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[pigeon] C++ generator makes uncompilable code for returning a list #105057
Description
I defined an API of the form:
List<String?> foo(...);
and the resulting C++ code doesn't compile for me. It gets translated to
virtual ErrorOr<std::unique_ptr<flutter::EncodableList>> Foo(...);
which doesn't work because ErrorOr<T> has a const T& rhs constructor that calls T(rhs), which is a deleted copy constructor for std::unique_ptr.
I'm confused about how this new variant of ErrorOr is intended to work now that I'm looking at it in actual use. v is a variant whose types include a T and a std::unique_ptr<T>, which in this case would be a nested double unique_ptr which seems unlikely to be the intent. @azchohfi what was the intended behavior here?
Activity
azchohfi commentedon May 31, 2022
This is how I tested this on the video player:
std::unique_ptr<TextureMessage> textureMessage = std::make_unique<TextureMessage>(); textureMessage->set_texture_id(textureId); return ErrorOr<TextureMessage>::MakeWithUniquePtr(std::move(textureMessage));azchohfi commentedon May 31, 2022
I'm sorry, but I didn't understand what you meant by nested double unique_ptr.
Why is the generated code an issue? I'm trying to understand the scenario here.
The only things not being passed by reference are bool, int64_t and double. Everything else is by reference.
I tried not to use unique_ptr for everything, but a variant is not permitted to hold references, neither is std::optional.
stuartmorgan-g commentedon May 31, 2022
It sounds like the intent was that the generated return type in my case should have been
ErrorOr<EncodableList>rather thanErrorOr<unique_ptr<EncodableList>>then. I'll take a look soon to see what went wrong (and get more tests spun up to cover return types).The issue is that once
TforErrorOr<T>is itself aunique_ptr, as happened for me, things fall apart.stuartmorgan-g commentedon Jun 1, 2022
Hm, the more I look at
ErrorOrthe more confused I am. I missed thatMakeWithUniquePtrreturns a different type than the type it's called with. And there doesn't appear to ever be a way to use theunique_ptrvariant flavor ofvgiven the implementation ofget?It looks like the intent of
vwas that it could have a copy or a unique ptr and either would work as a value, but that's not actually what the implementation does. I'll try adjusting it to fit that approach and see how it works.stuartmorgan-g commentedon Jun 1, 2022
I think I have a workable version of this using a move-based alternate constructor instead of a
unique_ptrversion. (I'm still working through some unrelated issues to get and end-to-end test going.)github-actions commentedon Aug 4, 2022
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