February 2005 - Boost - lists.preview.boost.org

best way to make {reverse_}fold a sequence
by Larry Evans 04 Mar '05

04 Mar '05

After creating: typedef range_c<int,0,3> types; typedef fold < types , empty_base , inherit< arg<2>, arg<1> > >::type tuple_type; I'd like to use for_each on tuple_type. For example: struct print_integral_c { template<int i, typename Right> void operator()(inherit<integral_c<int,i>, Right>)const { std::cout<<i<<"\n"; } }; for_each<tuple_type>(print_integral_c); However, tuple_type is not a sequence; hence, for_each won't work. I could define the necessary specializations for mpl::begin, etc., to make it a sequence, but I was wondering if there's any way or any plans to somehow automagically make fold or reverse_fold into a sequence so I could use for_each on it.

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Singleton is documented!
by Jason Hise 04 Mar '05

04 Mar '05

The singleton library is finally documented! A complete tutorial in using the library along with an extensive reference section are provided. A tutorial on extending the library is forth-coming. The new zip is available in the <a href="http://boost-sandbox.sourceforge.net/vault/">sandbox</a> under the singleton folder, and should be extracted into the boost root directory. As always, suggestions are welcome. -Jason

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regexes matching empty strings
by Michael Goldshteyn 03 Mar '05

03 Mar '05

Why can I not construct a regex of the form: "|abc" (throws exception based on REG_EMPTY) for use with regex_match, if I want to match either "abc" or the empty string "". I understand thatn in a regex_search the empty string would always match (per Perl like semantics), and abc would never be considered. But, I want to use this with regex_match, where the empty string would have to match the entire string being searched. Work arounds such as "(?:)|abc" and "^$|abc" seem to get around this deficiency, but I am wondering why it is there in the first place. Thanks, Michael Goldshteyn

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Preview of the Function Types library (function_traits replacement)
by Tobias Schwinger 03 Mar '05

03 Mar '05

Dear Boosters, a preview of the Function Types library is available, now. [ Online documentation: http://tinyurl.com/4fw9n ] [ Downloadable archive: http://tinyurl.com/6u77y ] It is the third (nearly) complete rewrite and the design finally feels right for me. However, I am very curious to hear any of your opinions about it ! So, if you have been hoping for the retirement of function_traits, please take a few minutes to find out if this would suit your needs. Regards, Tobias

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mpl::begin<mpl::vector<...> > performance problem
by Arkadiy Vertleyb 02 Mar '05

02 Mar '05

Hi all, I am having a huge compile performance problem trying to use mpl::begin for an mpl::vector of a large size -- the complexity seems to be far from constant (or even linear) time. The following example demonstrates it (I am using GCC 3.3.3 or 3.4.2) ---------- #define BOOST_TYPEOF_LIMIT_SIZE 200 #include "boost/mpl/vector.hpp" #include "boost/mpl/size_t.hpp" #include "boost/mpl/begin_end.hpp" #include "boost/mpl/aux_/config/ctps.hpp" #include "boost/preprocessor/iterate.hpp" #include "boost/preprocessor/if.hpp" #include "boost/preprocessor/repetition/enum.hpp" #include "boost/config.hpp" #if (BOOST_TYPEOF_LIMIT_SIZE > BOOST_MPL_LIMIT_VECTOR_SIZE) #warning started generating namespace boost { namespace mpl { #define BOOST_PP_ITERATION_PARAMS_1 (3,( \ BOOST_PP_INC(BOOST_MPL_LIMIT_VECTOR_SIZE), \ BOOST_TYPEOF_LIMIT_SIZE, \ "boost/mpl/vector/aux_/numbered.hpp")) #include BOOST_PP_ITERATE() } } #warning done generating #endif//BOOST_TYPEOF_LIMIT_SIZE > BOOST_MPL_LIMIT_VECTOR_SIZE #define MACRO(z, n, _)\ boost::mpl::size_t<n> #warning started getting begin typedef boost::mpl::begin<BOOST_PP_CAT(boost::mpl::vector, BOOST_TYPEOF_LIMIT_SIZE)< BOOST_PP_ENUM(BOOST_TYPEOF_LIMIT_SIZE, MACRO, ~) > >::type it; #warning done getting begin main() {} --------------- Thanks in advance for any help. Regards, Arkadiy

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Select overload revisited
by Daniel James 01 Mar '05

01 Mar '05

I was thinking about the overload resolution functions suggested a while back by Arturo Cuebas[1] and had no idea why the compiler needs to be told the number of parameters in the first place. So I took his code and removed the numbers from functions, and it worked (in gcc). I guess that it didn't work for him because of his compiler. Well, it almost worked. Sometimes there is more than the one function that fits the template parameters. So I kept the numbered overloads to be able to match them. It might be possible to get these to work without the number of parameters by making the compiler prefer overloads for functions with less parameters. I've attached a demo. Daniel [1] http://lists.boost.org/MailArchives/boost/msg69451.php

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Added "restore" member functions to ios_state
by Daryle Walker 01 Mar '05

01 Mar '05

See "BOOST_ROOT/boost/io/ios_state.hpp," CVS version 1.3, and "BOOST_ROOT/libs/io/doc/ios_state.html," CVS version 1.7, if they uploaded OK. (I had a temporary[?] wait on my own lock.) -- Daryle Walker Mac, Internet, and Video Game Junkie darylew AT hotmail DOT com

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[mpl] sequence details
by Tobias Schwinger 01 Mar '05

01 Mar '05

Hi, What is the best way to convert the result of a mutating mpl intrinsic metafunction (such as 'mpl::push_back', for example) to numbered form ('vectorN<a1,a2,..>') ? I tried using the type-member (which only works on some compilers - not with GCC for example) and figure 'copy' could probably do the trick (but is a quite complex operation) so there might be a better recipe for this. In a library I'm currently working on, I use a decorator pattern to make a class model of a sequence it holds (i.e. give it a tag type member and specialize *_impl with delegate semantics). After applying the delegate metafunction I need the sequence in a form I can match with partial specialization. Thanks for your help, Tobias

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[numeric/conversion] doc's example not up-to-date
by Samuel Krempp 01 Mar '05

01 Mar '05

Is the documentation for numeric/conversion finalized yet ? It seems that at least the code sample in numeric/conversion/doc/numeric_cast.html hasn't been kept up-to-date. This sample 1/ includes inexistent header 2/ uses boost::bad_numeric_cast instead of boost::numeric::bad_numeric_conversion and same for the other exception classes and most of all : 3/ // This works, because unsigned integral types cannot cause overflow. unsigned char c=numeric_cast<unsigned char>(l); But this line does raise an overflow exception (as it should, I guess). I guess the comment was about unsigned types implementing modulo-arithmetics, but in the end does numeric_cast check for unsigned overflows, or not ?? -- Samuel

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mpl::vector & push_back question ?
by Zhang, Karl 01 Mar '05

01 Mar '05

Hi: When I am reading mpl source code. I notice that push_bck algorithm, Look as following: template<> struct push_back_traits< aux::vector_tag > { template< typename Vector, typename T > struct algorithm { typedef vector_node<T,Vector> type; }; }; My question is: when generate new sequence, vactor_node<T, vector>, why skip "size" argument ? How does it increase sequence size in push_back? About push_front, It increase sequence size when generating new sequence. Regard Karl

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