Re: [boost] Non-allocating future-promise

Subject: Re: [boost] Non-allocating future-promise
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-10-21 01:57:40

On 19/10/2014 14:18, Niall Douglas wrote:
> However, sometimes you will always do:
>
> promise<int> &p;
> auto f=p.get_future().then([]); // always continue
>
> There a memory allocation and std::function wrap is totally
> unnecessary because the compiler has sufficient information to
> construct f statically, except that the present design in N4123
> requires it.
>
> This is what I mean by static continuations - ones which always
> happen unconditionally.

I don't think code like the above would actually occur in the wild.  (Or 
at least if it did, I would regard it as a bug until proven otherwise.)

There should be no reason to attach a continuation to the future in code 
that still has the promise in scope -- it can simply execute that code 
directly itself before setting the promise's value.

In "normal" code, the future is returned from a method to outside code 
that has no access to the promise, and it is that outside code that 
attaches continuations.  I don't think that the optimisation you refer 
to applies in that case.