Re: [LEAF] Loss of error_id

From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2020-05-26 22:58:51

On Tue, May 26, 2020 at 2:53 PM Bjorn Reese via Boost <boost_at_[hidden]>
wrote:
>
> On 2020-05-25 22:21, Emil Dotchevski via Boost wrote:
>
> > You mean, as in:
>
> Good example.
>
> > leaf::result<float> f(); // Returns a float or an error
> >
> > leaf::result<int> g() // Returns an int or an error
> > {
> >    (void) f(); // Discard the float or the error returned by f
> >    return 42;
> > }
>
> Suppose there is also an h() function in the call stack that is being
> called by the error_handler() function instead of g():
>
>    leaf::result<int> h()
>    {
>        auto res = g();
>        // How to detect that f() raised an error without knowing
>        // its error_id?
>        return res;
>    }
>
> Can this function detect that someone further up the call stack has
> already raised an error? Similar to std::uncaught_exceptions()?

If g() still does (void) f(); (that is, discards the error), by analogy
this is like try { f(); } catch(...) { }, so I don't understand why h()
would care.

That said, you can detect that f() reported an error. This is exposed
through the augment_id API: https://zajo.github.io/leaf/#augment_id. You
can instantiate this class, and then later call check_error() to see if
some function (from this thread) has reported any errors since then:

leaf::augment_id id;
(void) f();
(void) g();
if( id.check_error() ) {
  // f() or g(), or something they call, reported at least one error.
  // The error(s) may have been discarded or handled.
}

Naturally, this is not common. Generally, we handle, rather than discard,
all reported errors.