- Boost - lists.preview.boost.org

01 Jul '04

I'm trying to use iter_fold with a vector_c to compute the sum of all of the integers in the container. What I have is: template<typename Sequence> struct sum { typedef typename mpl::iter_fold< Sequence, typename mpl::begin<Sequence>::type, mpl::plus< mpl::deref<_1>, mpl::deref<_2> >; // ^ what should this really be? > // what do i put after here? }; Which obviously won't work. I don't know how to keep a running total as I iterate over the sequence with plus. Help! Thanks, -Dan

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ccref
by alnsn＠yandex.ru 01 Jul '04

01 Jul '04

This is not a typo, I'm trying to emphasize that ref, cref, ... sequence lacks one element which imposes more strict requirements on its argument. According to IQ tests logic that sequence should be continued with ccref :) If seriously, cref allows silent binding of a temporary to a reference. Apparently, this is not always fine. Inspired by N1610 (thanks to Dave and Gary) I've tried to detect both const and non-const rvalues. I didn't yet opened my copy of the standard to check it but the code seems to work on g++ 3.2.2, g++ 3.4.0, Intel 8.0 (all under linux) and Comeau online. #include <boost/ref.hpp> template<class T1, class T2> struct enable_if_same { }; template<class T> struct enable_if_same<T,T> { typedef int type; }; template<class T> boost::reference_wrapper<T const> const ccref(T volatile& r, typename enable_if_same<T,T const>::type = 0) { return boost::reference_wrapper<T const>(const_cast<T const&>(r)); } struct convertible_to_int { operator int() const { return 0; } }; template<class T> boost::reference_wrapper<T const> const ccref(T const volatile& r) { return boost::reference_wrapper<T const>(const_cast<T const&>(r)); } int rvalue() { return 0; } int const const_rvalue() { return 0; } int main() { int i; int const j = 0; ccref(i); ccref(j); // Try uncommenting any of these lines: // ccref(rvalue()); // ccref(const_rvalue()); } -- Alexander Nasonov

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Links at ACCU
by David Abrahams 01 Jul '04

01 Jul '04

Hi, I recently got this request from the ACCU website admins: Please take a look at www.accu.org/terse/cpp.htm and let me know how the Boost library/people would like to be represented on www.accu.org/terse/cpp.htm. So I figured I'd ask. Anyone have an opinion? -- Dave Abrahams Boost Consulting http://www.boost-consulting.com

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RE: [boost] Re: Cvs tag dependency
by Rozental, Gennadiy 01 Jul '04

01 Jul '04

> "Rozental, Gennadiy" <gennadiy.rozental(a)thomson.com> writes: > > > Hi, > > > > Inn a light of recent discussion on earlier freezing, I > have following > > question/proposition: > > > > Is it exist in Boost.Build or could we add feature that allow us to > > specify cvs tag dependency as a dependency grist? > > I _think_ what you're proposing is interesting, but the means > of getting there may be completely inappropriate for > Boost.Build, if I understand it at all. Why don't you try to > say what you're trying to accomplish without trying to design > the Boost.Build feature? I want to be able to use regression testing facility not only for HEAD cvs state. > Are you trying to get cvs checkout > actions into the build system? Namely. I want build system be aware about different versions of files/components and want to be able to specify which version I prefer to use for this particular test. > If so, why? What's wrong with doing > > cvs update -rwhatever_tag > before kicking off the test? Nothing. I just want it happened automatically on a computer that performs regression testing, depends on a rule defined in a Jamfile. > Dave Abrahams Gennadiy.

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[MPL] rational_c and fixed_c
by Cromwell Enage 30 Jun '04

30 Jun '04

A while back it was mentioned that boost::mpl::rational_c and boost::mpl::fixed_c needed redesigning. Before I attempt to tackle this task anymore than I already have, I should have some questions answered. 1. For those compilers for which BOOST_NO_LIMITS is defined, I've learned that there is no way to include a floating-point number in an integral constant expression. Does this mean that no matter how I define fixed_c::value, I cannot pass fixed_c as a template argument to a metafunction that ordinarily takes integral_c as input? Do I have to duplicate the functionality as a result? If so, what should be the naming convention for these new metafunctions? 2. Should I define fixed_c in terms of rational_c, to reduce the amount of functionality I may need to duplicate? Something like this: template <typename IntType, IntType N, IntType PowerOf10> struct fixed_c { typedef rational_c<IntType,...,...> type; }; 3. Eventually the compile-time rational-number representation will need to be converted to a runtime floating-point value. Should I define this conversion function within rational_c (which means an additional template parameter, with either function or class scope, defining the return value type), within a wrapper struct, or somewhere else? And how? 4. I only have to deal with rational numbers, right? I don't have to worry about accurately representing a transcendental number or a complex-number expression? TIA! Cromwell Enage __________________________________ Do you Yahoo!? Yahoo! Domains � Claim yours for only $14.70/year http://smallbusiness.promotions.yahoo.com/offer

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Implicit failures in MetaComm regression tests?
by Doug Gregor 30 Jun '04

30 Jun '04

The Boost.Function test case "lambda_test" for intel-win32-7.1-vc6 (and its STLport variant) come out as unexpected passes. I'd like to move these over to explicit passes, but they don't seem to be marked as expected failures in status/explicit-failures-markup.xml. How can I make these expected passes, to get that nice cushy all-green feeling? Doug

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Cvs tag dependency
by Rozental, Gennadiy 30 Jun '04

30 Jun '04

Hi, Inn a light of recent discussion on earlier freezing, I have following question/proposition: Is it exist in Boost.Build or could we add feature that allow us to specify cvs tag dependency as a dependency grist? Example1: Jamfile ------- MY_SRCS = .... tag_to_test = my_lib_dev # HEAD run my_test : <tag:$(tag_to_test)>$(MY_SRCS) Now If I want to test my new development branch I just set tag to appropriate value. Example2: Jamfile ------- exe my_test : ...sources... <tag:1_31_0><lib>../build/boost_unit_test_framework Now I want stable version of library - I specify which one I want. Example3: Jamfile ------- exe my_test : ...sources... <tag:1_30_0>...abc.h Here I know my code works with that version of header file I want to use it specifically. And so on. This feature could allow us to: 1. Test development branches for regressions 2 .Test new features before moving it in cvs HEAD 3. Depend on stable version of dependency, without need to keep up with changes in it. 4. Test development version Boost.Test with stable version of it e.t.c I am sure it's not easy to do. But may be it worth a shot. Gennadiy.

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boost::variant visitor member instantiation question
by craig perras 30 Jun '04

30 Jun '04

Hello - I want to do type conversions (checked at compile-time) using boost::variant. So I naively made a 'convert' visitor, which has designed to (hopefully) safely convert the variant value to another type (eg: a client might try to assign the variant's int value to an unsigned short). For instance: typedef boost::variant < std::string, int, double > ParamValue; // ----- ConvertParamValueVisitor //T is the type which will be assigned the variant's value //V is the Variant/Visitor template<typename T> struct ConvertParamValueVisitor<T> { typedef ConvertParamValueVisitor self; typedef void result_type; self( T*& val ) : value_(val) {} template<typename V> result_type operator()( const V& v, typename boost::enable_if<TypeTraits::is_arithmetic<V> >::type* dummy = 0 ) const { *value_ = boost::numeric_cast<T>(v); } template<typename V> result_type operator()( const V& v, typename boost::enable_if<TypeTraits::is_string<V> >::type* dummy = 0 ) const { *value_ = v; } private: T*& value_; }; It seems boost::variant will instantiate all members for all variant types, even the ones I try to suppress using enable_if. To work around this, I've tried various things, like specializing ConvertParamValueVisitor based on T, using lazy_enable_if, etc. Here's one attempt: // ----- ConvertParamValueVisitor //T is the type which will be assigned the variant's value //V is the Variant/Visitor //not implemented template<typename T, class Enable = void> struct ConvertParamValueVisitor; //ConvertParamValueVisitor int, double: use numeric cast template<typename T> struct ConvertParamValueVisitor<T, typename boost::enable_if<TypeTraits::is_arithmetic<T> >::type > { typedef void result_type; typedef ConvertParamValueVisitor self; self( T*& val ) : value_(val) {} template<typename V> result_type operator()( const V& val, typename boost::enable_if<TypeTraits::is_arithmetic<V> >::type* dummy = 0 ) const { *value_ = boost::numeric_cast<T>(val); } //not implemented template<typename V> result_type operator()( const V& val, typename boost::enable_if<TypeTraits::is_string<V> >::type* dummy = 0 ) const; private: T*& value_; }; //ConvertParamValueVisitor string template<typename T> struct ConvertParamValueVisitor<T, typename boost::enable_if<TypeTraits::is_string<T> >::type > { typedef void result_type; typedef ConvertParamValueVisitor self; self( T*& val ) : value_(val) {} //not implemented template<typename V> result_type operator()( const V& val, typename boost::enable_if<TypeTraits::is_arithmetic<V> >::type* dummy = 0 ) const; template<typename V> result_type operator()( const V& val, typename boost::enable_if<TypeTraits::is_string<V> >::type* dummy = 0 ) const { *value_ = val; } private: T*& value_; }; Is there any way to get something like this to work? Am I simply mis-applying boost::variant? I could of course just throw an exception in the unsupported/not implemented methods above, but I'd like this checked at at compile time if possible. (note that my question is generally applicable to visitors, not just to boost::variant). thanks! --craig __________________________________ Do you Yahoo!? Yahoo! Mail - 50x more storage than other providers! http://promotions.yahoo.com/new_mail

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Boost Signal Bug
by Neal Coombes 30 Jun '04

30 Jun '04

I've experienced the same problem discussed on June 3rd titled "Bug, signals, use of default Combiner" http://thread.gmane.org/gmane.comp.lib.boost.devel/103903 . I would have just replied except its a little old, thus a new thread. Doug Gregor wrote: > We intentionally did not return a default-constructed value because > the returned type might not have a default constructor. Granted, this > case might be sufficiently rare to justify using your version of > last_value. I would like to say a few things about this and offer an alternative solution. First, I think not returning a default constructed value is the correct approach with the last_value combiner for the reasons you've mentioned. I don't think the bug is with the last_value combiner, but with the fact that the last_value combiner is the default combiner (ok an exception rather than an assert would probably be better). However, I would venture to claim that a signal being called without any slots is not a rare case. In fact I'd guess that might be the most likely case. Certainly, as a library writer, I cannot assume that all the signals I provide to my users are going to be used in every application. My suggestion is to provide a default combiner that does the bare minimum, just call the slots: namespace boost { struct default_combiner { typedef void result_type; template<typename InputIterator> result_type operator()(InputIterator first, InputIterator last) const { while (first != last) *first++; } }; } I believe this would solve the problem without restricting use of default signals to objects with default constructors, or to signals with at least one slot connected.

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RE: [boost] Re: Boost.Threads: Do we need all those mutexes?
by Batov, Vladimir 30 Jun '04

30 Jun '04

1. Oh, c'mon, I never implied that your design was "dubious and likely incorrect". It was said in the context of operator[] for std:list. 2. IMHO for a library to be accepted, embraced, popular, survive it has to be suitable/convenient/intuitive for 80% of the users. The library should not go out of its way to satisfy esoteric requirements of 20% at the expense of clarity/simplicity/good design. Or example, std:auto_ptr is not for everyone and not always work where I'd like it to work. It does not mean std::auto_ptr needs to sacrifice its efficient and minimalistic beauty to make me happy. Getting back to Boost.Threads I'd still like to see a compelling example where I'd need to declare try_lock but twist its intuitive functionality into behaving like scoped_lock (that is blocking until locked). -----Original Message----- From: boost-bounces(a)lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Matt Hurd Sent: Wednesday, 30 June 2004 10:42 AM To: boost(a)lists.boost.org Subject: Re: [boost] Re: Boost.Threads: Do we need all those mutexes? On Wed, 30 Jun 2004 10:17:04 +1000, Batov, Vladimir <vladimir.batov(a)ca.com> wrote: > > Why give the user an alternative with is > dubious and likely incorrect? It is a pattern of need for multithreaded programming where I work to require single and multithreaded awareness for objects where performance excludes the option of using separate object instances. I don't think the design is dubious or incorrect it is mandated by the business requirements. Though most of my designs usually are incorrect or dubious ;-) I just call those tactical :-) matt. _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost

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