Kostas Michalopoulos via fpc-devel <fpc-devel@lists.freepascal.org> schrieb am Mi., 22. Feb. 2023, 10:37:
> > Because Delphi doesn't have them and when constraints were implemented > > they were implemented for Delphi compatibility. > > Can they be added? The original announcement ~13 years ago mentioned > that those could be added at some point. "object" and "operator X" would > be quite useful for me. > Some additional constraints might be added. However I have none of them planned currently and enough other things to do. > I tried to use constraints at some point in my code but they proven too > limited in functionality - the biggest issue i faced was that i couldn't > use a specialization with a forward declared class (which is the entire > point of the forward declaration). For example i have a generic > collection that only works on TSerializable subclasses so i gave it that > constraint, but in another unit i need to define a specialization for it > before it was defined (so the class was available with a forward class > declaration) since that specialization also needs to be used by the > class itself. In general it seems like having constrained generic work > with a tree structure of derived types that use the generic itself is > impossible. > > Wouldn't storing a list of "specializations to confirm later in this > unit" work (with later being when the class is actually defined in the > unit)? The language already has forward declarations for a bunch of > other things. > Forward declarations can not be used for constraints, because a specialization of the generic might be used before the forward declared type is fully defined and the compiler *must* be able to check whether the parameter in question is compatible at the time the specialization is declared. Regards, Sven >
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