Object-safe trait {up,down}casting? | Rust Programming Language Community | Page 1

teal siren Jul 2, 2022, 8:33 PM

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I'm making a GTK app that I want to interact with multiple different online services, but some online services offer more features than others. So, what I would like to do is somehow test the features that a service offers at runtime.

Lets say I have a trait -- Service, that is the supertrait of several other traits, AccountService, FeedService, etc. How might I make it so a Box<dyn Service> could be downcasted to a Box<dyn AccountService>?

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Object-safe trait {up,down}casting?

humble nest Jul 2, 2022, 9:13 PM

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Ono, subtyping detected 🚨

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To my understanding you can't do it in Rust, you would have to resort to full-on dynamic typing using Any

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(Because the methods from Any trait only work on dyn Any, so a subtrait of Any won't work)

teal siren Jul 2, 2022, 9:15 PM

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Oh wonderful

humble nest Jul 2, 2022, 9:15 PM

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You don't want subtyping

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Well in some edge cases like servo yes

teal siren Jul 2, 2022, 9:16 PM

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What might work better in this case?

humble nest Jul 2, 2022, 9:16 PM

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Is there anything stopping you from using enums?

teal siren Jul 2, 2022, 9:16 PM

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Hmmm... now that I think about it not really

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I'm too used to writing library code, lol

#Object-safe trait {up,down}casting?