Motivation for atomic_future

[Scott Meyers]

The only motivation I can find for atomic_future is this (from N2997):

The need for an atomic_future arose from the fact that the move constructor
and both assigments made the shared_future much more error prone to use a
single instance from multiple threads. But such use is required when more
than one thread scans a global list of futures for results
provided by worker
threads.

That's a bit terse. Can somebody please elaborate on the motivation
for atomic_future?

Thanks,

Scott

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[Anthony Williams]

Scott Meyers <use...@aristeia.com> writes:

> The only motivation I can find for atomic_future is this (from N2997):
>
> The need for an atomic_future arose from the fact that the move constructor
> and both assigments made the shared_future much more error prone to use a
> single instance from multiple threads. But such use is required when more
> than one thread scans a global list of futures for results
> provided by worker
> threads.
>
>
> That's a bit terse. Can somebody please elaborate on the motivation
> for atomic_future?

Every member function of atomic_future is fully synchronized, so if you
call get() whilst another thread assigns a new value to the future then
either you block on the original future or you block on the newly
assigned one, but there is no data race. With a shared_future it is just
the associated state which is synchronized, so calling get() and
operator= concurrently is a data race.

This means that e.g. you can have a global array of atomic_future
objects where each future is assigned individually by a thread as it
creates the task to determine the future value. Another thread that
waits for the values can thus wait on each future without worrying about
the potential for a data race.

Anthony
--
Author of C++ Concurrency in Action | http://www.manning.com/williams
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