January 2005 - Boost-users - lists.preview.boost.org

Exercice 3-7 (C++ Templates Metaprogramming)
by Philippe Mori 02 Feb '05

02 Feb '05

I do not understand well the semantic of some constructs. 1) For t1, does the external mpl::lambda has any effect on some use of t1 or mpl::lambda<_1>::type and also mpl::lambda<_1> are always equivalent? 2) Does the uses of lambda in t8 has any effect or the result is always the same as with t2. 3) Does there are other tests that the ones below (i to vii) that would shows some other differences in the semantics of the expression? 4) Same question but for to_be_filled part of my tests. Anything different that the four ones belows (a to d)? 5) In any ot the tests with a to_be_filled part, is it possible to have more than one template argument and that the extra ones are used? The thing I have some of the following (what I have tried and what compile depends on the construct - that is t1, t2,...) : i) display_type(tn); ii) display_type(tn::type); iii) display_value(tn::type::value); iv) display_type(mpl::apply<tn, to_be_filled>::type) v) display_value(mpl::apply<tn, to_be_filled>::type::value) vi) display_type(tn::apply<to_be_filled>::type) vii) display_value(tn::apply<to_be_filled>::type::value) where tn is one of the type t1, t2,... t8 and to_be_filled is something similar to one of those : a) int b) X where X is a metafunction class c) mpl::plus<int_<1>, int_<2> > d) Y where Y is a simple metafunction similar to always_int in section 3.5.4. I have look at the documentation but it is hard to see the relation between each construct, the possible uses of each ones and their prmary purpose. I would be nice to provide more samples and more equivalences. Philippe

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mpl -> integral sequence wrappers -> wrapping enums ?
by Martin Pasdzierny 02 Feb '05

02 Feb '05

Hello MPL experts, I'd like to use meta - vectors of integral constants defined as enums. But unfortunately the code below does not compile. <code> #include <iostream> #include <boost/mpl/vector_c.hpp> #include <boost/mpl/size.hpp> enum MY_ENUM {X,Y}; // (*) typedef boost::mpl::vector_c<MY_ENUM,X,Y> my_vector_c; static const int size = boost::mpl::size<my_vector_c>::value; int main(int argc, char** argv) { std::cout << "\nsize:" << size << "\n... bye" << std::endl; } </code> After replacing line (*) with typedef boost::mpl::vector_c<int,X,Y> my_vector_c; the code compiles and runs fine, but this does not look type safe any more. Any suggestions ?! regards, Martin.

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equality fails with C++ bools
by Ross Boylan 02 Feb '05

02 Feb '05

I have a function declared bool that is acting rather non-bool. std::cout << obj.fn() gives me "76" and BOOST_CHECK_EQUAL(obj.fn), true) fails, with the message /home/ross/peter/R/mspath/src/test/Covariates_test.cc(178): error in "obs_covariates": test env.matchesObservation() == true failed [76 != 1] std::cout << true prints "1". obj.fn() works OK as an argument to if (obj.fn()). I am using g++ 1:3.3.5-5 on Debian GNU/Linux. I've tried -O2 and -O1; same behavior either way. boost is 1.31.0-9 The function is declared bool matchesObservation() const Maybe bool is being typedef'd or #define'd to int somewhere, for compatibility with old code? Probably not; I tried printing typeid(xxx).name() and got "b" for true and the function return value, while I got "i" for the number 1. I tried tracing into the code with limited success. Some mix of the fact that the underlying libraries weren't a debug version, that the code was optimized, and that there are #line directives (the source files were generated out of another file) made it hard to follow. Any suggestions? -- Ross Boylan wk: (415) 502-4031 530 Parnassus Avenue (Library) rm 115-4 ross(a)biostat.ucsf.edu Dept of Epidemiology and Biostatistics fax: (415) 476-9856 University of California, San Francisco San Francisco, CA 94143-0840 hm: (415) 550-1062

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Two issues with lexical_cast
by Yuval Ronen 01 Feb '05

01 Feb '05

Hi. I'm probably just another one out of a million who make these remarks, but I found nothing in the documentation about them, so I'll mention them anyway. 1. lexical_cast(Source arg) takes the source argument by value. Why not by const& ? Calling lexical_cast(some_basic_string) now makes a copy of the source argument for no reason. 2. lexical_cast ignores trailing whitespaces, but not leading whitespaces. Meaning that int a = lexical_cast<int>("3 "); // three-space will work and return 3, but int a = lexical_cast<int>(" 3"); // space-three will throw. Why? There's no rationale here. Both cases should be treated in the same manner, and IMHO, that manner should be throwing. After a look at the code, it seems that the trailing-whitespace-ignore code ('stream >> std::ws' in line 151) was specifically added, so there was supposed to be a reason for this. But as I said, I believe this is wrong, at least as the default behaviour when converting a string to something else. Maybe some policies (IgnoreLeadingWhitespace, IgnoreTrailingWhitespace) can be added to control this? And again, I suppose I'm not the first to think of it... Thanks, Yuval

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implicitly_convertible<a,b>
by Benjamin Schmeling 31 Jan '05

31 Jan '05

Hi, I have exposed a bigint class which has no constructor for long_ . I added an additional constructor by defining .def("__init__", make_constructor(bigint_constructor)) This works fine. Now I want to use the implicitly_convertible function which expects from the class to have an constructor which takes the first argumen (in this case a long_) I used implicitly_convertible<boost::python::long_ ,bigint> but now the problem is that the original c++ class has no such constructor, but the wrapped class. How can I make him use my new constructor in implicitly_convertible? Benjamin

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Compile problems with multi_index on CW 9.3 (mac)?
by Marshall Clow 31 Jan '05

31 Jan '05

I'm trying to use boost::multi_index for the first time, and I'm getting odd compile errors. I have a class with two "keys", a text one and a numeric one, and I want to have two indices into the collection. So, I tried several things, and all of them get the same error: 'index_class' is not a member of class 'boost::multi_index::ordered_unique<boost::multi_index::identity<Foo>, boost::multi_index::detail::null_arg, boost::multi_index::detail::null_arg>' (and then an amazing cascade after that) Here's the basic idea: typedef boost::multi_index_container < Foo, boost::multi_index::indexed_by < boost::multi_index::ordered_unique < boost::multi_index::identity < Foo >, LessThanNumeric >, boost::multi_index::ordered_unique < boost::multi_index::identity < Foo >, LessThanText > > > Foo_; where LessThanNumeric (and LessThanText, too) looks like this: struct LessThanNumeric: std::binary_function< Foo, Foo, bool > { bool operator () ( const Foo& x, const Foo& y ) const { return x.NumericKey () < y.NumericKey (); } }; My source is at <http://www.idio.com/misc/mi-test.cpp> Anyone got any tips? Thanks! -- -- Marshall Marshall Clow Idio Software <mailto:marshall@idio.com> It is by caffeine alone I set my mind in motion. It is by the beans of Java that thoughts acquire speed, the hands acquire shaking, the shaking becomes a warning. It is by caffeine alone I set my mind in motion.

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boost::filesystem -- "Aborted"
by matthias 31 Jan '05

31 Jan '05

Hi, I am currently writing a gtkmm component which acts as a file browser, much like a small filemanager which can be embedded in gtkmm applications as a scrolled window. I have just rewritten the component to use the boost::filesystem classes instead of the C functions for interacting with the filesystem. However I am getting a strange error when I start my app: When iterating through a directory I suddenly get the message: Aborted. void FileBrowser::rebuild_file_list() { namespace boostfs = boost::filesystem; boostfs::path cur_path( m_cur_dir ); FileContainer tail; // m_files will first only contain directories, // while tail will contain only files m_files.clear(); try { boostfs::directory_iterator end; for( boostfs::directory_iterator dir( cur_path ); dir != end; ++dir ) { std::cout << (*dir).string() << std::endl; if( boostfs::is_directory( *dir ) ) m_files.push_back( *dir ); else tail.push_back( *dir ); } } catch( std::runtime_error & e ) { std::cerr << e.what() << std::endl; } m_files.sort( SortedByName() ); tail.sort( SortedByName() ); drx::list_concat( m_files, tail ); } OUTPUT: [...] /home/mkay/.acrobat /home/mkay/.synce /home/mkay/.gimp-2.0 /home/mkay/.fonts /home/mkay/.designerrctb2 /home/mkay/.eclipse /home/mkay/.gftp /home/mkay/.ntrc_2 /home/mkay/.DCOPserver_anthrax__0 /home/mkay/.DCOPserver_anthrax_:0 /home/mkay/.kpackage /home/mkay/.fonts.conf /home/mkay/.xscreensaver /home/mkay/xdefaults_old /home/mkay/.reslisarc /home/mkay/.aMule /home/mkay/.gtkrc Aborted At this point the application exits. Any idea what is causing this? Thanks in advance, Matthias

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enabling boost::threads for HP-UX
by Oliver Kowalke 31 Jan '05

31 Jan '05

Hi, I want to use boost on HP-UX together with gcc-3.3.2. As I figured out that thread-support is disabled (BOOST_DISABLE_THREADS is defined in file boost/config/platform/hpux.hpp). Up gcc-3.3 posix-threads are supported on HP-UX-plattform (http://gcc.gnu.org/install/specific.html#hppa*-hp-hpux11). Is it then save to remove #define BOOST_DISABLE_THREADS in boost/config/platform/hpux.hpp? with best regards, Oliver -- "Der Spezialist ist ein Barbar, dessen Unwissenheit nicht allumfassend ist." Master's Voice, S.LEM

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Setting the path to use Boost
by Grek 31 Jan '05

31 Jan '05

Hi fellow Boost users! I am new to the Boost Graph Library (BGL) and I have a query about setting the path to access the library. My platform is Cygwin. I want to use the g++ command and incorporate the BGL in it. How do I go about it? Let's say that I want to use the dijkstra's algorithm. I set the path as follows: boost_1_32_0/libs/graph/example Then I typed the following. g++ -g dijkstra-example.cpp But it displays error messages and I do not know how to proceed. It would be of great help if anyone could tell me how one has to set the path to use the dijkstra's algorithm in a program. Thanks for taking the time to read through this! Cheers Grek --------------------------------- Do you Yahoo!? Yahoo! Search presents - Jib Jab's 'Second Term'

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atlanta statics...
by Jeffrey Holle 31 Jan '05

31 Jan '05

http://houseandhome.msn.com/pickaplace/comparecities.aspx?FromState=WA&From… Hope this link works for you. It takes 5 min. to load a page from website for me. Microsoft's MSN uses windows technology now I guess... The big thing that stands out for me is crime. Seattle is above the national average by about 2x, Atlanta appears to be between 7 and 8 times! I'll have to consider such a move with care....

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