[fpc-pascal] Compatibility of very similar generic types

[Sven Barth]

Am 18.08.2016 15:09 schrieb "Felipe Monteiro de Carvalho" <
felipemonteiro.carvalho at gmail.com>:
>
> On Thu, Aug 18, 2016 at 2:44 PM, Sven Barth <pascaldragon at googlemail.com>
wrote:
> > generic procedure WriteVector_To_DM<T>(ADest: TDataManager; ASelf:
> > specialize TVector<T>);
> > generic procedure ReadVector_From_DM<T>(AFrom: TDataManager; var ASelf:
> > specialize TVector<T>);
> >
> > and use them like this:
> >
> > specialize WriteVector_To_DM<TSubClass1>(...);
>
> Aha, great, didn't know we had something in this direction. But
> correct me if I am wrong, but this wouldn't work in my case, because
> my function writes the class to a stream in the end of it, so it needs
> to know which type T is. If the type is TSomeClass or a descendent,
> then it will write each field of the class to the stream.
>
> But in a generic class/function there is no way to ask if T is an int,
> or is an TSomeClass, or am I missing some way of writing code that is
> different for various possible types?

You can enforce that T needs to be a subclass of TSomeClass by declaring it
as "T: TSomeClass"; like this:

generic procedure WriteVector_To_DM<T: TSomeClass>(ADest: TDataManager;
ASelf: specialize TVector<T>);

In that case the compiler knows that T will at least be a TSomeClass so you
can use methods and fields of it without problems.

Note: these generic constraints also work for generic types and is already
in 3.0.0 for them.

Additionally you can play around with TypeInfo() which works on variables
of a generic type as well.

Regards,
Sven
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