[emval] Support dynamically-typed conversion to/from val #24790
Conversation
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I'm having trouble reasoning about this code, to be honest, but then, I'm not that familiar with C++ RTTI and also hard to say what it's supposed to do without a test / example. Could you add one please and maybe @brendandahl can also take a look? Just wanted to say that I also wanted to submit one final PR that changes caller mechanism further that I've been working towards, but had a few setbacks that delayed it. I don't want to block this PR on mine, just saying we might end up having some further merge conflicts. |
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Yeah, sorry for not adding a test example from the get-go. Added one now, hope it clarifies things a bit (it might still look a bit unmotivated that it wants to use |
...for cases where a dynamically given user-defined type is known to be ABI-compatible with a statically given wire type (e.g., an enum type being compatible with its underlying type, or a struct type being compatible with `const void*`; for the latter, a returned `const void*` needs to be transformed somehow into the desired dynamic type before destructors are run and the pointed-to object is destroyed).
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I haven't looked too deeply at this yet, but a few initial thoughts:
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...for cases where a dynamically given user-defined type is known to be ABI-compatible with a statically given wire type (e.g., an enum type being compatible with its underlying type, or a struct type being compatible with
const void*; for the latter, a returnedconst void*needs to be transformed somehow into the desired dynamic type before destructors are run and the pointed-to object is destroyed).In LibreOffice, I have a scenario where I combine
valwith a C++Anytype that dynamically knows what type it holds, which does not fit with the statically typedvalconstructor andasgetter. Prior to d8dda21 "[embind] Use a single invoker mechanism (#24577)", I had tapped into the internals ofemscripten/val.h(https://git.libreoffice.org/core/+/c35985a4b42d6a7d654b4fd1f99a966b75ee28a6%5E%21 "Embind construction of UNO Any enum", https://git.libreoffice.org/core/+/2995a0e0785911322c9d57e98b925073ff6cb6bd%5E%21 "Embind construction of UNO Any sequence/struct/exception/interface"), which no longer works. So I'm proposing here adding the relevant code (which only makes sense for__has_feature(cxx_rtti)) to Emscripten.(The JS code now needs to keep track and reuse generated method callers. I have no idea how better to implement in JS such a lookup map from signatures (tuples of
EM_INVOKER_KINDvalues and argument types) to method caller IDs, other than going via a cheesy string representation?)