Detlef Vollmann <dv@vollmann.ch>

N3071: Renaming launch::any and what asyncs really might be (Rev.)

Introduction

N2996 introduced two new function calls async to start some function asynchronously and return a future to obtain the result of the function. N2996 allowed for two mechanisms to run that function: in another thread or as deferred function in the same thread.

Discussion

The two launching mechanisms defined by N2996 make sense for the current revision of the standard, but future revisions of C++ might allow for a much larger variety of launching mechanisms. Some potential mechanisms might be:

• launching in a specific thread pool
• launching in a separate process
• launching on a different computing unit (such as GPU)
• launching on a different machine (e.g. through MPI)
• ...

Some of these future mechanisms might want to use the existing async function, just with a different launch policy. With this in mind, the naming of launch::any is unfortunate, as it only allows for "calling in another thread" and "calling in the same thread".

So I propose to change launch::any to launch::sync_or_async.

Proposed Changes

In 30.6.1 [futures.overview], header <future> synopsis, enum class launch:
Change:

anysync_or_async

In 30.6.9 [futures.async], paragraph 2, Effects clause:
Change:

Effects:

The first function behaves the same as a call to the second function with a policy argument of launch::anysync_or_async and the same arguments for F and Args.

And in the tird bullet:
Change:

• launch::anysync_or_async — the implementation may choose either policy above at any call to async.