Blog Archive for / 2010 / 10 /

Coming Soon: Just::Thread Pro

Friday, 29 October 2010

Multithreaded code doesn't have to be complicated.

That's the idea behind the Just::Thread Pro library. By providing a set of high level facilities in the library, your application code can be simplified — rather than spending your time on the complexities of multithreading and concurrency you can instead focus on what it is your application is trying to achieve.

Building on the Just::Thread C++0x thread library, Just::Thread Pro will provide facilities to:

• Encapsulate communication between threads to avoid deadlocks and race conditions
• Easily scale your application to make use of multi-core processors
• Parallelize existing single-threaded code without a major rewrite

Just::Thread Pro will be available for all platforms supported by Just::Thread.

Head over to the Just::Thread Pro website and sign up to receive further news about the library and notification when it is released.

Posted by Anthony Williams
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Tags: concurrency, cplusplus, multithreading
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October 2010 C++ Standards Committee Mailing

Thursday, 21 October 2010

The October 2010 mailing for the C++ Standards Committee was published earlier this week. This is the pre-meeting mailing for the November 2010 committee meeting.

As well as the usual core and library issues lists, this mailing also includes a Summary of the status of the FCD comments, along with a whole host of papers attempting to address some the remaining FCD comments.

To move or not to move

The big issue of the upcoming meeting is looking to be whether or not the compiler should implicitly generate move constructors and move assignment operators akin to the copy constructors and copy assignment operators that are currently auto generated. The wording in the FCD requires this, but people are concerned that this will break existing code when people start using their code with a C++0x compiler and library. There are two papers on the subject in the mailing: N3153: Implicit Move Must Go by Dave Abrahams, and N3174: To move or not to move by Bjarne Stroustrup.

There seems to be consensus among committee members that the FCD requires compilers to generate the move constructor and move assignment operator in cases that will break existing code. The key question is whether the breakage can be limited by restricting the cases in which the move members are implicitly generated, or whether implicit generation should be abandoned altogether. The various options are explained very clearly in the papers.

Exceptions and Destructors

N3166: Destructors default to noexcept is another potentially controversial issue. It is generally acknowledged that throwing exceptions from destructors is a bad idea, not least because this leads to termination if the destructor is invoked whilst the stack is being unwound due to another exception. Herb Sutter wrote about this way back in 1998 when the original C++ standard was hot off the presses, in GotW #47: Uncaught Exceptions.

The proposal in the paper comes from a Finnish comment on the FCD, and is quite simple: by default all destructors are assumed to be marked noexcept(true) (which is the new way of saying they cannot throw an exception, similar to an exception specification of throw()), unless they explicitly have a non-empty exception specification or are marked noexcept(false).

Since it is generally good practice not to throw from a destructor, you'd think this would be uncontroversial. Unfortunately it is not the case — there are currently situations where throwing from a destructor has defined behaviour, and even does exactly what people want. The example most frequently cited is the SOCI project for accessing databases from C++. This library provides an easy syntax for constructing SQL queries using the << operator. The operator builds a temporary object which executes the SQL in the destructor. If the SQL is invalid, or executing it causes an exception for any other reason then the destructor throws. Changing destructors to be noexcept(true) by default will make such code terminate on a database error unless the destructor is updated to declare that it can throw exceptions. Working code with defined behaviour is thus broken when recompiled with a C++0x compiler.

Concurrency-related papers

There are 3 concurrency-related papers in this mailing, which I've summarised below.

N3152: Progress guarantees for C++0x (US 3 and US 186)

The FCD does not make any progress guarantees when multiple threads are used. In particular, writes made by one thread do not ever have to become visible to other threads, and threads aren't guaranteed ever to actually run at all. This paper looks at the issues and provides wording for minimal guarantees.

N3164: Adjusting C++ Atomics for C Compatibility

This is an update to N3137 from the last mailing, which provides detailed wording updates for the required changes to regain compatibility with C1X atomics.

N3170: Clarifying C++ Futures

There were a few FCD comments from the US about the use of futures; this paper outlines all the issues and potential solutions. The proposed changes are actually fairly minor though:

• future gains a share() member function for easy conversion to the corresponding shared_future type;
• Accessing a shared_future for which valid() is false is now required to throw an exception rather than be undefined behaviour;
• atomic_future is to be removed;

A few minor changes have also been made to the wording to make things clearer.

If you have any opinions on any of the papers listed here, or the resolution of any NB comments, please add them to the comments for this post.

Posted by Anthony Williams
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Tags: C++0x, C++, standards, concurrency
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just::thread C++0x Thread Library V1.4.2 Released

Friday, 15 October 2010

I am pleased to announce that version 1.4.2 of just::thread, our C++0x Thread Library has just been released.

The big change with this release is the new support for gcc 4.5 on Ubuntu Linux. If you're running Ubuntu Lucid then you can get the .DEB files for gcc 4.5 from yesterday's blog post. For Ubuntu Maverick, gcc 4.5 is in the repositories.

Other changes:

• Overflow in ratio arithmetic will now cause a compilation failure
• Ratio arithmetic operations derive from the resulting std::ratio instantiation as well as providing the ::type member to better emulate the C++0x working draft
• On Windows, just::thread can now be used in MFC DLLs

Purchase your copy and get started with the C++0x thread library now.

As usual, existing customers are entitled to a free upgrade to V1.4.2 from all earlier versions.

Posted by Anthony Williams
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Tags: multithreading, concurrency, C++0x
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gcc 4.5 Packages for Ubuntu Lucid

Thursday, 14 October 2010

Ubuntu Maverick was released earlier this week. Amongst other things, gcc 4.5 is available in the repositories, whereas for previous versions you had to build it yourself from source.

In order to save you the pain of compiling gcc 4.5 for yourself (which can take a while, and overheated my laptop when I tried), I've built it for Ubuntu Lucid, and uploaded the .deb files to my website. The .debs are built from the Maverick source packages for gcc 4.5.1, binutils 2.20.51, cloog-ppl and mpclib, and I've built them for both i386 and amd64 architectures.

• binutils_2.20.51.20100908-0ubuntu2_amd64.deb
• binutils_2.20.51.20100908-0ubuntu2_i386.deb
• cpp-4.5_4.5.1-7ubuntu2_amd64.deb
• cpp-4.5_4.5.1-7ubuntu2_i386.deb
• g++-4.5_4.5.1-7ubuntu2_amd64.deb
• g++-4.5_4.5.1-7ubuntu2_i386.deb
• gcc-4.5-base_4.5.1-7ubuntu2_amd64.deb
• gcc-4.5-base_4.5.1-7ubuntu2_i386.deb
• gcc-4.5_4.5.1-7ubuntu2_amd64.deb
• gcc-4.5_4.5.1-7ubuntu2_i386.deb
• libcloog-ppl0_0.15.9-2_amd64.deb
• libcloog-ppl0_0.15.9-2_i386.deb
• libgcc1_4.5.1-7ubuntu2_amd64.deb
• libgcc1_4.5.1-7ubuntu2_i386.deb
• libgomp1_4.5.1-7ubuntu2_amd64.deb
• libgomp1_4.5.1-7ubuntu2_i386.deb
• libmpc2_0.8.2-1build1_amd64.deb
• libmpc2_0.8.2-1build1_i386.deb
• libstdc++6-4.5-dev_4.5.1-7ubuntu2_amd64.deb
• libstdc++6-4.5-dev_4.5.1-7ubuntu2_i386.deb
• libstdc++6_4.5.1-7ubuntu2_amd64.deb
• libstdc++6_4.5.1-7ubuntu2_i386.deb

Enjoy!

Posted by Anthony Williams
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Tags: gcc, lucid, ubuntu
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Concept Checking Without C++0x Concepts

Wednesday, 06 October 2010

My latest article, Concept Checking Without Concepts in C++ was published on the Dr Dobb's website a couple of weeks ago.

One of the important features of the now-defunct C++0x Concepts proposal was the ability to overload functions based on whether or not their arguments met certain concepts. This article describes a way to allow that for concepts based on the presence of particular member functions.

The basic idea is that you can write traits classes that detect particular sets of member functions. Function overloads that require these concepts can then be enabled or disabled by using std::enable_if with these traits.

The example I use is checking for a Lockable type which has lock(), unlock() and try_lock() member functions, but the same technique could easily be used for other concepts that required other member functions.

Read the article for the full details.

Posted by Anthony Williams
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Tags: concepts, cplusplus
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