Re: [boost] Noexcept

Subject: Re: [boost] Noexcept
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-06-12 20:50:44

2017-06-12 22:28 GMT+02:00 Emil Dotchevski via Boost <boost_at_[hidden]>
:

> On Mon, Jun 12, 2017 at 1:15 PM, Andrzej Krzemienski via Boost <
> boost_at_[hidden]> wrote:
>
> > 2017-06-12 20:07 GMT+02:00 Emil Dotchevski via Boost <
> > boost_at_[hidden]>
> > :
> >
> > > On Mon, Jun 12, 2017 at 2:42 AM, Niall Douglas via Boost <
> > > boost_at_[hidden]> wrote:
> > >
> > > > On 12/06/2017 09:22, Emil Dotchevski via Boost wrote:
> > > > > The lively debates during the Outcome review show that there is a
> > great
> > > > > deal of interest in solving the problem of error handling in
> > > environments
> > > > > where C++ exception handling is unavailable.
> > > > >
> > > > > Noexcept is a new C++11 library that implements a different
> approach
> > to
> > > > > solving the same problem. Any feedback is welcome.
> > > > >
> > > > > https://zajo.github.io/boost-noexcept/
> > > >
> > > > The use of functional throw(), try() and catch() was a design
> approach
> > > > rejected very early by me and most who have looked into this problem.
> > > >
> > > > Nobody wants to reimplement via a library exception handling with
> > > > exceptions disabled. It's an impoverished experience, and leads to
> > > > brittle code.
> > >
> > >
> > > Can you elaborate? My understanding is that the problem with exception
> > > handling is the unpredictability of the performance you'll get.
> Noexcept
> > > directly addresses that issue by not introducing the unpredictability
> of
> > > its own return type which may or may not get optimized.
> > >
> > > It also removes the redundancy of requiring types which already have a
> > > useful empty state to be wrapped into something like outcome<>. Nobody
> > > would return optional<FILE *> from a function that may fail, they'll
> just
> > > return FILE */nullptr. Returning outcome<FILE *> is similarly redundant
> > and
> > > possibly inefficient.
> > >
> > >
> > > > Just enable C++ exceptions if you want exceptions.
> > > >
> > >
> > > I agree, the question is what to do if you can't.
> > >
> >
> > There is a number of expectations people have or might have form
> > error-handling framework:
> >
> > 1. Predictable times.
> > 2. When I forget to check for error, the computation should not silently
> > proceed.
> > 3. Not polluting the function return type
> > 4. Explicit control flows.
> > 5. No explicit control flows.
> > 6. Neutrality for some functions, elspecially those extern "C".
> > 7. Being fast.
> > 8. Being able to carry any payload.
> >
> > Obviously, a framework cannot guarantee all of these, and trade-offs need
> > to be made. One thing that both exceptions, and outcome<>/expected<> have
> > is #2: when you forget that the function might fail, and it fails, the
> > dependent functions will not get called.
>
>
> What do you mean "dependent functions"?
>
>
> > In case of exceptions this is
> > owing to stack unwinding. In case of outcome, it is because the program
> > will not compile.
>
>
> Can you post an actual example so we're not talking in the abstract?
>

```
int job(int x)
{
  int y = f(x); // f might fail, but I forgot
  int z = g(y); // g might fail, but I forgot
  return h(z);
}
```

If for some reason, I have forgotten that f() might fail (and signal
failure), does function g() get called? In case of exceptions no, because
if f() throws, then g() is never called. In case of outcome<>: no, because
f() returns `outcome<int>, so the above will fail to compile, an I will be
forced to rewrite function job().

Does that make sense?

Regards,
&rzej;