One possible workaround is to use the derived class in the bindings, where
one of its base classes is required. That even works without changing the
underlying C++ library code. In my case I changed:
class_<CommonTokenStream, base<BufferedTokenStream>>("CommonTokenStream")
.constructor<TokenSource *>()
.constructor<TokenSource *, size_t>();
to:
class_<CommonTokenStream, base<BufferedTokenStream>>("CommonTokenStream")
.constructor<Lexer *>()
.constructor<Lexer *, size_t>();
That limits the acceptable token source to lexers only, but in my case this
is acceptable. Any other suggestion is still welcome!
On Monday, August 14, 2023 at 11:16:29 AM UTC+2 Mike Lischke wrote:
Hi all,
C++ multiple inheritance is not discussed in the documentation and there
are only very old issues in the emscripten issue tracker. From what I read
so far is that multiple inheritance is not supported in embind because JS
does not support that. That's not false, but MI is used so often in C++
there must be a workaround (and I'm looking for one).
In my case I have `class Lexer : public Recognizer, public TokenSource {}`.
In some places I need that lexer as a recognizer and in some I need it as
token source. My bindings are like this (simplified):
class_<Lexer, base<Recognizer>>("Lexer$Internal")
.property("mode", &Lexer::mode);
class_<LexerHelper, base<Lexer>>("Lexer")
.constructor<CharStream *>()
.allow_subclass<LexerWrapper>("LexerWrapper", constructor<CharStream *>());
Since I cannot specify both classes as base in the bindings call, I started
with `base<Recognizer>` which works fine. However, when I try to use the
lexer class in a place where a token source is required, I get the error:
BindingError: Expected null or instance of TokenSource, got an instance of
Recognizer at throwBindingError (file:...)
Side note: that approach to use `Lexer$Internal` and `LexerHelper` comes
from the fact that `Lexer` is an abstract class, but I need to bind its
constructor to allow calling that from JS where I use the Lexer to derive a
JS class from. So, I created the (non-abstract) `LexerHelper` class and
pretend it's the actual lexer class.
IIUC the error is bogus, because the checked class indeed derives from both
base classes, only JS doesn't know that. What can I do to make this
scenario work?
I experimented with leaving out the `Lexer` binding entirely (to avoid the
MI dilemma), but that only caused the generated JS class to be not
extendable.
Mike
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