Hi
I have been busy porting a large Delphi app to FP & Lazarus. This issue
took me quite a while to find. My RTTI unit tests kept on failing for
no obvious reason.
Finally I found the issue. It is a difference between how Delphi and
FPC handles Forward Declarations and the {$M+} directive.
I have a workaround for my code, but I'm not sure which compiler handles
this correctly and which one doesn't. I did get this info from the
Delphi 7 help titled 'Runtime type information': "Note that if a class
is forward declared, the first declaration of the class must be declared
with the $M switch."
This works under Delphi, but not in FPC. Puting the {$M+} around the
forward declaration. You don't get any compiler error in FPC, but
calling the GetPropList fuction to retrieve a list of published
properties for example will thow an AV error at runtime. See example #1
To get this to work in FPC (but then it fails in Delphi) is to put the
{$M+} around the actual class declaration and not around the forward
declaration. See example #2
Works in Delphi, but not in FPC.
---[Example #1]---------------------------------------
type
{$M+}
TMyTestObject = class;
{$M-}
TSomeOtherClass = class(TObject)
...
end;
TMyTestObject = class(TObject)
private
FIntProp: integer;
FStringProp: string;
public
published
property StringProp: string read FStringProp write FStringProp;
property IntProp: integer read FIntProp write FIntProp;
end;
------------------------------------------
Works in FPC, but doesn't in Delphi.
---[Example #2]---------------------------------------
type
TMyTestObject = class;
TSomeOtherClass = class(TObject)
...
end;
{$M+}
TMyTestObject = class(TObject)
private
FIntProp: integer;
FStringProp: string;
public
published
property StringProp: string read FStringProp write FStringProp;
property IntProp: integer read FIntProp write FIntProp;
end;
{$M-}
------------------------------------------
Regards,
- Graeme -
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