December 2004 - Boost - lists.boost.org

variant==dependent pair, dependent function possible?
by Larry Evans 06 Dec '04

06 Dec '04

IIUC, the variant library is similar to a "dependent product" which can be thought of as a pseudo_pair<Index,TypeList>. where an instance: pseudo_pair<I,T> dpIT; is a pair of values (i,t) where: I i; T<i> t; IOW, there is only one value, but it can be one of many types depending on the valid values of I. BTW, would variant be made for readable by making variant take an enumeration: enum I{i0, i1, ...., in-1} as the discriminator? Anyway, back to the original question. Is there any interest in a "dependent function" where: dependent_function<I,T> df; where: T<i0>& df<i0>(); T<i1>& df<i1>(); ... T<in-1>& df<in-1>(): IOW, this would be like a regular struct (a record type), however, instead of: df.ix there would be: df<ix>(); BTW, this idea comes from Nuprl: http://www.cs.cornell.edu/Info/Projects/NuPrl/nuprl.html and I've recently had a need for it because I've got a template which is specialized on an enumeration; hence, this would come in handy if I wanted to have a structure containing all instances of the specializations.

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Unexpected behaviour of boost::filesystem::equivalent
by Daniel Krügler 06 Dec '04

06 Dec '04

Hi booster, its quite a long time ago since I could follow the threads here more thoroughly :-( , so the following information might not be new one. During tests of the current Win32 implementation of boost::filesystem::equivalent I observed that the function unexpectingly fails (my test environment was Win XP SP 1 with MSVC7.1) if it compares a "soft link" created via the DOS command "subst" (or its API analogon ::DefineDosDeviceA/W) with the corresponding mapped path object (Lets say I map M: to the existing directory C:\temp). To get an understanding of the test failure I debugged the implementation and found that actually the first test part (getting the handles and comparing the VSN's + further data) gave the expected positive result, but that the very last step, i.e. the comparison of both st_dev values, gave a final unequality. Did others also observe this failure and don't you think that the expected result should show the identity of the underlying path objects? Of course there might be legitimate arguments for the decision for the current behaviour, but I would like to know about them. I am not an expert to argue about the comments in the source code which say that "In theory, volume serial numbers are sufficient to distinguish between devices, but in practice VSN's are sometimes duplicated, so device id is also checked.[..]" The question now is: Should different (MS DOS) device names pointing to the same underlying (Windows NT) device not be compared equivalent? I could think of the following test extension (at least under Windows, regrettably I am not a posix expert to provide a similar solution): If the VSN' and further data have returned a positive result, but the check of both st_dev values fails, one should try to check whether the underlying devices are actually the same, e.g. using the ::QueryDosDeviceA/W API function. What do you think? Greetings, Daniel Krügler

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3D logo
by Joaquín Mª López Muñoz 06 Dec '04

06 Dec '04

Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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! Wiki Spam
by Christopher D. Russell 06 Dec '04

06 Dec '04

I just found the start page of the Wiki completely replaced with xanax drug spam. I removed the offending content and left a note in its place. Sorry - I'm not sure where the original content is archived or I would go fix it myself right now. Perhaps we need a password mechanism on the Wiki? This seems to work well over at http://www.program-transformation.org/ where you need to register and login to edit. - Chris

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[Review Results] Named Parameters Library
by Doug Gregor 06 Dec '04

06 Dec '04

The Named Parameters Library has been accepted into Boost. Congratulations to the authors, David Abrahams and Daniel Wallin. The vast majority of reviews were positive, although most reviewers requested additional documentation. Here is a list of the most important/commonly-requested changes or additions to the library: - More documentation! Specifically, + Automatic overload generation should go into the tutorial + Document operator, + Performance data (what's the cost?) + Reference section + "Best practices" section. - Add a metafunction for getting the type of a parameter, e.g., param_type<Params, width_t>::type. - Add test cases for the automatic overload generation macros. - Fix the BOOST_NAMED_PARAMS_MAX_ARITY bug; Cromwell Enage provided a solution. Doug Gregor Review Manager

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Anyone interested in a virtual constructor lib?
by Roland Schwarz 06 Dec '04

06 Dec '04

As a byproduct of my current work I factored out a small utility library that implements a virtual constructor. The implementation is based on boost::any and typeid(...).name(). It features dynamic creation of objects by name (string), and allows for default and custom constructors beeing called. It could be used as the basic step to object deserialization from a file, creation of operating system dependent implementations, or allow a library to instantiate user-pluggable classes. The design is inspired by the Io_obj example from Stroustrup's book on C++ programming. A small example to show the usage: #include <string> #include <iostream> #include "dynanew.hpp" using namespace dynanew; // a simple class hierarchy with abstact base class struct shape { virtual void draw() = 0; }; struct square : public shape { virtual void draw() { std::cout << "I am a square" << std::endl; } }; struct circle : public shape { circle() : radius(0) {} circle(double dr) : radius(dr) {} virtual void draw() { std::cout << "I am a circle of radius: " << radius << std::endl; } double radius; }; // create the type map TYPE_MAP(shape); // square with default constructor TYPE_REGISTER(shape, square); // circle with custom constructor taking a double parameter TYPE_REGISTER1(shape, circle, double); // circle with default constructor TYPE_REGISTER(shape, circle); int main(int argc, char* argv[]) { std::string type; type = "square"; shape* ps1 = dynamic_new<shape>(type); shape* ps2 = dynamic_new<shape>("circle", 20.0); shape* ps3 = dynamic_new<shape>("circle"); shape* ps4 = dynamic_new<shape>("square", 10.0); if (ps1) ps1->draw(); // prints: I am a square if (ps2) ps2->draw(); // prints: I am a circle of radius: 20.0 if (ps3) ps3->draw(); // prints: I am a circle of radius: 0.0 if (ps4) ps4->draw(); // does nothing since no ctor available delete ps1; delete ps2; delete ps3; delete ps4; return 0; } regards, Roland

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date-time out of whack?
by Victor A. Wagner Jr. 06 Dec '04

06 Dec '04

The program that causes this error used to compile (until a few days ago) I'm using the cvs HEAD c:\boost\include\boost-1_32\boost\iterator\iterator_facade.hpp(326) : error C2664: 'boost::implicit_cast' : cannot convert parameter 1 from 'const std::string *__w64 ' to 'boost::mpl::identity::type' with [ T=boost::detail::operator_arrow_result,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::value_type,boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::reference,boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::pointer>::type ] Conversion loses qualifiers c:\boost\include\boost-1_32\boost\iterator\iterator_facade.hpp(325) : while compiling class-template member function 'boost::detail::operator_arrow_result::type boost::detail::operator_arrow_result::make(Reference)' with [ ValueType=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::value_type, Reference=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::reference, Pointer=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::pointer ] c:\boost\include\boost-1_32\boost\iterator\iterator_facade.hpp(642) : see reference to class template instantiation 'boost::detail::operator_arrow_result' being compiled with [ ValueType=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::va lue_type, Reference=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::reference, Pointer=boost::iterator_facade,std::basic_string,std::allocator>::const_iterator,std::string>,std::string,boost::detail::minimum_category::type>::type,const std::string &>::pointer ] c:\boost\include\boost-1_32\boost\token_iterator.hpp(41) : see reference to class template instantiation 'boost::iterator_facade' being compiled with [ Derived=boost::token_iterator,std::basic_string,std::allocator>::const_iterator,std::string>, Value=std::string, CategoryOrTraversal=boost::detail::minimum_category::type>::type, Reference=const std::string & ] c:\boost\include\boost-1_32\boost\date_time\date_parsing.hpp(119) : see reference to class template instantiation 'boost::token_iterator' being compiled with [ TokenizerFunc=boost::char_delimiters_separator, Iterator=std::basic_string,std::allocator>::const_iterator, Type=std::string ] c:\boost\include\boost-1_32\boost\date_time\gregorian\parsers.hpp(24) : see reference to function template instantiation 'date_type boost::date_time::parse_date(const std::string &,int)' being compiled with [ date_type=boost::gregorian::date ] Victor A. Wagner Jr. http://rudbek.com The five most dangerous words in the English language: "There oughta be a law"

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Boost.FileSystem, problem with exists()
by Martin Slater 06 Dec '04

06 Dec '04

Hi, I've been using boost.filesystem and have come up on a problem with exists(). Basically on a windows system it will return true if asking whether a file exists on a non-existent share. The problem is at, operations_posix_windows.cpp:327. There should (?) be an additional check against ERROR_BAD_NETPATH as well. The fuzziness of this function is quite disturbing as shown by the following comment in the code return false; // GetFileAttributes failed because the path does not exist // for any other error we assume the file does exist and fall through, // this may not be the best policy though... (JM 20040330) The windows docs don't really help that much by not specify exactly what the failures can be though. Personally I think it should always return false on GetAttributes() error to avoid false positives and the expense of the possible false negatives, but we need a list of all possible errors to be sure that this is the right thing to do. Oh and any chance you can put in serialization support for boost::filesystem::path? I've done it locally and the code is trivial but it would be nice to have it out the box. Anyhow, thanks for a great library (filesystem and the rest of boost). cheers martin

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A few more filesystem documentation corrections
by Edward Diener 06 Dec '04

06 Dec '04

Here are a few more grammatical corrections to the excellent filesystem documentation. Default Name Check mechanism Dangers second sentence: "Since global variables are considered harmful [Wulf-Shaw-73], class path allows the default name check function can be set only once, and only before the first use." should be: "Since global variables are considered harmful [Wulf-Shaw-73], class path allows that the default name check function can be set only once, and only before the first use." or "Since global variables are considered harmful [Wulf-Shaw-73], class path allows the default name check function to be set only once, and only before the first use." Rationale Const vs non-const returns last sentence: "A requirement that path::iterator not be a non-class type was added to eliminate errors non-const iterator errors." should be: "A requirement that path::iterator not be a non-class type was added to eliminate non-const iterator errors." or better: "A requirement that path::iterator be a class type was added to eliminate non-const iterator errors." Finally, in operations.hpp, links on "class directory_iterator" and "other functions" to their explanations further down the page do not work using IE, Netscape, or Opera, so I imagine some HTML error.

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Filesystem documentation error
by Edward Diener 06 Dec '04

06 Dec '04

The documentation to filesystem is outstanding. Congratulation Mr. Dawes ! I did find an error, however. In the class documentation to path.hpp there are two default_name_check member functions. In the specific documentation for default_name_check, there is only a single default_name_check mentioned and the return value in the declaration of the specific documentation has 'void' even while the "Returns:" says otherwise and correctly matches the return value given for either one of the default_name_checks given in the class documentation.

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