February 2005 - Boost - lists.boost.org

[range?] Re: Partial specialization matching
by David Abrahams 28 Feb '05

28 Feb '05

The following message is a courtesy copy of an article that has been posted to comp.lang.c++.moderated as well. [I'm Cc'ing this to Boost because IIRC some people were struggling with a similar issue recently. Of course it may not apply ;-)] "Nicolas" <nicolas.tisserand(a)gmail.com> writes: > Ben Hutchings wrote: > > [snip] >> I believe it was considered unreasonable to require implementers to >> do that kind of analysis, given that it could involve checking for >> specialisations across the whole program. > > Got that. Thanks :) > > In the code I posted, I was trying to specialize a class template > (IteratorTraits) according to the relationship between an Iterator and > its Container. > For instance, I wanted to be able to produce a specialization of > IteratorTraits for what std::vector<T, Alloc> advertises as being its > iterator type. It's fundamentally impossible [unless the core language was to make exceptions based on special knowledge about certain standard library components]. Forget everything you know about vectors, and now consider: template <class Iterator> struct IteratorTraits; template <class T> struct vector { typedef char* iterator; }; // This would be the syntax, if it were legal. template <class T> struct iterator_traits<vector<T>::iterator> { // What is T? }; It's illegal because there's no way for the compiler to deduce T. > Is there a way to do that without having to examine a given STL > implementation and find out what the exact type of std::vector<T, > Alloc>::iterator is? Technically, no. *If* you're willing to assume that std::vector<T,A>::iterator is the same type for any A (which is not quite correct in any real implementation because of vector<bool>), you could do this: template <class Iterator, class Enable = void> struct IteratorTraits; template <class Iterator> struct IteratorTraits< Iterator , boost::enable_if< boost::is_same< Iterator , typename std::vector< typename std::iterator_traits<Iterator>::value_type >::iterator > >::type > { // whatever you want here. }; This specialization of IteratorTraits applies iff the iterator type of a vector of the argument's value_type is the same as the argument. I'm sure you can figure out how to deal with the vector<bool> anomaly. See http://www.boost.org/libs/utility/enable_if.html for information about how enable_if works with class template specializations. HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com

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RE: [boost] Re: IoStreams Question: How do I get "tee" functionality
by Brian Braatz 27 Feb '05

27 Feb '05

-----Original Message----- From: boost-bounces(a)lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Jonathan Turkanis Sent: Sunday, February 27, 2005 12:20 PM To: boost(a)lists.boost.org Subject: [boost] Re: IoStreams Question: How do I get "tee" functionality Jonathan Turkanis Wrote: <<<<<<<<<<<<<<<<<<<<< I apparently didn't test that code, because it contained two errors (write takes a const char* instead of a char*, and only multi-character filters implement write() instead of put()). At the end of this message is an updated and tested version. I plan to include a tee filter in the first released version of the library. It should be slightly more efficient than the example. Jonathan <<<<<<<<<<<<<<<<<<<<< BRIAN BRAATZ WRITES: THANK YOU !

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lazy exact arithmetic
by Brandon Kohn 27 Feb '05

27 Feb '05

Hello, I was just posting to see if there is any interest in developing a library to help with exact arithmetic using the lazy arithmetic strategy? I've been working for the last couple of years with computational geometry algorithms, and have found that such a library is really essential to overcoming problems due to floating point precision/round off errors in most computational geometry algorithms with non-brute force complexity. The idea behind the lazy paradigm is that exact computation (using rational arithmetic with arbitrary sized integers) is too costly to be used all the time. So the exact computations are delayed until such a time as you cannot reliably make decisions with the information you have from the 'lazy' filters. The lazy filters are implemented as a layer of calculation that uses interval arithmetic to compare numerical quantities. If the intervals being compared are non-overlapping, the result is considered to be sufficient and exact computation is avoided. If the intervals do overlap, exact computation is required to unambiguously decide how the quantities compare. This is just a rough outline of how the method works, and I'm sure there will be plenty of kinks to work out (my background is in physics and computer science... so perhaps a mathematician in the group would step up to help guide my efforts :). A library such as this one exists in the CGAL kernel library which is available under the LGPL. I find that while the LGPL can be useful in cases where the library is already organized as a single DLL/shared object library, it is generally difficult to implement into commercial solutions due to the dynamic linking constraints imposed by the library. Further, I think that this type of algorithm (which solves a very big problem with floating point arithmetic in computing) is really something that should be standardized and freely available. Sorry for the long post :) What do you guys think? Kind regards, Brandon

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IoStreams Question: How do I get "tee" functionality
by Brian Braatz 27 Feb '05

27 Feb '05

However, I am NOT (yet) an expert in the Iostreams library so please excuse me if I am missing something obvious. I add 2 sinks filtering_ostream out; out.push(file_sink("Report.txt")); out.push(cout); I get an exception thrown on the second one (regardless of which one goes in first) In searching the archives, I noticed this mail: http://tinyurl.com/6soeb where Johnathon Turkis writes: "For this we need to define an OutputFilter which stores a reference to an ostream and has a member function 'write' which forwards all characters to the stored ostream as well as to the downstream sink (you don't need to understand the following code -- the point is that it's just a few lines): struct tee : boost::io::output_filter { tee(std::ostream& dest) : dest(dest) { } template<typename Sink> void write(Sink& snk, char* s, std::streamsize n) { // Write to the downstream Sink boost::io::write(snk, s, n); // Write to the stored ostream: dest.write(s, n); } std::ostream& dest; }; We use the tee as follows. Given two ostreams ostream first; ostream second; we define a filtering_ostream which passes data through the tee to both streams: filtering_ostream out; out.push(tee(first)); out.push(second); out << "this gets written to both ostreams\n"; " Which looks "good" except I suspect the library has changed since the above is not compiling for me under VC71. (it complains about wanting a Put() instead of a write()).. Any help appreciated- and also - a request- if "tee" functionality could be officially implemented in the streaming lib for output sinks. OR if it already is, I request that how do to it be spelled out more clearly in the documentation. Thanks Brian Braatz

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Re: [boost] boost::multi_index::key_extractors status
by JOAQUIN LOPEZ MU?Z 27 Feb '05

27 Feb '05

----- Mensaje original ----- De: Loïc Joly <loic.actarus.joly(a)wanadoo.fr> Fecha: Domingo, Febrero 27, 2005 7:04 pm Asunto: [boost] boost::multi_index::key_extractors status > Hello everybody, > > I'm discovering boost::multi_index, and something strikes me: I've > been > dreaming of something like boost::multi_index key extractors (and > especially member) for most of my std::set or std::sort uses. > > I therefore think that this part of boost::multi_index does not > belong > in this library, but is more general purpose, and deserves a > library of > its own, along with a way to get a suitable less predicate from > the > result of those. All the arguments that make such a framework > usefull > for boost::multi_index can equally be applied to STL containers > and > algorithms. > > I wonder if such a proposal has already been made (maybe it even > already > exists, boost has so many parts that I may be unaware of this), > and in > the positive case what were the arguments in favor or in > opposition of it. > This issue was considered during the review, and those who gave an opinion about it thought it was better to leave like it is now. Anyway, there's nothing preventing you from using the key extraction sublib for your own purposes, as it is headerwise separate from the rest. You'll only need some compose classes to plug key extractors into comparison predicates, something like this: template<typename KeyFromValue> struct key_less { typedef result_type bool; template <typename Arg> bool operator()(const Arg& x,const Arg& y) { return key(x)<key(y); } private: KeyFromValue key; }; ... typedef std::set< employee, key_less<member<employee,std::string,&employee::name> > > employee_set; HTH Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

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boost::multi_index::key_extractors status
by Loïc Joly 27 Feb '05

27 Feb '05

Hello everybody, I'm discovering boost::multi_index, and something strikes me: I've been dreaming of something like boost::multi_index key extractors (and especially member) for most of my std::set or std::sort uses. I therefore think that this part of boost::multi_index does not belong in this library, but is more general purpose, and deserves a library of its own, along with a way to get a suitable less predicate from the result of those. All the arguments that make such a framework usefull for boost::multi_index can equally be applied to STL containers and algorithms. I wonder if such a proposal has already been made (maybe it even already exists, boost has so many parts that I may be unaware of this), and in the positive case what were the arguments in favor or in opposition of it. Best regards, -- Loïc

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Reading the contents of a directory
by Dark Haruka 27 Feb '05

27 Feb '05

Hello :) I'm searching a library for C/C++ which can access directorys. I have read that the boost library can it. So I have downloaded and installed it. But I'm searching a function with which I can look up which files are in a directory. I'd like to say a directory to scan to the function. The function should return the names of the files in that directory and if possible the name of subdirs. Does anyone know this function? Thank you in advance :)

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Object-oriented preprocessor programming :-)
by Arkadiy Vertleyb 27 Feb '05

27 Feb '05

Hi all, If one needs to manage a lot of PP code, some code structuring technique might be helpful. So, how about the OO? The example below demonstrates this techinique with the canonical shapes example. An object is represented as a PP sequence of properties. The head of the sequence contains the "class name" and is used for dispatching. I believe this proved very useful in the BOOST_TYPEOF project where we polymorphicaly treat type, integral, and template template parameters. Comments? Regards, Arkadiy -- #include <boost/preprocessor/seq/transform.hpp> #include <boost/preprocessor/seq/enum.hpp> #include <boost/preprocessor/seq/cat.hpp> #include <boost/lambda/lambda.hpp> #include <algorithm> #include <iostream> // #define VIRTUAL(Fname, This) BOOST_PP_SEQ_CAT(\ (BOOST_PP_SEQ_HEAD(This))(_)(Fname)) // #define CIRCLE(r) (CIRCLE)(r) #define CIRCLE_RADIUS(This)\ BOOST_PP_SEQ_ELEM(1, This) #define CIRCLE_AREA(This)\ 2 * 3.14159 * CIRCLE_RADIUS(This) // #define RECTANGLE(h, w) (RECTANGLE)(h)(w) #define RECTANGLE_HIGHT(This)\ BOOST_PP_SEQ_ELEM(1, This) #define RECTANGLE_WIDTH(This)\ BOOST_PP_SEQ_ELEM(2, This) #define RECTANGLE_AREA(This)\ RECTANGLE_HIGHT(This) * RECTANGLE_WIDTH(This) // #define SQUARE(x) (SQUARE)(x) #define SQUARE_SIDE(This)\ BOOST_PP_SEQ_ELEM(1, This) #define SQUARE_AREA(This)\ SQUARE_SIDE(This) * SQUARE_SIDE(This) // #define MY_SHAPES\ (CIRCLE(5))\ (RECTANGLE(3, 4))\ (CIRCLE(3))\ (SQUARE(3))\ (SQUARE(5))\ (RECTANGLE(2, 6)) #define ENUM_AREAS(shapes)\ BOOST_PP_SEQ_ENUM(BOOST_PP_SEQ_TRANSFORM(OP, ~, shapes)) #define OP(s, data, elem)\ VIRTUAL(AREA, elem)(elem) double MyAreas[] = { ENUM_AREAS(MY_SHAPES) }; // int main() { using namespace std; using namespace boost::lambda; for_each( MyAreas, MyAreas + sizeof(MyAreas)/sizeof(MyAreas[0]), cout << _1 << "\n"); return 0; }

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Autoconfiguration problem with wcs* functions?
by Julio M. Merino Vidal 27 Feb '05

27 Feb '05

Hi all, I'm using the autoconfiguration framework under NetBSD 2.99.14 to generate an user.hpp file. The libs/config/test/boost_no_cwchar.ipp test passes successfully, so Boost ends up thinking that wcs* functions work. In fact, they do, but in C (defined in wchar.h). The problem is that those functions (I'm referring to wcscmp and wcslen, but maybe there are some others) are not (yet) defined in the std:: namespace. (The wchar stuff is being worked on at the moment, AFAICT; at least, it's very recent.) So, what happens? As the BOOST_NO_CWCHAR constant is not defined, the build will try to use cwchar functions. And it will fail at multiple places. The "problem" is that the code is using std::wcscmp and std::wcslen because it thinks it can. I don't know which is the correct fix. Just temporally, I've changed the test to use the functions in the std:: namespace rather than the other ones, so that I get a failure and cwchar is disabled. But this seems suboptimal (basically because the functions do exist). Any comments on how this should be handled? Thanks, -- Julio M. Merino Vidal <jmmv84(a)gmail.com> http://www.livejournal.com/users/jmmv/ The NetBSD Project - http://www.NetBSD.org/

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Type of Review for Unordered Associative Containers & Hash Functions
by Daniel James 27 Feb '05

27 Feb '05

Hello everyone, I've just uploaded a new version of the unordered associative containers, this time in the 'Hash' folder on the file vault. There's also the file 'hash.tar.gz', which is just the hash function object. The hash function objects are ready for review, the container library is almost ready, it mainly just needs more documentation. The implementation has been split into two libraries. Boost.Hash implements the hash function objects which would go into <functional>, so it is proposed that these are added to the functional library. This is needed for Joaquín's MultiIndex library, so it would be good to add it to Boost as soon as possible. The other part is Boost.Unordered, which implements the unordered associative containers. This is a little larger and more complicated, and there's no immediate need for it (unless you count Boost.Tr1). We're not sure what kind of review these libraries will need. Since they're both based on the draft standard, their design isn't really up for review here (right?), but the documentation and implementation is. Do you think they require a full review, or will a fast track review be appropriate? A fast track review is tentatively scheduled for the hash function objects after the review of the finite state machine, with Thorsten managing it. The full container library could be included in this, if that is considered acceptable. Or it could be added to the queue for a full review? Any opinions? Daniel

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