Document number: N4313 Revises: N4123 Date: 2014-11-21 Project: Programming Language C++ Reference: ISO/IEC IS 14882:2011(E) Reply to: Artur Laksberg, Microsoft Corp. arturl@microsoft.com
This document proposes improvements to N4107, the current Working Draft of the Concurrency TS.
This document is a revision of N4123, which was presented to SG1 and LEWG at the Urbana meeting.
This document is motivated by N4032 and N4048, which were presented to SG1 at the Rapperswil meeting. This document collects the parts of these papers that received positive feedback from SG1 and presents them as a combined set of proposed changes to the Concurrency TS.
The paper also addresses numerous issues discovered and reported to the author by various individuals, namely Agustín Bergé, Vicente J. Botet Escriba, Hartmut Kaiser and Anthony Williams.
In addition to the changes proposed in N4123, this document implements the following suggestions from SG1 and LEWG received at the Urbana meeting:
std::experimental::async
function. This avoids
having to specify execution policies. This also
avoids the question of whether a future
created by
async
should block in the destructor.
when_any_back
, and change the return type of
when_any
to return the sequence of future
s
as well as the index of the ready future
.
Finally, the paper implements a small fix to the signature of make_ready_future
to support reference_wrapper
types. This fix was
requested
during the Urbana meeting but was not reviewed by SG1 or LEWG.
The author seeks feedback from LWG on this document. If approved by LWG, the changes proposed in this document will be incorporated into the next Working Paper of the Concurrency TS.
The proposed changes are presented as "diffs" to N4107 marked as insertions and
deletions.
Some of the extensions described in this Technical Specification represent
types and functions that are currently not part of the C++ Standards Library,
and because these extensions are experimental, they should not be declared
directly within namespace
std
. Instead, such extensions are
declared in namespace std::experimental
.
std
.
Unless otherwise specified, references to such entities described in this
Technical Specification are assumed to be qualified with
std::experimental
, and references to entities described in the C++
Standard Library are assumed to be qualified with std::
.
Since the extensions described in this technical specification
are experimental and not part of the C++ standard library, they
should not be declared directly within namespace
std
.
Unless otherwise specified, all components described in this technical specification either:
::experimental::concurrency_v1
to a namespace defined in the C++ Standard Library,
such as std
, or
std
.
Each header described in this technical
specification shall import the contents of
std::experimental::concurrency_v1
into
std::experimental
as if by
namespace std {
namespace experimental {
inline namespace concurrency_v1 {}
}
}
Unless otherwise specified, references to other entities
described in this technical specification are assumed to be
qualified with std::experimental::concurrency_v1::
,
and references to entities described in the standard are assumed
to be qualified with std::
.
Extensions that are expected to eventually be added to an
existing header <meow>
are provided inside the
<experimental/meow>
header, which shall include
the standard contents of <meow>
as if by
#include <meow>
New headers are also provided in the
<experimental/>
directory, but without such an
#include
.
|
This section describes tentative plans for future versions of this technical specification and plans for moving content into future versions of the C++ Standard.
The C++ committee intends to release a new version of this
technical specification approximately every year, containing the
library extensions we hope to add to a near-future version of the
C++ Standard. Future versions will define their contents in
std::experimental::concurrency_v2
,
std::experimental::concurrency_v3
, etc., with the
most recent implemented version inlined into
std::experimental
.
When an extension defined in this or a future version of this
technical specification represents enough existing practice, it
will be moved into the next version of the C++ Standard by
removing the experimental::concurrency_vN
segment of its namespace and by removing the
experimental/
prefix from its header's path.
For the sake of improved portability between partial implementations of various C++ standards,
WG21 (the ISO technical committee for the C++ programming language) recommends
that implementers and programmers follow the guidelines in this section concerning feature-test macros.
Implementers who provide a new standard feature should define a macro with the recommended name,
in the same circumstances under which the feature is available (for example, taking into account relevant command-line options),
to indicate the presence of support for that feature.
Implementers should define that macro with the value specified in
the most recent version of this technical specification that they have implemented.
The recommended macro name is "__cpp_lib_experimental_
" followed by the string in the "Macro Name Suffix" column.
Programmers who wish to determine whether a feature is available in an implementation should base that determination on
the presence of the header (determined with __has_include(<header/name>)
) and
the state of the macro with the recommended name.
(The absence of a tested feature may result in a program with decreased functionality, or the relevant functionality may be provided in a different way.
A program that strictly depends on support for a feature can just try to use the feature unconditionally;
presumably, on an implementation lacking necessary support, translation will fail.)
Doc. No. | Title | Primary Section | Macro Name Suffix | Value | Header |
---|---|---|---|---|---|
N3875 | Improvements to std::future |
future_continuations |
201410 | <experimental/future> |