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Posts Tagged ‘cplusplus0x

C++0x: No concepts no fun?

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“Concepts” was a proposed C++0x feature that was supposed to make generic programming more enjoyable. The intention was to render many guru-level “template tricks” superfluous, produce much better compile-time error messages and provide “modular type checking” so errors can be caught early (even before a template is instantiated).

  • Fact 1: Concepts have been removed from the C++0x standardization effort. See Doug Gregor’s blog post.
  • Fact 2: C++0x still includes a whole lot of other core language and library features.

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Written by pizer

August 16, 2009 at 6:29 pm

C++0x: Rvalue references 101

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If found a very nice article about rvalue references and move semantics (primarily) by Dave Abrahams. It also considers return value optimizations, exception safety and in what state an object should be left in after its resources have been “stolen”. Here it is.

[Edit 2009-08-18: The link doesn’t seem to work anymore. However, Dave Abrahams has started to refine, extend, and convert it into a series of blog posts on his new site ]

– P

Written by pizer

May 27, 2009 at 2:22 pm

C++0x: Do people understand rvalue references?

with 8 comments

[last update: 2009-05-27]

Occasionally I happen to see a piece of C++ code from an eager author that tries to utilize rvalue references but doesn’t understand them and gets it wrong. I remember that I had trouble fully grasping the rvalue reference concept. But now, it seems like that was way in the past. I can hardly remember why it was so difficult to understand rvalue references.

It may have something to do with the sources people use to learn about this feature. For example I wouldn’t consider the C++0x wikipedia article to be a good introduction for rvalue references. It seems too short for that. But this wikipedia article is a nice overview of what kind of features can be expected.

Let me just mention the real sources worth reading:

  • N1377: Move Proposal (2002)
  • N1385: Perfect Forwarding Problem (2002)
  • N2027: A Brief Introduction to Rvalue References (2006)
  • N2812: A Safety Problem with Rvalue References (2008)
  • N2844: Fixing a Safety Problem with Rvalue References (Revision 1, 2009)
  • Rvalue References 101by Dave Abrahams

I would recommend N2027 as an introduction and Dave Abraham’s article for more practical details including exception safety and return value optimizations.

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Written by pizer

April 13, 2009 at 12:50 pm

A little C++0x finger exercise

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Since the C++ compiler of GCC 4.3.2 comes with partial C++0x support and I had not tested all of these features yet, I decided to do a little finger exercise: “function”.

“function” is a useful library feature of Boost, TR1, and the upcoming C++ standard library. It’s basically a class template mimicing the behaviour of a function pointer. But it’s much more flexible because it applies to any “callable object” — function pointers and objects with overloaded function call operator. Also, the signatures don’t have to match perfectly as long as the parameters and return values are implicitly convertible. I’m going to show you how this library feature can be implemented with C++0x features — including variadic templates and rvalue references. But before going into any details let me give an example of how code that uses this “function” library might look like. In case you’re already familiar with this library feature you can skip the following piece of code and paragraph of text.

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Written by pizer

March 28, 2009 at 9:59 pm

Surprizing C++ reference-binding rules

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Isn’t it amazing? You see a very simple piece of code that looks perfectly fine and wonder why it is supposed to be ill-formed:

  struct foo {
    foo(const foo& rhs);

  void func1(const foo&);

  void func2() {
    foo o;
    func1( o ); // ok
    func1( foo() );  // ill-formed, foo's copy-ctor is private

Although an rvalue is allowed to bind to a reference-to-const, the C++ standard allows a compiler to copy an rvalue to another temporary rvalue before binding it to a reference-to-const. Obviously, you need to have a public copy constructor for that.

I really fail to see the merit in this rule. It seems that every available C++ compiler does not create an extra temporary object here and some of these compilers don’t even bother to report an error in this situation (g++ version 3.4.5 accepts the above code).

Fortunately, the upcoming version of the C++ standard removes the CopyConstructible requirement for reference binding. It makes the code above well-formed.

Source: this usenet thread

– P

Written by pizer

March 25, 2009 at 1:56 pm

C++0x: concepts & rvalue references — 2nd round

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Last update: 2009-02-1

Concepts and rvalue references are two new features that are going to be included in the next official version of the C++ language with high probability. I’m not going to mention the basics behind those two features because it has been done before in other articles. Try this google video or this article for an introduction to concepts by Doug Gregor and this article for an introduction to rvalue references by Howard Hinnant et al.

I already wrote a post on concepts and rvalue references here which discussed some problems when concepts are combined with rvalue references. It turned out that David Abrahams and Doug Gregor were also aware of the first issue I addressed. They published an article one day before I wrote about it. In this post I’m going to address other arguments (consistency and intuitivity) for why I think their proposed resolution is a good idea and what other language modification can increase consistency. Before I go into the last bit I will shed some light on what function requirements actually mean and how they can be satisfied.

Update 2009-02-1:[ It turns out you can’t exprapolate the meaning of function requirements from the concept_map checking rules because they don’t cover all use cases in constrained contexts (see hole in concepts system). So, keep this in mind while reading the rest of this article.]

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Written by pizer

January 8, 2009 at 7:04 pm

C++0x: Interaction between rvalue references & concepts

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Last update: 2008-12-08

I noticed some unfortunate gotchas when using two of the upcoming C++ features together: Rvalue references and concepts/templates. One of these problems can actually be seen in the currend draft of the future C++ specification. As far as I can tell, it looks like a subtle but nasty bug. Check out chapter 23.2.6 [vector, page 826]:

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Written by pizer

December 6, 2008 at 6:44 pm