New paper: N3536, C++ Sized Deallocation -- Lawrence Crowl

By Blog Staff | Mar 6, 2013 06:02 PM | Tags: None

A new WG21 paper is available:

Document number: N3536

Date: 2013-03-05

C++ Sized Deallocation

by Lawrence Crowl

Excerpt:

Problem

With C++11, programmers may define a static member function operator delete that takes a size parameter indicating the size of the object to be deleted. The equivalent global operator delete is not available. This omission has unfortunate performance consequences.

Modern memory allocators often allocate in size categories, and, for space efficiency reasons, do not store the size of the object near the object. Deallocation then requires searching for the size category store that contains the object. This search can be expensive, particularly as the search data structures are often not in memory caches.

Solution

Permit implementations and programmers to define sized versions of the global operator delete. The compiler shall call the sized version in preference to the unsized version when the sized version is available.

...

New paper: N3526, Uniform initialization for arrays and class aggregate types -- Michael Price

By Blog Staff | Mar 6, 2013 06:02 PM | Tags: None

A new WG21 paper is available:

Document number: N3526

Date: 2013-01-21

Uniform initialization for arrays and class aggregate types

by Michael Price

An excerpt:

This document proposes a slight relaxation of the rules for eliding braces from aggregate initialization in order to make initialization of arrays and class aggregates more uniform. This change is required in order to support class aggregate types with a single member subaggregate that behave similarly to their array counterparts (i.e. std::array).

[...]

However, when we begin to mix the aggregate types, uniform initialization begins to break down.

  struct aggr_t {
      int a;
      int b;
  }  array_of_aggr[2] = {{1, 2}, {3, 4}};

  struct aggr_ex_t {
      int x[2][2];
  };

  aggr_ex_t bad  = {{1, 2}, {3, 4}};      // Error: Too many initializers, see below for details
  aggr_ex_t good = {{{1, 2}, {3, 4}}};

New paper: N3529, Minutes of SG5 (Transactional Memory) Teleconferences

By Blog Staff | Mar 1, 2013 08:50 AM | Tags: None

A new WG21 paper is now available, and will appear in the pre-Bristol meeting. This paper contains minutes of the regular (usually bi-weekly) teleconferences of Study Group 5 working on Transactional Memory.

Document Number: N3529

Date: 2013-02-14

SG5: Transactional Memory (TM) Meeting Minutes 2012/10/30-2013/02/04

by Michael Wong

Concepts Lite: Constraining Templates with Predicates -- Andrew Sutton, Bjarne Stroustrup

By Blog Staff | Feb 24, 2013 07:34 AM | Tags: experimental

During the C++11 standards development cycle, much work was done on a feature called "concepts" which aimed at providing systematic constraints on templates. Concepts was deferred from C++11 for lack of time to complete it, but work has continued.

In January 2012, the results of a major "concepts summit" were published as a 133-page report titled "A Concept Design for the STL" (WG21 paper N3351).

Now, a draft of new paper is available proposing a very useful subset of concepts, dubbed "Concepts Lite", for near-term consideration including at the spring ISO C++ meeting in Bristol, UK, this April. For example, imagine writing this template:

template<Sortable Cont>
void sort(Cont& container);

and when you call it like this:

list<int> lst = ...;   // oops, bidirectional iterators
sort(lst);             // today, results in very long "template spew" error message

getting this short and non-cryptic error message:

error: no matching function for call to ‘sort(list<int>&)’
   sort(l);
         ^
note: candidate is:
note: template<Sortable T> void sort(T)
   void sort(T t) { }
        ^
note: template constraints not satisfied because
note:   'T' is not a/an 'Sortable' type [with T = list<int>] since
note:     'declval<T>()[n]' is not valid syntax

That's an actual error message from the prototype GCC implementation linked below.

We're very excited about this feature and its continued progress. Here are links to the draft of the new paper:

Concepts Lite: Constraining Templates with Predicates (PDF) (Google Docs)

From the Introduction:

In this paper, we introduce template constraints (a.k.a., “concepts lite”), an extension of C++ that allows the use of predicates to constrain template arguments. The proposed feature is minimal, principled, and uncomplicated. Template constraints are applied to enforce the correctness of template use, not the correctness of template definitions. The design of these features is intended to support easy and incremental adoption by users. More precisely, constraints:

• allow programmers to directly state the requirements of a set of template arguments as part of a template’s interface,
• support function overloading and class template specialization based on constraints,
• fundamentally improve diagnostics by checking template arguments in terms of stated intent at the point of use, and
• do all of this without any runtime overhead or longer compilation times.

This work is implemented as a branch of GCC-4.8 and is available for download at http://concepts.axiomatics.org/~ans/. The implementation includes a compiler and a modified standard library that includes constraints. Note that, as of the time of writing, all major features described in this report have been implemented.

Related links:

• Bjarne Stroustrup, Andrew Sutton, et al., "A Concept Design for the STL" (ISO/IEC JTC1/SC22/WG21 N3351, January 2012).

New forum active: SG8 (Concepts)

By Blog Staff | Feb 24, 2013 07:27 AM | Tags: None

A new publicly readable committee forum is now available on the Forums page:

SG8 : Concepts

Near-term focus is on a convergence between the static if proposals and the parameter-type-constraints subset of concepts.

Note that this forum is for standardization of a new feature; do not expect the features mentioned in this forum to be available yet in your own compiler. However, if you're interested in seeing the shape of the feature as it advances to being approved by the committee likely in the near future, this forum is for you.

For information about this and other SGs, see also:

• Standardization | The Committee -- Subgroups and Study Groups
• Standardization | Standing Documents | SD-3 -- Study Group Organizational Information

Open and Efficient Type Switch for C++ -- Solodkyy, Dos Reis, and Stroustrup

By Blog Staff | Feb 13, 2013 06:41 PM | Tags: intermediate advanced

Here's a recent highlight from the pre-Portland mailing that you might have missed:

Open and Efficient Type Switch for C++

Yuriy Solodkyy, Gabriel Dos Reis, Bjarne Stroustrup

... we implement a type switch construct as an ISO C++11 library, called Mach7. This library-only implementation provides concise notation and outperforms the visitor design pattern. ... For closed sets of types, its performance roughly equals equivalent code in functional languages, such as OCaml and Haskell.

C++ is a powerful library-building language. Whenever possible, we prefer to add new functionality as a library rather than in the language. This is an excellent example of where a C++ library-only solution can get equivalent performance to the language support included in some popular functional languages.

New paper: N3528, Minutes of Feb 5 2013 SG1 (concurrency) teleconference

By Blog Staff | Feb 8, 2013 09:45 AM | Tags: None

As an experiment to make C++ standardization papers more accessible, we're trying out a new idea: posting links to papers as the papers become available. This does not replace the committee mailings; the papers will still be collected and made available in the regular mailings (about four times a year) and those mailings will also still be announced here. The purpose of this experiment is to try to publish papers earlier and more often, so as to make it easier to get feedback sooner from a wider audience and reduce the glut of papers that need to be absorbed before meetings. If you are not a committee member, please use the comments section below or the std-proposals forum for public discussion.

Note that some papers, like this one, will be mainly informational rather than proposal papers for technical discussion. 

Here's the first:

Document Number: N3528

Date: 2013-02-05

 

Minutes of Feb 5 2013 SG1 Phone Call

by Pablo Halpern

Online C++ compilers

By Blog Staff | Jan 31, 2013 11:29 AM | Tags: None

Many people don't realize how many web pages offer access to try out C++ compilers, including many of the latest compilers with burgeoning C++11 language support. So we thought we'd publish a list.

Do you:

• want to try out C++, but don't have a compiler installed?
• want to try out a C++11 feature your compilers don't yet support?
• want to compare the results of compiling a test program using different compilers?

Then try one of these online compilers! Some are compile-only to let check whether your code is legal, and some let you also run your test programs to see their output. For each, we include a list of the compilers that the page currently supports -- they include the latest from Clang (3.2, Dec 2012), GCC (4.8.0 prerelease), Intel (13.0, Oct 2012), and Microsoft (VC++ alpha CTP, Nov 2012).

• LiveWorkspace (Clang 3.2, GCC 4.6.3 - 4.7.2)
• gcc.godbolt.org (Clang 3.0, GCC 4.5.3 - 4.8.0 prerelease, Intel ICC 13.0.1)
• Rise4Fun (Microsoft VC++ 2012 and November 2012 CTP)
• Stacked-Crooked (GCC 4.7)
• ideone.com (GCC 4.3.4 and 4.5.1)
• Comeau Test Drive (EDG 4.3.3)

This list is now also available on the Get Started! page.

Reminder: Registration deadline for spring ISO C++ meeting

By Blog Staff | Jan 21, 2013 02:05 PM | Tags: None

Today on the committee reflector, the Bristol standards meeting host, Roger Orr, reminded us of the registration deadline.

This is also of interest to people who are not regular standards meeting. If you are in the Bristol area or otherwise interested in attending part of the standards meeting to see what's going on, you're welcome to attend -- see "How to Participate at Face-to--Face Meetings" on the Meetings and Participation page for more details.

Roger Orr writes:

Hello all, here is a reminder of booking details for the meeting.

 

Many of you have already booked for the Apr 15-20 (6-day) meeting of WG21 in Bristol, UK, and need read no further.

If you have not yet booked but are intending to come, please note that the room reservations at the hotel are time-limited. To ensure a room please book before the release date early in February.

 

• The hotels will not claim the cost until the conference.
• There is no time limit for registering for attendance if you are making other arrangements for accommodation.

The following link will take you to the WG21 tab of the ACCU booking site:

 

http://www.cvent.com/events/accu-2013/custom-22-09ec03b22c4f4a0a832e28126a4585fc.aspx

As mentioned in N3397, the event is colocated with the ACCU conference (Apr 9-13).

You can book hotel accomodation at the time of registration: either in the conference hotel -- the Bristol City Centre  Marriott -- or the nearby Bristol Hotel (Cabot Circus) which is a little cheaper.

 

If you are only attending WG21 select registration type "WG21 conference only" in online registration.

If you are attending both the ACCU conference and the WG21 meeting you can book both at the same time (and therefore get continuous hotel room occupation) select registration type ACCU Member or ACCU Non-Member as appropriate.

 

The site also contains some tabs with information about Bristol, the hotels, and travel.

If you have any questions about Bristol please contact me (host), or Herb Sutter (convenor) as appropriate.

 

I look forward to seeing many of you in Bristol in April!

Regards,

Roger.

January standards papers mailing available

By Blog Staff | Jan 21, 2013 11:46 AM | Tags: None

The between-meetings standards papers mailing is now available. It includes the spring meeting agenda, updated issues lists, and a number of new papers including at least one that came through the public std-proposals forum. (Update: Please also direct discussion about these papers to that forum.)