Re: [boost] Noexcept

Subject: Re: [boost] Noexcept
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2017-06-20 01:38:40

On Mon, Jun 19, 2017 at 2:41 PM, Andrzej Krzemienski via Boost <
boost_at_[hidden]> wrote:

> 2017-06-19 20:33 GMT+02:00 Emil Dotchevski via Boost <
> boost_at_[hidden]>
> :
>
> > On Mon, Jun 19, 2017 at 2:58 AM, Andrzej Krzemienski via Boost <
> > boost_at_[hidden]> wrote:
> >
> > > 2017-06-14 21:52 GMT+02:00 Richard Hodges via Boost <
> > boost_at_[hidden]
> > > >:
> > >
> > > > Exception return paths are not infinite. There are a finite number of
> > > > places in code that an exception can be thrown.
> > > >
> > > > The exception path is one path, the non-exception path is another.
> > That’s
> > > > two in total. Exactly equivalent to an outcome<>.
> > > >
> > > > It is a fallacy to say that there are an indeterminate number of
> paths.
> > > >
> > > > If developers do not understand RAII, then an afternoon of training
> can
> > > > solve that.
> > > >
> > > > RAII is the foundation of correct c++. It is the fundamental
> guarantee
> > of
> > > > deterministic object state. A program without RAII is not worthy of
> > > > consideration. The author may as well have used C.
> > > >
> > > > Perhaps there is an argument that says that RAII adds overhead to a
> > > > program’s footprint. If things are that tight, fair enough.
> > > >
> > > > Otherwise there is no excuse to avoid exceptions. I’ve never seen a
> > > > convincing argument.
> > > >
> > >
> > > The above statement almost treats RAII and exception handling as
> > > synonymous. But I believe this gives the false picture of the
> situation.
> > >
> > > RAII is very useful, also if you do not use exceptions, but have
> multiple
> > > return paths. You want to acquire the resource in one place and
> schedule
> > > its future release in one place, not upon every return statement.
> > >
> >
> > If you adopt this programming style as a rule, there is no downside to
> > using exceptions.
> >
> >
> > > In case of using things like Outcome, you still want to follow RAII
> > idioms.
> > >
> > > People who choose to use Outcome do understand RAII and will still use
> > it.
> > > But RAII does not handle all aspects of failure-safety, and this is
> about
> > > these other aspects that people may choose to go with Outcome rather
> than
> > > exceptions. One example: propagating information about failures across
> > > threads, or "tasks".
> > >
> >
> > There is exception_ptr for transporting exceptions between threads, which
> > was the only primitive that was missing for being able to accumulate
> > results from multiple workers.
> >
> > Outcome and Noexcept are simply better alternatives for users who write
> or
> > maintain code that is not exception-safe -- better not compared to
> > exception handling (which in this case can not be used), but compared to
> > what they would likely do otherwise.
> >
>
> To somewhat challenge this statement, The following is an example of how I
> would use Boost.Outcome if I had it available at the time when I was
> solving this parsing problem:
> https://github.com/akrzemi1/__sandbox__/blob/master/outcome_
> practical_example.md
> This tries to parse (or match) the string input, where I expect certain
> syntax, into a data structure.
> It is not performance-critical, I do not mind using exceptions, but I still
> prefer to handle situations where the input string does not conform to the
> expected syntax via explicit contrl paths. A number of reasons for that:
>
> 1. I want to separate resource acquisition errors (exceptions are still
> thrown upon memory exhaustion) from input validation.
>

Why?

> 2. Some debuggers/IDEs by default engage when any exception is thrown. I do
> not want this to happen when an incorrect input from the user is obtained.
>

"By default", so turn off that option.

> 3. I want validation failers to be handled immediately: one level up the
> stack. I do not expect or intend to ever propagate them further.
>

You can catch exceptions one level up if you want to. Right? :)

However, if you're only propagating errors one level up, it really doesn't
matter how you're handling them. I mean, how much trouble can you get into
in this case? It's trivial.

Error handling libraries are needed in more complex use cases where errors
must be propagated across multiple levels, across threads, across API
boundaries. The important design goals are:

1) The error object created by reporting code should be able to be
propagated across (potentially many) error-neutral contexts which should
not be required to "translate" it (that is, turn it into a different error
object.) The idea of translation of errors gave us exception specifications
which are notoriously one of the more embarrassing aspects of C++.

2) Error-neutral contexts should be able to ignore any errors reported by
lower level code but also intercept _any_ error, augment it with relevant
information (which may not be available at the point the error is detected)
and let it propagate up the call stack, intact.

3) Error-handling contexts should be able to recognize the errors they can
deal with but remain neutral to others.

Your use of outcome is probably fine in this simple case but

out::expected<Range, BadInput> parse_range (const std::string& input)

looks much too close to exception specifications:

Range parse_range(const std::string& input) throw(BadInput)

While this doesn't matter if your error handling is only one level up, it's
going to create a lot of problems in more complex cases because it makes
error-neutral contexts impossible and, as the exception specifications
fiasco showed, in general it is not possible for functions to even "know"
what errors might propagate through them. Assuming we agree that it is not
acceptable for error-neutral contexts to kill errors they don't recognize,
this is a problem.