Exceptions (C++ and JavaScript) in Cheerp

Cheerp supports C++ exceptions, enabled when passing -fexceptions to the compiler. Cheerp also supports dealing with JavaScript exceptions, whether catching JavaScript exceptions raised by external JavaScript code, or throwing JavaScript exceptions from C++ code.

‘Internal’ C++ exception caught by C++ code

This is the simplest case of using exceptions in Cheerp. In this case, your C++ code behaves in the same way that it would do when compiling natively, and any exceptions-enabled C++ code will work out of the box.

Example:

#include <iostream>
using namespace std;

int main () {
  try
  {
    throw 20;
  }
  catch (int e)
  {
    cout << "An exception occurred. Exception Nr. " << e << '\n';
  }
  return 0;
}

C++ exceptions caught in C++ work as expected, whether compiling to WebAssembly, JavaScript, or both. As long as both sides are present on the C++ side it will follow the ‘regular’ rules for C++ exceptions, (eg. catch(…) included).

‘External’ exception caught by C++ code

In this case, C++ code invokes some JavaScript method that, in turn, generates an exception. The exception is caught in C++.

Example:

#include <iostream>
#include <cheerp/jsexception.h>
#include <cheerp/clientlib.h>
using namespace std;

namespace client{
    void someUndefinedExternalFunc();
}

int main () {
  try
  {
          client::someUndefinedExternalFunc();
  }
  catch (cheerp::JSException& ex)
  {
          client::Error* error = (client::Error*)ex.get();
          client::console.log(error->get_name());
          client::console.log(error->get_stack());
          client::console.log(error->get_message());
  }
  return 0;
}

Here say that we don’t implement someUndefinedExternalFunc. Executing it will throw a JavaScript Error, that can be caught using the cheerp-specific syntax:

catch (cheerp::JSException& ex)

Note that using the throw value requires: calling get() + casting to the appropriate value.

Here we casted to a pointer to client::Error, but see for example here: cheerp-utils/tests for catching an integer value.

‘Internal’ (C++) exception catched by external JavaScript code

If your code (eg. a jsexported function) lets a C++ exception bubble up, you can still catch it on the JavaScript side by wrapping the call of the relevant jsexported-method in a try {} catch {} statement. This is ‘just’ regular JavaScript.

[[cheerp::jsexport]]
void throwIfOdd(int n)
{
  if (n % 1)
    throw n;
}

try {
  throwIfOdd(3);
} catch (e) {
  console.log("Catched C++ exception ", e);
}

This is expecially useful during development.

Throw builtin

Also possibly useful for testing (and for internal usage) there is a __builtin_cheerp_throw( someObj ) that codegen to the equivalent JavaScript throw.

#include <cheerpintrin.h>
client::Object* obj = new client::Object();

__builtin_cheerp_throw(obj);

Throws via the builtin behave exactly as the object was thrown via the JavaScript throw mechanism (so it will be considered as ‘external’ by the Cheerp compiler).

__asm__ sections can’t throw

This is the very last ‘catch’, there can’t be throws (either directly or indirectly) in __asm__ sections (see asm sections).