embind: Add helper for registering a std::optional type. #21076
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| @@ -88,6 +88,7 @@ export interface MainModule { | |||
| a_bool: boolean; | |||
| an_int: number; | |||
| global_fn(_0: number, _1: number): number; | |||
| optional_test(_0: Foo | undefined): number | undefined; | |||
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It could probably be interesting to add logic to detect std::optional<...> at the end of parameter list (only when not followed by non-optional types) and transform them into _0?: Foo instead, as in, actual optional params. Can be a separate PR though.
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Calling `register_optional<std::string>("Optional");` results in:
```
error: no matching function for call to 'register_optional'
8 | emscripten::register_optional<std::string>("Optional");
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/path/to/emsdk/upstream/emscripten/cache/sysroot/include/emscripten/bind.h:1880:6: note: candidate function template not viable: requires 0 arguments, but 1 was provided
1880 | void register_optional() {
| ^
1 error generated
```
As shown in later in the doc and in usage in
#21076,
`register_optional` should not take in an argument. Also see the source
code:
https://github.com/emscripten-core/emscripten/blob/df2200953b57af106dc8a597692580b796f542ef/system/include/emscripten/bind.h#L2006
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This is an alternative implementation of #20359. Now we instead use
undefinedfor empty optionals and the value otherwise in JS.Currently this uses a
valto send optionals back and forth. This is probably not the most efficient way of doing it. However, I started writing a custom wire type value that was malloc'd for optionals and it added a ton of code to be able to serialize them back and forth. This seems much cleaner.Fixes #20082