT) hotmail (DOT) com[/email]

Replace '-' with '_' to reply


Back to top
Sebastian Moleski
Guest
PostPosted: Thu Jul 10, 2003 1:04 am    Post subject: Re: Incompatible types ISerializableObject and TObject... Reply with quote


"Paul Nicholls" <paul-nicholls (AT) hotmail (DOT) com> wrote


Quote:
var
SerializableObject: ISerializableObject;

begin
RegisterClassByClassID('TestID',TSerializableObject);
SerializableObject := CreateNewObjectFromClassID('TestID');

Even though TSerializableClass is stored in the RegisteredClasses array and
inherits the interface, the
compiler complains with

How is the compiler supposed to know that the object that
CreateNewObjectFromClassIID returns implements your interface? It can't do so
statically which is why you get that compiler error. One way to get around that
is to use Supports or TObject's GetInterface function.

sm



Back to top
Paul Nicholls
Guest
PostPosted: Thu Jul 10, 2003 1:16 am    Post subject: Re: Incompatible types ISerializableObject and TObject... Reply with quote


Have you an example on how to do this?
Thanks...

"Sebastian Moleski" <sebmol (AT) yahoo (DOT) com> wrote

Quote:
"Paul Nicholls" <paul-nicholls (AT) hotmail (DOT) com> wrote in message
news:3f0cb3ba$1 (AT) newsgroups (DOT) borland.com...

var
SerializableObject: ISerializableObject;

begin
RegisterClassByClassID('TestID',TSerializableObject);
SerializableObject := CreateNewObjectFromClassID('TestID');

Even though TSerializableClass is stored in the RegisteredClasses array
and
inherits the interface, the
compiler complains with

How is the compiler supposed to know that the object that
CreateNewObjectFromClassIID returns implements your interface? It can't do
so
statically which is why you get that compiler error. One way to get around
that
is to use Supports or TObject's GetInterface function.

sm





Back to top
Ritchie Annand
Guest
PostPosted: Thu Jul 10, 2003 8:56 am    Post subject: Re: Incompatible types ISerializableObject and TObject... Reply with quote
In article <3f0cb3ba$1 (AT) newsgroups (DOT) borland.com>, [email]paul-nicholls (AT) hotmail (DOT) com[/email]
says...
Quote:
I do this

var
SerializableObject: ISerializableObject;

begin
RegisterClassByClassID('TestID',TSerializableObject);
SerializableObject := CreateNewObjectFromClassID('TestID');

Even though TSerializableClass is stored in the RegisteredClasses array and
inherits the interface, the
compiler complains with

Incompatible types: 'ISerializableObject' and 'TObject'

Is there anyway I can get around this?

CreateNewObjectFromClassID creates TObjects, and TObject doesn't
implement *any* interfaces Smile It should complain the same way if you
tried to do:

var
AObject : TButton;
begin
RegisterClassByClassID('TestID',TSerializableObject);
AObject := CreateNewObjectFromClassID('TestID');

That's an "up-cast" from TObject to TButton, and Delphi won't do it
without explicit instruction. Same goes for interfaces.

What you want is to use the likes of GetInterface to try and pull an
interface out of the object...

var
SerializableObject: ISerializableObject;
begin
RegisterClassByClassID('TestID',TSerializableObject);
if CreateNewObjectFromClassID('TestID').GetInterface(
ISerializableObject,SerializableObject) then
begin
// here, you can use SerializableObject
end;
end;

Supports does nearly the same thing, except it involves a GetInterface to
IUnknown first.

Both work fine for getting interfaces from objects that DO implement
IUnknown (TInterfacedObject is one Smile, but Supports will not work with,
say, TComponents unless you specifically declare support for IUnknown in
the class.

The one advantage of Supports is that it will return False if the object
is nil, whereas, of course, GetInterface will return an access violation
:)

Hope that helps! :)

-- Ritchie Annand

Senior Software Architect
Malibu Software & Engineering Ltd.
Business: http://www.malibugroup.com
Personal: http://nimble.nimblebrain.net
Wiki: http://wiki.nimblebrain.net

Back to top
Paul Nicholls
Guest
PostPosted: Thu Jul 10, 2003 10:57 pm    Post subject: Re: Incompatible types ISerializableObject and TObject... Reply with quote
Thanks for the info Ritchie, it does help :)

"Ritchie Annand" <nimble_minusthisbit (AT) shaw (DOT) ca> wrote

Quote:
In article <3f0cb3ba$1 (AT) newsgroups (DOT) borland.com>, [email]paul-nicholls (AT) hotmail (DOT) com[/email]
says...
I do this

var
SerializableObject: ISerializableObject;

begin
RegisterClassByClassID('TestID',TSerializableObject);
SerializableObject := CreateNewObjectFromClassID('TestID');

Even though TSerializableClass is stored in the RegisteredClasses array
and
inherits the interface, the
compiler complains with

Incompatible types: 'ISerializableObject' and 'TObject'

Is there anyway I can get around this?

CreateNewObjectFromClassID creates TObjects, and TObject doesn't
implement *any* interfaces Smile It should complain the same way if you
tried to do:
SNIP




Back to top
Display posts from previous:   
Post new topic   Reply to topic    BorlandTalk.com Forum Index -> Delphi OO design All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2006 phpBB Group
SEO toolkit © 2004-2006 webmedic.