Comparing pointers to base objects

[paulvanderlinden]

Hi,

First off, not sure if this is the correct subforum, please inform me if not.
In delphi, this was perfectly valid code:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. type
4. Pbbase = ^Obbase;
5. Obbase = object
6. end;
7.  
8. Pbderived = ^Obderived;
9. Obderived = object(Obbase)
10. end;
11.  
12. var base:PbBase;
13.       Derived:Pbderived;
14. begin
15.   new(Base);
16.   new(Derived);
17.   if Base=Derived then
18.     writeln('yes');
19. end.

However compiling in lazarus/freepascal gives me an error about incompatible types Pbbase and Pbderived.
I know I could put a cast around it (

Code: Pascal  [Select][+][-]

1. if Base=PbBase(Derived) then

but that is a bit to non-restrictive, if f.e. I decouple base from derived the cast would still compile....

Paul

[marcov]

It seems that Delphi does some IS like functionality here. Possibly an extension on Turbo Pascal ?!)?!

You can try to file a bug, but as it is very legacy functionality, it might lay a while.

[paulvanderlinden]

Ok, thanks.
So definitely not a setting I missed.

Turned out, assigning the derived to a tempvar of type base does the trick too.
Not the cleanest solution, but at least it's the same functionality. and with a bit of luck, this will be optimized out by the compiler anyway

[ASerge]

It seems that Delphi does some IS like functionality here. Possibly an extension on Turbo Pascal ?!)?!

From Turbo Pascal 5.5 docs:

Type compatibility also operates between pointers to object types, under the same general rules as with instances of object types: Pointers to descendants can be assigned to pointers to ancestors.

The Delphi compiler still supports this.

[marcov]

It seems that Delphi does some IS like functionality here. Possibly an extension on Turbo Pascal ?!)?!

From Turbo Pascal 5.5 docs:

Type compatibility also operates between pointers to object types, under the same general rules as with instances of object types: Pointers to descendants can be assigned to pointers to ancestors.

The Delphi compiler still supports this.

This is not an assignment, but a comparison. But it would be nice to know what TP/BP 7 (our reference version) does with this code.

[Thaddy]

@Marco
TP does a deep comparison, so a full run over both objects. as opposed to a shallow comparison, where just the pointers and size need to be equal.
Note that deep comparisons are rather slow, especially in a loop. Delphi does still a deep comparison on objects. FPC doesn't.
So it is a - afaik not documented! - compatibility issue. Either that, or if this was not intentional it is a bug.