JS String to std::float conversion

[jmarcosfer]

I have a class method in my Embind bindings that takes a std::float as an argument. However, the method doesn't throw when I pass it a String from JS as long as that String contains numeric characters. Is there some sort of implicit type casting done under the hood that I'm missing?

I've checked this part of the docs about type conversions, but the table makes it look like it's about C++ to JS conversions and not the other way around (plus with the behaviour I've found I know definitely that JS Strings don't always become std::string on the other side).

For what it's worth, I've also found that JS Strings with text characters are converted to NaNs on C++.

Anything I'm missing?
Thanks!

[sbc100]

I suspect this is just an artifact of the automatic conversion rules that apply when calling from JavaScript into Wasm.

I believe the same thing would happen if you directly called an export C function from JS (i.e. without embind at all).

Assuming I'm correct, then the next question might be: Should emscripten attempt to do more strict type checking at the boundary than the normal JavaScript API does? I guess it could provide more debugging in debug build perhaps? @brendandahl @kripken WDYT?

[brendandahl]

We currently have a check for this if assertions are enabled

emscripten/src/embind/embind.js

Lines 482 to 486 in a589a22

	#if ASSERTIONS
	if (typeof value != "number" && typeof value != "boolean") {
	throw new TypeError(`Cannot convert ${embindRepr(value)} to ${this.name}`);
	}
	#endif

[sbc100]

Oh nice. @jmarcosfer can you confirm this is firing in a debug build for you?

@brendandahl @kripken should we perhaps move this check to be in a more generic place so it works the same for all Wasm exports (not just embind'd ones).

[jmarcosfer]

Thank you all for the quick reply.

The test I did used our "sync" build which has default ASSERTIONS=1.
Should I try with ASSERTIONS=2?

Just for context, these are our emcc commands for building an "async" and a "sync" version.

@emcc -Oz $(BINDING_ESSENTIAJS) $(INCLUDE_ESSENTIAJS) \
	-lembind -lessentia -L $(LIB_DIR_ESSENTIA) \
	-I $(EIGEN_PATH) -I $(INCLUDE_DIR_ESSENTIA)\
	-s WASM=1 \
	-o $(ESSENTIA_JS_WEB) \
	-s EXCEPTION_DEBUG \
	-s ASSERTIONS=2 \
	-s ENVIRONMENT=web \
	-s MODULARIZE=1 \
	-s EXPORT_NAME="EssentiaWASM" \
	--post-js $(POST_JS_WEB_WASM) \
	-s ALLOW_MEMORY_GROWTH=1 || exit 1
@echo "Done ..."

@echo "... compiling sync builds..."
@emcc -Oz $(BINDING_ESSENTIAJS) $(INCLUDE_ESSENTIAJS) \
	-lembind -lessentia -L $(LIB_DIR_ESSENTIA) \
	-I $(EIGEN_PATH) -I $(INCLUDE_DIR_ESSENTIA)\
	-s WASM=1 \
	-o $(ESSENTIA_JS_MODULE) \
	-s BINARYEN_ASYNC_COMPILATION=0 \
	-s ALLOW_MEMORY_GROWTH=1 \
	-s SINGLE_FILE=1 || exit 1

PS: I realise this is probably suboptimal, so also feel free to suggest improvements.

[brendandahl]

The example below seems to work fine for me. Do you see the assertion code in the output JS file for your project?

#include <stdio.h>
#include <emscripten.h>
#include <emscripten/bind.h>

using namespace emscripten;

void foo(float f) {}

int main() {
EM_ASM(
  Module.foo("hello");
);
}

EMSCRIPTEN_BINDINGS(xxx) {
  function("foo", &foo);
}

built with:

emcc -Oz -sASSERTIONS=1 -lembind main.cpp -o a.out.js

gives me:
TypeError: Cannot convert hello to undefined