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boost::bind and member template functions
by Matt Schuckmann 21 Aug '06

21 Aug '06

I can't seem to make boost::bind work with a member template function and it seems like is should. The following won't compile with MSVC 7.0, struct Test_t { template<typename T> void testV( int i, std::vector<T>& b ) { b.push_back( static_cast<T>(i) ); } char m_c; }; void MyTest() { int i(1); Test_t t; std::vector<long> v; boost::bind( &Test_t::testV<long>, t, _1, v )(i); } I get a whole bunch of wrong number of arguments errors. I tried the boost::bind<void> variation and that also won't compile, just with a little different error of the form "cannot convert parameter 1 from 'void (int,std::vector<_Ty> &)' to 'boost::type<T>'" If testV is just a regular template function, not a member of a class or struct, it works fine. Anybody have any idea how to make this work? Thanks, Matt S.

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Header file granularity policy?
by Scott Meyers 21 Aug '06

21 Aug '06

Some Boost libraries, such as SmartPtr and Variant, offer a single header with declarations for all library functionality as well as a set of headers for subsets of functionality. For example, smart_ptr.hpp declares all of shared_ptr, shared_array, weak_ptr, scoped_ptr, scoped_array, and intrusive_ptr. If I don't want all these declarations, I can use more specific headers, e.g., shared_ptr.hpp or scoped_array.hpp. Such libraries allow me to choose between the convenience of a single comprehensive header or the precision of multiple targeted headers. Other libraries seem to offer only the collection of targeted headers. For example, FileSystem offers five headers, but there doesn't seem to be a comprehensive filesystem.hpp. Similarly, Lambda seems to offer only targeted headers; there doesn't seem to be a single "declare everything in Lambda" header. (There is a lambda.hpp, but it's not comprehensive, e.g., it doesn't declare Lambda's bind.) I find that this kind of inconsistency among libraries degrades my experience as a Boost user, especially when I'm first learning to use a library. More than once I've spent time puzzling over a compilation error, only to discover that my mistake was not including a needed header. (I seem to make this error most commonly when using Lambda.) Is there some kind of Boost convention or policy on whether a library should offer a comprehensive "declare everything in the library" header in addition to more targeted headers? Thanks, Scott

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Compile 32 bits on 64 arch how-to ?
by david v 21 Aug '06

21 Aug '06

Hello, I would like to compile boost on 64 opteron arch but create the libraries in 32 bits mode (using -m32 cxxflags). The reason for that is that i have quite a lot libraries compiled in 32 that i can't port for the moment to 64. I'm not sure how to do that. I read about modifying user.hpp usung the configure script and that is what i did but it still doesn't seem to compile with the -m32 compiler flag. Can anyone help me ?

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bind depending on the type
by Minkoo Seo 19 Aug '06

19 Aug '06

Hi list. I'm trying to find out the way to bind depending on the types in a lambda expression. Let's take the following as an example: #include <iostream> #include <vector> #include <numeric> #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace std; using namespace boost::lambda; int main() { vector< pair<string, int> > subjects; subjects.push_back(make_pair("Math", 90)); subjects.push_back(make_pair("English", 86)); subjects.push_back(make_pair("Physics", 88)); subjects.push_back(make_pair("Biology", 97)); long total; total = accumulate(subjects.begin(), subjects.end(), 0, _1 + bind(&pair<string, int>::second, _2)); cout << "total score: " << total << endl; return EXIT_SUCCESS; } This is a simple score card application that computes the total score. As you can see, I've used bind in calling accumulate and that seems to be ugly. So, I modified the code as follows: #define get_score(p) (bind(&pair<string, int>::second, p)) .. total = accumulate(subjects.begin(), subjects.end(), 0, _1 + get_score(_2)); This seems to be pretty. But the problem is that this code depends on the fact that _1 is int while _2 is pair<string, int>. I don't know whether this is platform independent, and this code still seems to be weird anyway. So, what I want to do is write a function get_score that successfuly compiles and run the following: total = accumulate(subjects.begin(), subjects.end(), 0, get_score(_1) + get_score(_2)); I've tried my best only to fail. Could anybody write get_score functions for me? Thanks in advance. Sincerely, Minkoo Seo

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Convenience.hpp extension core drops on ~ (tilda)
by Bob J 19 Aug '06

19 Aug '06

I ran across a problem in the convenience extension function. If the filename happens to have the the tilda character in it the function core drops. I discovered the problem when a file named ~trash was inadvertently created in a data directory. After creation of that file the system started core dropping. After much debugging I narrowed the problem to the extension function and the ~trash file. I was able to replicate the problem by std::string filename = "~test.txt"; string ext = extension(filename); Does anyone know a work around or if there are any other gotcha characters in this function. Bob

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[SmartPointers] weak_ptrs, raw pointers, and cycles
by Scott Meyers 18 Aug '06

18 Aug '06

I've recently realized that I just don't get something. Cycles of shared_ptrs are bad, because they can lead to resource leaks. To break a cycle, we're encouraged to use weak_ptrs. Okay, but we could use raw pointers to break the cycle, too, and I can't see any advantage offered by the weak_ptr. If A uses a shared_ptr to B and B uses a weak_ptr to A, B's life will be shorter than A's (its destructor will be invoked from A's destructor and will run to completion before A's does), so there is no advantage to a weak_ptr being able to tell if it dangles. In this context, it never will. So it seems that using a raw pointer to break cycles has no disadvantage over a weak_ptr, and it's more efficient (i.e., smaller and faster). Clearly, I'm missing something. Can somebody please explain what it is? I understand that weak_ptrs have other uses (e.g., as observers), so my question is only about the advice to use weak_ptrs to break cycles. Thanks, Scott

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[BGL] Maps as vectors x parameterized maps
by Leandro Melo 18 Aug '06

18 Aug '06

Hi! Some of boost algorithms require a vertex_index_map. Since I'm not very familiar with bgl, I don't know if there're any which also require an edge_index_map. If the algorithms were parametrized by the above maps, we could use vectors indexed by the the values in this maps to create capacity_maps, distance_maps, weight_maps and any other maps required by the algorithm (however, to maintain the same level of flexibility we would have to allow the users to parameterize the value types of this vectors). And we would still have constant time access to the maps (like the default boost map implementations in the algorithms). Just for a matter of curiosity, what are advantages between using boost's strategy and the one I described above (in which all maps would be vectors)? The major benefit I can see on boost's current strategy is flexibility. The user can use any map implementation he/she wants (maybe this maps comes from legacy applications). There's also the possibility of implementing readable and writable maps. What are others advantages? And what could be the drawbacks of using this vector aproach? Thanks. -- Leandro Terra C. Melo

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[BGL]container traits
by Louis Lavery 18 Aug '06

18 Aug '06

Would it be much easier to implement the boost graph lib if C++ containers could reference themselves? I mean if something like std::list_traits<T> was available so that the iterator class could be got without instantiating the actual list. Simplistically, some thing like... struct vertex; struct edge; typedef std::list_traits<vertex>::iterator Vit; typedef std::list_traits<edge>::iterator Eit; struct edge { Vit source; Vit target; }; struct vertex { std::list<Eit> edges; }; I'm asking because container traits seem to be useful in the work I do, so I'm trying to garner support for them. Thanks, Louis.

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matrix<double> initialization
by Minkoo Seo 18 Aug '06

18 Aug '06

Hi list. I got a problem of matrix initialization. Please see the following code: #include <iostream> #include <iterator> #include <boost/numeric/ublas/matrix.hpp> #include <boost/numeric/ublas/io.hpp> using namespace std; using namespace boost::numeric::ublas; int main () { matrix<double> m(3, 3); for (unsigned i = 0; i < 3; i++) m(i, i) = -i; cout << m << endl; return EXIT_SUCCESS; } In this code, I'm trying to initialize the diagonal with -1, -2, -3. But, the result is obviously different from what I expected: [3,3]((0,0,0),(0,4.29497e+09,0),(0,0,4.29497e+09)) This must be due to type conversion from unsigned from double that occurs at 'm(i, i) = -i'. IMHO, the code must works as follows i // unsigned -i // (-1) * i = int * unsigned = int m(i, i) = -i // lhs is double, so int on rhs should be converted // into double. But the result implies that I'm wrong. Anybody knows what I'm missing here? How would you correct the above code? Sincerely, Minkoo Seo

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Re: [Boost-users] matrix<double> initialization
by james.jones＠firstinvestors.com 18 Aug '06

18 Aug '06

From: Minkoo Seo <minkoo.seo(a)gmail.com> [code snipped] > for (unsigned i = 0; i < 3; i++) > m(i, i) = -i; > > In this code, I'm trying to initialize the diagonal with > -1, -2, -3. But, the result is obviously different from > what I expected: > > [3,3]((0,0,0),(0,4.29497e+09,0),(0,0,4.29497e+09)) If you must use unsigned, then change to: m(i, i) = -1.0 * static_cast<double>(i); What it's doing now is this: i = 1; -i = 4294967295; // it's unsigned, after all double(-i) = 4.295e+9 Probably clearer to use int instead of unsigned, though. - James Jones Administrative Data Mgmt. Webmaster 375 Raritan Center Pkwy, Suite A Data Architect Edison, NJ 08837

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