Martin Frb via fpc-devel <fpc-devel@lists.freepascal.org> schrieb am Sa., 5. Apr. 2025, 12:02:
> See the below code. (tested 3.2.3 and 3.3.1) > > Is this intentionally allowed? > > Technically it should work. TSubTest.Test always returns something that > fits the inherited requirement. > > > program Project1; > type > TFoo = class end; > TBar = class(TFoo) end; > > TTest = class > function Test: TFoo; virtual; abstract; > end; > > TSubTest = class(TTest) > function Test: TBar; override; > end; > > function TSubTest.Test: TBar; > begin end; > > begin end. > Yes, this is intentional and is called "covariant return type" ( https://en.m.wikipedia.org/wiki/Covariant_return_type ). > > And if that is intentional, then why not also > > TTest = class > procedure Test(a: TBar); virtual; abstract; > end; > > TSubTest = class(TTest) > procedure Test(a: TFoo); override; > end; > > This fails. Even though any TBar passed will be fine to the base and the > subclass. > > And any > var sub: TSubTest > will accept TFoo too, but such a var can never hold the base class. > This is called contra-variance and is in general not something that programming languages support. See for example here: https://stackoverflow.com/questions/2995926/why-is-there-no-parameter-contra-variance-for-overriding Regards, Sven >
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