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C++ Template Metaprogramming - factorial example
by Brad Austin 23 Jan '05

23 Jan '05

I recently purchased David Abrahams' and Aleksey Gurtovoy's new book on the MPL, "C++ Template Metaprogramming" (obviously recently since it's only been out a couple of weeks), and am working through some of the examples. I'm having trouble with the factorial example in section 8.3.1, page 160: #include <boost/mpl/int.hpp> #include <boost/mpl/multiplies.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/eval_if.hpp> #include <boost/mpl/prior.hpp> #include <boost/static_assert.hpp> namespace mpl = boost::mpl; template <class N> struct factorial : mpl::eval_if< mpl::equal_to<N,mpl::int_<0> > // check N == 0 , mpl::int_<1> // 0! == 1 , mpl::multiplies< // N! == N * (N-1)! N , factorial<typename mpl::prior<N>::type> > > { BOOST_STATIC_ASSERT(N::value >= 0); // for nonnegative N }; There's a minor issue in that equal.hpp is (apparently) mistakenly included instead of equal_to.hpp, but even after fixing that, I can't get this to compile in the form in which it's written. Should it, or is it broken? I'm using gcc 3.3-23 on Linux. The first meaningful line in the error novel I get is: error: no type named `tag' in `struct factorial<mpl_::int_<0> >' I was able to get a modified version to work, but only by explicitly invoking the eval_if metafunction and wrapping the result in a constant wrapper, like so: template <class N> struct factorial : mpl::int_< mpl::eval_if< mpl::equal_to<N,mpl::int_<0> > , mpl::int_<1> , mpl::multiplies< N , factorial<typename mpl::prior<N>::type> > >::type::value > { }; But as this is a less-than-trivial departure from the book version, I don't feel comfortable moving on without first gaining a better understanding of what's going wrong. Thanks in advance to anyone who can offer some insight.

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[program_options] How do I read a vector of strings?
by Matthew P. Cashdollar 22 Jan '05

22 Jan '05

Hello, I have been using program_options for a while and it works great. However, I don't know how to make a program which behaves like this: programname -blah abc -blah def -blah ghi In my code I would read the option blah as a vector of strings, like: po::options_description desc("Configuration"); desc.add_options() ("blah", po::value<vector<string>>(), "the location of the blah device") ; po::store(po::parse_config_file(config_file, desc), vm); po::notify(vm); etc. When I try to compile the code under MSVC++ 7.0 I get this error (among others): error C2947: expecting '>' to terminate template-argument-list, found '>>' Is it possible for me to do what I am trying to with the program_options library? Thanks, -Matt

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date_time::microsec_clock questions
by Donato Petrino 22 Jan '05

22 Jan '05

Hello, 2 questions regarding the microsec_clock: - Why it just have 'local_time()' but not 'universal_time()' member? (Analogous to the second_clock. I know that it can be converted to UTC, but its inefficient and I have to know in which time zone the program runs.) - For what reason "::std::time(&t)" in "microsec_time_clock.hpp:58" is called? gettimeofday()'s "tv->tv_sec" already has the sec's since 1.1.1970, so there is no need for this extra call. Is it a bug? Thank you in advance! -- Donato Petrino Petrino EDV-Beratung Darmstädter Str. 29 64331 Weiterstadt T +49 6150 5418-11 F +49 6150 5418-18 email: dpetrino(a)petrino-edv.de

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date_time and negative time_durations
by Erik Thiele 22 Jan '05

22 Jan '05

hi the docs of time_duration say that minutes() returns 0..59 and seconds() return 0..59. this is not true, the correct answer is, minutes() returns -59..+59 and seconds() also. the handling of negative durations is not fully documented. i.e. is it true that the sign of hours(),minutes() and seconds() is always equal to each others sign? x=time_duration(anything...); y=x.invert_sign(); is the following true for all values of x: abs(x.hours()) == abs(y.hours()) abs(x.minutes()) == abs(y.minutes()) abs(x.seconds()) == abs(y.seconde()) but whatever way you document it, it should somehow be documented. to continue on my work i now make the assumption that in the following assertions are true for all values of x: if (x.is_negative()) x = x.invert_sign(); assert(x.hours()>=0); assert(0<=x.minutes()<=59); assert(0<=x.seconds()<=59); i hope this assertions are correct for all values of x !!!!! :) cya! erik

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boost::tuple problem
by heroboy 22 Jan '05

22 Jan '05

hello everyone,     what's the difference between "tuple<int> t;"  and  " tuple<int> t();" why tuple<int> t; cout<<t; is ok. But tuple<int> t(); cout<<t; make a link error. I'm using vs.net2003__________________________________________________________________欢迎使用ACMAIL 邮件系统

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Re: strange filesystem dll behaviour (Beman Dawes)
by Christian Hoffmann 22 Jan '05

22 Jan '05

Hi, I guess, you don't define BOOST_ALL_DYN_LINK Rgds -- Christian Hoffmann Trema Laboratories SARL -----Original Message----- From: boost-users-request(a)lists.boost.org [mailto:boost-users-request@lists.boost.org] Sent: Friday, January 21, 2005 6:12 PM To: boost-users(a)lists.boost.org Subject: Boost-users Digest, Vol 437, Issue 3 Send Boost-users mailing list submissions to boost-users(a)lists.boost.org To subscribe or unsubscribe via the World Wide Web, visit http://lists.boost.org/mailman/listinfo.cgi/boost-users or, via email, send a message with subject or body 'help' to boost-users-request(a)lists.boost.org You can reach the person managing the list at boost-users-owner(a)lists.boost.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Boost-users digest..." Today's Topics: 1. Re: strange filesystem dll behaviour (Beman Dawes) 2. Re: unordered_map/hash_table bug? (Daniel James) 3. Re: Re: (newbie) Linking problem using unit testframework(Mac OSX 10.3.7 gcc 3.3 XCode 1.5). (Gennadiy Rozental) ---------------------------------------------------------------------- Message: 1 Date: Fri, 21 Jan 2005 10:18:02 -0500 From: Beman Dawes <bdawes(a)acm.org> Subject: Re: [Boost-users] strange filesystem dll behaviour To: boost-users(a)lists.boost.org, boost-users(a)lists.boost.org Message-ID: <6.0.3.0.2.20050121100502.049f35e0(a)mailhost.esva.net> Content-Type: text/plain; charset="us-ascii"; format=flowed At 01:12 AM 1/20/2005, Paul wrote: >I looked deeper into the problem, and hacked up boost a little to find >out that the problem is the following check: > > if ( (checker == no_check || checker == native) && src.size() >= 2 ) >at path_posix_windows.cpp:243 > >specifically checker==native. > >I had constructed path with boost::filesystem::native, but when I hacked >it up to do the following: > ># ifdef BOOST_WINDOWS > std::stringstream ss; ss << "checker=" << >reinterpret_cast<unsigned long>(checker) << std::endl > << "native=" << reinterpret_cast<unsigned long>(native) << >std::endl; > boost::throw_exception( filesystem_error( > "boost::filesystem::path", > ss.str() )); > > if ( (checker == no_check || checker == native) && src.size() >= 2 ) > > >I got the following results from calling the line: >boost::filesystem::path p(s,boost::filesystem::native); > > >linking to libboost_filesystem-mgw-d-1_32.lib: >checker=4201048 native=4201048 > >linking to boost_filesystem-mgw-d-1_32.dll: >checker=4263776 native=1649966248 > > >so, it appears that testing the checker using the address of a function >is unreliable in a DLL context? Have I compiled my program/library >incorrectly? I am not too familiar with DLL details etc, can someone >else please shed some light on this? I'm not a DLL expert either, but it certainly looks like there is some problem with address of functions across DLL boundaries. This code will be going away in the next version of the library, so I'm not sure if it is worth putting a lot of effort into fixing it. But I'll try to reproduce the problem here. If it does reproduce, it shouldn't be too hard to come up with an interim fix. Thanks for the report, --Beman ------------------------------ Message: 2 Date: Fri, 21 Jan 2005 16:15:48 +0000 From: Daniel James <daniel(a)calamity.org.uk> Subject: [Boost-users] Re: unordered_map/hash_table bug? To: boost-users(a)lists.boost.org Message-ID: <csr9ro$ma$1(a)sea.gmane.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Thorsten Ottosen wrote: > "Martin Wartens" <martin.wartens(a)my-mail.ch> wrote in message > news:380-22005121815022188@my-mail.ch... > Hi, > has anybody tried the implementation of unordered map that can be found at > http://groups.yahoo.com/group/boost/files/jtc1-sc22-wg21-2003-n1456.tar. gz ? > I believe there is a bug in the rehash function of the underlying > hash_table. The new bucket indices are computed using the old number of > buckets. That means that the items stored in these buckets cannot be found > anymore later. > ------- > > Btw, would you mind to submit a patch to me to fix this bug? > > Thanks > > Thorsten Hi Thorsten, I'm working a new version which fixes this (and a few other) bugs. I'll upload it the file vault soon. Daniel ------------------------------ Message: 3 Date: Fri, 21 Jan 2005 11:21:43 -0500 From: "Gennadiy Rozental" <gennadiy.rozental(a)thomson.com> Subject: [Boost-users] Re: Re: (newbie) Linking problem using unit testframework(Mac OSX 10.3.7 gcc 3.3 XCode 1.5). To: boost-users(a)lists.boost.org Message-ID: <csra6n$1sg$1(a)sea.gmane.org> > c++filt _ZTv0_n12_NSoD0Ev > virtual thunk to std::basic_ostream<char, std::char_traits<char> >::~basic_ostream() > Then I tried to run just this: > #include <iostream> > > int main (int arc, char** argv) > { > std::cout << "test"; > } > ... and it failed with the same error. > However, if I remove from the project the unit test framework files, it runs fine. Hmm... > I am using libstdc++.a. I don't understand what is happening. So do I. > Why when I add the unit test framework code it doesn't find the standard library ~basic_ostream() anymore ? Most probably looks like compiler bug > It seems these last 5 years doing only Java got me really rusty with C++ (I'd rather work with C++ but the market forced me to go to Java). I don't think 'last 5 years doing only Java' has anything to do with this. Let's try couple more thigs: 1. Could you locate where this undefined symbol referenced from? 2. Could you build framework as a standalone library and link with it? Gennadiy ------------------------------ _______________________________________________ Boost-users mailing list Boost-users(a)lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users End of Boost-users Digest, Vol 437, Issue 3 ******************************************* This message, including any attachments, is intended only for the person(s) to whom it is addressed. If you received it in error, please let us know and delete the message from your system. This message may be confidential and may fall under the duty of non-disclosure. 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How to automatically release legacy resources?
by Aaron W. LaFramboise 22 Jan '05

22 Jan '05

I was writing some simple code today, and rediscovered this general sort of problem, and realized I have not yet decided on an optimal solution. Given: 1) During a particular function, a type-int variable is initialized with a value corresponding to a POSIX file descriptor. 2) There are multiple paths of exit from the function, including both returns and exceptions. Problem: What is the best way to ensure that the file descriptor is automatically close()ed when the function is exited? An additional thorn is that there must be code for handling that case where close() fails. The solution must handle both cases where I'd like to throw an exception, or cases where I'd just like to print an error message and continue. I have considered various solutions, some involving throw away classes, and none of them appear to be satisfactory. So far, the best solution I have is to create a reusable helper (based on templates and boost::function) that is created as an automatic variable and is passed the file descriptor, and a functor (which may be a boost.lambda expression) that calls close(), and contains any other necessary error-processing code. But seriously, for such a simple problem, this is insane! Solutions I don't particularly like involve creating a non-reusable throw-away class that handles just this one particular case, or lack sufficient mechanisms for handling errors. Any ideas? Aaron W. LaFramboise

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[Boost-users - newbie]Conditional insertion into a mpl::list_c
by suresh.t＠philips.com 21 Jan '05

21 Jan '05

hi, I am new to boost. I have a class that takes in 10 template parameters (std::size_t N1 to N10). I need to create a mpl::list_c using N1 to N10. I have an empty mpl::list_c (acc). I plan to check each of N1 to N10 and if the value > 0 i will insert into the list. Can someone please tell me how i could do this. I have tried for one N1 alone using an mpl::if_ but that doest seem to work template <std::size_t N1,std::size_t N2 =0,std::size_t N3 = 0, std::size_t N4 = 0,std::size_t N5 = 0, std::size_t N6 = 0,std::size_t N7 = 0, std::size_t N8 = 0,std::size_t N9 = 0,std::size_t N10 = 0> struct access_list{ typedef mpl::list_c<int> acc; //I tried for N1 this way but this does not word typedef mpl::if_< mpl::bool_<N1 != 0>, typename mpl::push_back<acc,mpl::int_<N1> >::type, mpl::false_ >::type type; }; --------------------------------------------------------------------------------------------------------- Suresh

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Boost.Preprocessor question
by Cory Nelson 21 Jan '05

21 Jan '05

I am initializing an array like: unsigned char tmp[256]={0, 1, 2, 3, ...} I feel like this is something Boost.Preprocessor could do for me, but am unable to find out how. Anyone know? -- Cory Nelson http://www.int64.org

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implicit conversion (long --> bigint)
by Benjamin Schmeling 20 Jan '05

20 Jan '05

Hi, thanks Ben your advice with radd worked fine. Now when I am going further for my bigint class I have the problem that methods which take an bigint as an parameter do not accept longs. For example: >>import _PythonLiDIA >>x=123L; >>c=_PythonLiDIA.bigint(); >>_PythonLiDIA.gcd(c,x); Traceback (most recent call last): File "test.py", line 132, in ? _PythonLiDIA.gcd(a,x); Boost.Python.ArgumentError: Python argument types in _PythonLiDIA.gcd(bigint, long) did not match C++ signature: gcd(LiDIA::bigint, LiDIA::bigint) I don't know how to achieve implicit convertion at this point, turning an long automatically into an bigint. The other way round, turning an bigint into long can be realized by defining __long__. Is it possible?/Any idea? Benjamin -- GMX im TV ... Die Gedanken sind frei ... Schon gesehen? Jetzt Spot online ansehen: http://www.gmx.net/de/go/tv-spot

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