April 2007 - Boost-users - lists.preview.boost.org

[fusion] error in example code
by Christian Henning 02 Apr '07

02 Apr '07

Hi there, the example code of the list sequence doesn't compile. list<int, float> l(12, 5.5f); std::cout << at<0>(l) << std::endl; std::cout << at<1>(l) << std::endl; It should be: list<int, float> l(12, 5.5f); std::cout << at<mpl::int_<0> >(l) << std::endl; std::cout << at<mpl::int_<1> >(l) << std::endl; I'm using MSVC 7.1. Regards, Christian

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Which is preferred? Static assertions with type traits or concept_check?
by Benjamin Lau 02 Apr '07

02 Apr '07

Hi all, I'm writing a template class and I have several requirements on the type parameters. A lot of it has to do with the inter-convertibility of the types. I have 2 options to choose from as to how I can check for these convertibility requirements: static assertions with the type traits selector is_convertible, or using concept_check to check if the assignment operation is defined for the types. Even though the former leads to shorter code, the latter works just as well as I already have concepts modeled for these types, so I could just dump the inter-convertibility checks inside as well without much hassle. In summary, I either choose this: BOOST_STATIC_ASSERT((boost::is_convertible<T1, T2>::value)); or this: template<class T1, class T2> class Concept { public: void constraints() { // insert other constraints a = b; b = a; } T1 a; T2 b; }; Thanks a lot, and have a good day Benjamin Lau

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[mpl] erase_key confusion
by Scott Meyers 02 Apr '07

02 Apr '07

I expect the following code to compile cleanly. It does not. #include <boost/mpl/set.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/erase_key.hpp> namespace mpl = boost::mpl; using mpl::_1; using mpl::_2; struct A {}; struct B {}; typedef mpl::set<A, B> set1; typedef mpl::set<B, A> set2; BOOST_MPL_ASSERT(( mpl::equal< mpl::erase_key<set1, A>::type, mpl::erase_key<set2, A>::type > )); I'm simply trying to assert that the sets {A,B} and {B,A} have the same elements after A has been removed from both. What am I doing wrong? Thanks, Scott

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[threads] Passing Data and Thread Joining
by Sami Lakka 02 Apr '07

02 Apr '07

I have an method in my application which is called by a third-party app by every 1 second. In my method I have to do some intensive math operations as well as persist data. Because I have to do the math operations in the one second period I'm creating a thread that will handle the file operations (as there are no hurry for these). My question is how should I pass the data to my file saving thread? The data is in shared_array<double> so should the data be passed by value or by reference? My code looks like as follows: void workerMethod() { // do some mathematics shared_array<double> data = .... thread saveThread(bind(&DataFileSaver::saveInThread, dataFileSaver,data)); // Thread is not joined } I think that if I pass by reference the data variable can go out of scope and my thread can end up saving garbage. Am I correct? My second question is that in my saveInThread method I'm using scoped_lock to guard the file operations. When I run some unit tests for this method I'm experiencing crashes and memory leaks. This is probably because I'm not joining the started thread in the workerMethod but in my unit tests I use sleep() enough so that the threads should have performed the operations. What am I doing wrong?

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[graph] adjacency_list with edges in mulitsetS does not seem to sort
by Carsten Scholtes 02 Apr '07

02 Apr '07

Hello! I'm trying to use boost::graph's adjacency_list<listS, multisetS,...>. The documentation ( boost_1_33_1/libs/graph/doc/adjacency_list.html ) states for the function edge_range(): > This function only works when the OutEdgeList for the adjacency_list > is a container that sorts the out edges according to target vertex, > and allows for parallel edges. The multisetS selector chooses > such a container. Attached you find a small program demonstrating a case where edges do not seem to get sorted accordingly. The last comment in the program contains a sample output. * The line below "All edges" shows the sequence of all edges. It does not seem to be sorted in the specified way. Probably as a consequnce of this, edge_range() does not work as expected: * The last line of the output is generated by using edge_range(). It should output edges from node 1 to node 0. There is only one such edge. Instead of that, edges (1,2) and (1,0) are returned. Am I using the interface in a wrong way or is there an error in the implementation of adjacency_list<>? Thank you in advance for any comment or advice. Carsten Scholtes

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[serialization] abstract templatized base class
by Ruediger Berlich 02 Apr '07

02 Apr '07

Hi again, apologies for the frequent questions. Here is another problem I've run into. I think I've found a solution, but would like to cross-check whether I'm right. I have a number of templatized base classes, that contain purely virtual member functions. As per the Boost.Serialization documentation, purely virtual classes should be labelled as "abstract". >From the documentation and the header file "is_abstract.hpp" it appears as if using the macro BOOST_IS_ABSTRACT would not work (indeed it produces lots of error messages) for templatized classes, but that I should instead add the following to the header: namespace boost { namespace serialization { template<class T> struct is_abstract<myClass<T> > { typedef mpl::bool_<true> type; BOOST_STATIC_CONSTANT(bool, value = true); }; }} Indeed this seems to work. It appears to me as if another solution might be to use the original macro in those files where the templatized classes are first instantiated (i.e. "filled" with a type). Please note, though, that they are base-classes for some other non-abstract classes, so that I'm not sure this would work. Anyway, the idea would be something along the lines of // [...] // myClass<myType> is no longer a template BOOST_IS_ABSTRACT(myClass<myType>) class derived :public myClass<myType> { ...}; Would these two options cause any problems ? Thanks and have a good day, Ruediger

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[lexical-cast] double not converted as it is to a string
by Digvijoy Chatterjee 01 Apr '07

01 Apr '07

Hello, I am seeing this behavious with boost::lexical_cast<std::string>(double) occasionaly numbers like 76.32 are being converted to strings of the form 76.319999999 The expected behavior is seen for almost all doubles except for a few which are reproducible e.g. 76.32.. here is a simple program, int main() { double d=76.32; std::string s = boost::lexical_cast<std::string>(d); cout << s << endl; } output 76.319999999... a) am i doing something wrong ?? b) if this is not wrong s this standard behavior ?? Thanks Digz

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Building for cygwin on Windows?
by John Harper 01 Apr '07

01 Apr '07

I can't get the boost libraries to build correctly using Cygwin and GCC on Windows XP. Boost seems determined to build for some kind of native Windows environment. I'm trying: bjam "-sTOOLS=gcc" "--prefix=/usr/local" and it complains "wide i/o not supported on this platform". If I omit --prefix it puts the libraries in the Windows file system under C:\Boost, but even if I copy them to somewhere under cygwin and point GCC at them, it still doesn't find them. SUrely I'm not the first person to want to do this? What is the magic formula? Thanks...

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[serialization] free load/save
by Ruediger Berlich 01 Apr '07

01 Apr '07

Dear all, I'd like to serialize a number of classes using the free-standing version of the serialize function. As I do not want to access internals of the classes directly, I have to work with set/get functions, which in turn implies having to write load/save functions. This seems to work for simple types, like in the following example. But will it circumvent any internal mechanisms of Boost.Serialization , if I do not give the library direct access to class data ? Will this work for complex types, e.g. vectors of shared_ptr objects ? /*************************************************************************/ class base{ public: base(void){ setSecret(1); } base(int val){ setSecret(val); } virtual ~base(){ /* nothing */ } virtual int getSecret(void) const{ return secret; } virtual void printInfo(void) = 0; void setSecret(int val){ secret = val; } private: int secret; }; BOOST_IS_ABSTRACT(base); namespace boost { namespace serialization { template<class Archive> void save(Archive & ar, const base & t, unsigned int version) { int secret = t.getSecret(); ar & make_nvp("secret", secret); } template<class Archive> void load(Archive & ar, base & t, unsigned int version) { int secret; ar & make_nvp("secret", secret); t.setSecret(secret); } template<class Archive> inline void serialize(Archive & ar, base & t; const unsigned int version) { split_free(ar, t, version); } } } /*************************************************************************/ Thanks and have a good day, Ruediger

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