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

bjam build of boost.thread on mac - does not work??
by Edward Kogan 02 Feb '05

02 Feb '05

We have been unable to get bjam to build boost::threads using Metrowerks CD 9.3 on Mac OS X. The command lines generated by bjam are invalid and rejected by the compiler (mwcc). 1) has anyone been able to get bjam to work correctly in the above set up? 2) or, does anyone have a valid version of the threads.mcp project? The one in the distribution and in CVS is not a usable project file. Thanks in advance Edward __________________________________ Do you Yahoo!? Yahoo! Mail - Helps protect you from nasty viruses. http://promotions.yahoo.com/new_mail

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accessing the underlying C array of a boost::array
by Frank Hess 02 Feb '05

02 Feb '05

Does anyone know what the preferred method of accessing the underlying C array of a boost::array is? In boost 1.32 there is a data() member which returns a const pointer, and a c_array() member which returns a pointer. Also, the elems member itself is public. But at http://www.josuttis.com/cppcode/array.html, the author states that data() can be used for both const and non-const pointers, and indeed it is so in the version he posts on his website http://www.josuttis.com/cppcode/array.hpp.html (which doesn't provide the c_array() member but overloads data() instead). -- Frank

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regex_token_iterator bug?
by samantha Quark 02 Feb '05

02 Feb '05

Dear all, I just played a little bit with the regex_token_iterator ( boost v 1.32) and modified the "html" example. I found this (strange?) behavior when i "tokenize" a string. while (i != j) { std::cout << i++->str().length() << std::endl; } crashes at runtime at the last sub-match of a match with: mtest: ../boost_1_32_0/boost/shared_ptr.hpp:253: T* boost::shared_ptr<T>::operator->() const [with T = boost::regex_token_iterator_implementation<__gnu_cxx::__normal_iterator<const char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, char, boost::regex_traits<char>, std::allocator<char> >]: Assertion `px != 0' failed. zsh: 24497 abort mtest test.html Whereas: while (i != j) { std::cout << i->str().length() << std::endl; i++; } works fine. Is this behavior a bug, or is it correct, that the first version does not work, if yes, why? Cheers, Joe. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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[BGL] implementing operator < for edge_descriptors
by Max Schoebinger 02 Feb '05

02 Feb '05

Hello, I am trying to create a std::map<edge_descriptor, unsigned int>. Unfortunaltely, there is no less operator defined for edge descriptors, so i'd like to implement my own version based on a custom internal property associated with the edge. Is it possible to access an arbitrary property, solely based on an edge_descriptor without having access to the graph object? Here some pseudo-code: struct compare_edge_descriptors { bool operator()(edge_descriptor e1, edge_descriptor e2) { unsigned int val1 = get_property_somehow(e1.get_property(), my_custom_property); unsigned int val2 = get_property_somehow(e2.get_property(), my_custom_property); return (val1 < val2 ); } }; Do you have an idea how to implement "get_property_somehow"? Thanks, Max

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boost-users: cant i use mpl::contains to find members of list_c ??
by suresh.t＠philips.com 02 Feb '05

02 Feb '05

hi, I was trying to use mpl::contains with list_c and it always seems to return mpl::false_. For example typedef mpl::list_c< int ,16,8 > mylist; typedef mpl::contains<mylist, mpl::int_<16> >::type tt; "tt::value" is zero though i thought 16 being in the mylist it would be 1. Can someone please let me know whether the usage is improper and if so what should be used to find elements in list_c? Thanks in Advance S.T

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Exercices 3-3 and 3-4 (C++ Templates Metaprogramming)
by Philippe Mori 02 Feb '05

02 Feb '05

I'm not sure of the meaning of "twice twice" and "twice on itself" respectively. My guess would be for exercice 3 that I should do something like : typedef twice<to_be_filled_1>::type T1; // T1 would be T ** // The second time, I replace T (in to_be_fiiled_1) by // T1 (in to_be_filled_2) , thus T2 is T1** which is also T**** typedef twice<to_be_filled_2>::type T2; while for exercice 4, we want something that would look like : typdef twice<twice<to_be_filled_3a>to_be_filled_3b>::type T3; Does I understand the questions correctly? For twice, I use the definition from section 3.3.2. Philippe

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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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uBLAS sparse_matrix "default" value
by Austin Bingham 02 Feb '05

02 Feb '05

Apparently the wheels of the ublas mailing list move slowly, so I'm asking this question here. Is there any way to set the "default" value of a sparse_matrix? sparse_matrix seems to guarantee that you always get a value when you access an index, even if that index hasn't been explicitly set. For sparse_matrix<float>, for instance, you get 0. Is there any way to specify this value? That is, if I wanted to get back, say, -1 for unset entries, is there a way to do this? There's no direct mention of this that I can find, although there were some discussions of zero-constructors or somesuch that may have been dealing with this question. Thanks. Austin Bingham

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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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Re: Rationale for intrusive_ptr?
by Brian Neal 02 Feb '05

02 Feb '05

Well to answer my own question, I think, it can be used when you already have something that is reference counted and you want to manage it yourself. Also in places where new and delete aren't the correct actions. Or when you need something with less resource requirements than shared_ptr. For example, when multi-threading is turned on, shared_ptr asks for a mutex. Intrusive_ptr leaves that up to you. Am I getting close here? I didn't think about using it when you have a function that takes a shared_ptr but you need to pass in this. I dunno, I tend to use shared_ptr sparingly. If I don't need a shared_ptr, I don't ask for one on the interface to a function. That way the user can pass in a naked pointer or a shared_ptr::get(). I make sure to document what the function does (and does not do) with the input pointer though. Thanks. __________________________________ Do you Yahoo!? Yahoo! Mail - Find what you need with new enhanced search. http://info.mail.yahoo.com/mail_250

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