Re: [boost] expected/result/etc

Subject: Re: [boost] expected/result/etc
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2016-01-26 05:03:46

On 26 Jan 2016 at 0:16, Michael Marcin wrote:

> Some months ago there was a lot of discussion regarding Rust style 
> return values for non-exceptional error handling.

Mine was one of many discussed. Its current name is Outcome.

> Did anything ever come of this?

Yes, Outcome continues to mature as it is being used to write AFIO 
v2. The non-futurey bits are quite mature by now and have seen very 
extensive usage. The futurey bits aren't being used in the AFIO v2 
rewrite yet, and so haven't had the same quantity of very fine level 
tweaking.

If I do say so myself, it is *spectacularly* well suited to this use 
case, producing seriously high quality assembler output. I am very, 
very pleased with it.

> Is there a useable library?

https://github.com/ned14/boost.outcome

It requires VS2015 Update 1 or newer, or any C++ 14 compiler. Some 
example AFIO v2 code using it:
 
https://github.com/BoostGSoC13/boost.afio/blob/master/fs_probe/include
/handle.hpp

https://github.com/BoostGSoC13/boost.afio/blob/master/fs_probe/include
/detail/impl/windows/handle.ipp

As you'll see, the enormous win is that almost every function apart 
from constructors is now noexcept. That causes the compiler to do a 
very good job indeed of inlining and eliminating code from the 
assembler output, plus static analysis tools spot when you forget to 
properly handle an errored return because it'll try throwing in a 
noexcept function. I expect AFIO v2 to have an undetectable overhead 
relative to syscalls, even no op syscalls.

Two big items remain todo for Outcome:

1. I want to write a clang-tidy extension which rewrites any outcome 
returning function to be noexcept unless commented otherwise. This 
prevents one forgetting to add noexcept.

2. I want another clang-tidy extension to mark up outcome returning 
functions with the compiler-specific markup to complain if you forget 
to do something with the returned value. Right now outcomes have the 
same problem as C error returns, one can forget to handle them 
altogether.

I also strongly emphasise that nothing in Outcome is fixed, things 
may yet change. Outcome is not in the Boost review queue, nor may 
never be.

Niall

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