throw exception with message in cpp

[robfors]

I have a cpp file test.cpp:

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

int main()
{
  throw std::invalid_argument("test error");
  
  printf("\ndone\n");
  return 0;
}

Compiled with emcc --bind -fexceptions -std=c++11 test.cpp -o test.js

I would like the code to print out the exception message in some way so the user has a description of the error.
When I run it I get exception thrown: 5246984 and Uncaught 5246984.
It does not appear as though emscripten will print any distinguishable message.
Does emscripten have this feature?

If not, I was thinking of adding a thow method to val that would accept a message and possibly a javascript exception class.

[kripken]

I don't think we have such a feature. In general I think people just add a try-catch at the highest level and print it from C++. If there's a way to add that feature it would be nice, but I'm not sure how easy it would be.

[jleni]

@kripken so there is no way of propagating the exception to js? Is there any way in which it could be wrapped or something?

[kripken]

I can't think of an easy way. But the thrown exception is a pointer, so in theory you could call from JS into C++ code like this, and give it that pointer:

// gets an exception object, and prints it out.
void print_exception(void* exception) {
  try {
    throw exception;
  } catch (...) {
    std::cout << exception.what() << '\n';
  }
}

I think that might work, but I'm not sure.

[jleni]

ok, I didnt know that was an actual pointer!
I will play a bit with the idea and write back, thanks!

[tuananh]

is it still open? i would like to know how to throw exception from c++ to js as well.

[Shachlan]

Hi, this is the current solution I use:

In C++:

namespace foo {
std::string getExceptionMessage(int exceptionPtr) {
  return std::string(reinterpret_cast<std::exception *>(exceptionPtr)->what());
}
}

EMSCRIPTEN_BINDINGS(fooBinding) {
emscripten::function("getExceptionMessage", &foo::getExceptionMessage);
};

In JS:

try {
  ... // some code that calls webassembly
} catch (exception) {
  console.error(Module.getExceptionMessage(exception));
} finally {
  ...
}

I guess you can also throw an exception from an EM_ASM block, if you wanted to rethrow c++ to JS.

[artemjackson]

Any updates?

[kripken]

I think adding an option like @Shachlan 's from the previous comment might be a good idea, if @Shachlan or someone else wants to submit a PR.