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Subject: Re: [boost] [next gen future-promise] What to call themonadicreturn type?
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-05-26 05:20:01

On 26 May 2015 at 2:36, Peter Dimov wrote:

> > In my implementation, if you never call promise.get_future() you never get
> > synchronisation.
> That's an interesting use case. When does it occur?


> > The constexpr folding has the compiler elide all that.
> I suspect that what you call "constexpr folding" has nothing to do with
> constexpr, it's just inlining and ordinary non-constexpr optimization. KAI
> C++ was famous for doing such folding miracles decades ago.

You're right it's not in the standard. Well, actually it is, but

Let me explain. If you read what is allowed for constexpr at, and then write
logic which provides outcome paths which do nothing not constexpr,
then the compiler will completely elide code entirely at compile time
when those paths are followed. If you examine my code closely, you'll
see I always predicate the construction of anything with a
non-trivial destructor (e.g. exception_ptr) because non-trivial
destructors appear to force code output in current compiler
technology, though it may also be the atomic writes that
exception_ptr does.

This probably is not by accident. The machinery inside the compiler
to implement constexpr is probably reused for optimisation, or
rather, vice versa. Maybe a compiler vendor might chime in here to
tell us?

> > Also if you call promise.set_value() before promise.get_future(), you
> > never get synchronisation as futures are single shot.
> Another interesting case for which I've trouble imagining a practical use.
> :-)

Its main benefit for me is as a unit test smoke test. However
resumable functions, as currently proposed, should hugely benefit
from this pattern. I've emailed Gor about it.


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