[fpc-pascal] FPC bug or brain bug ?

Jonas Maebe jonas.maebe at elis.ugent.be
Sat May 29 15:56:54 CEST 2010

On 29 May 2010, at 12:56, Frank Peelo wrote:

> On 27/05/10 23:13, Jonas Maebe wrote:
>> On 27 May 2010, at 23:31, Yann Bat wrote:
>>>> The compiler always adds a VMT if an object has a constructor or destructor. The reason is that the VMT also contains the instance size, which is used by the constructor helper to allocate the required amount of memory.
>>> Ok but why a different behaviour between [fpc | objfpc] mode and [tp |
>>> delphi] mode ?
>> It is allowed in Delphi and TP because they allow declaring typed constants of objects. I don't remember why it was disabled in the FPC modes, but that happened a long time ago (before the move from cvs to svn). Maybe someone else does.
> Definitely TP-style objects did not have a VMT unless you declared a virtual method, in TP. And a destructor was only virtual if it was declared virtual. Hence typed constant objects could be possible.

I made a typo in the comment above (my first quoted statement at the top of the mail is still correct though), it should have read "It is allowed in Delphi and TP because they allow declaring typed constants of objects *that contain a VMT*". As I explained, FPC adds a VMT for any object type that contains a constructor or destructor. This by itself may be another incompatibility with TP/Delphi.

> Presumably FPC decided compatibility with this feature was not desirable?

The reason for adding the VMT as soon as there is a constructor or destructor may be for implementation reasons, although it's strange that this is TP-incompatible since this was probably implemented by Pierre or Carl-Eric, and they generally paid a lot of attention to being binary compatible to TP as much as possible.

As mentioned before, I don't know the reason for not allowing typed constants of TP-style objects that have a VMT.

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