Passing exceptions across a C API boundary

Question

I am writing a library in C++ which uses an older C API. The client of my library can specify callback functions, which are indirectly called through my library which is called through the C API. This means that all exceptions in the client callbacks must be handled.

My question is this: how can I catch the exception on one side of the boundary and re-throw it once the C API boundary has been recrossed and the execution is back in C++ land so that the exception can be handled by client code?

Answer 1

With C++11 we could use:

std::exception_ptr active_exception;

try
{
    // call code which may throw exceptions
}
catch (...)
{
    // an exception is thrown. save it for future re-throwing.
    active_exception = std::current_exception();
}

// call C code
...

// back to C++, re-throw the exception if needed.
if (active_exception)
    std::rethrow_exception(active_exception);

Before C++11 these can still be used via Boost Exception.