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[variant] is_a/can_be
by Yuval Ronen 05 Feb '05

05 Feb '05

Hi. I encountered the need to query a boost::variant to which type it currently contain. I'm talking about something like: variant<int, string> v(3); assert(v.is_a<int>()); assert(!v.is_a<string>()); This can be done using the which() method, but its drawback is that I rely on the order of the types in the variant declaration rather on the type, which is really what I'm interested in. I can implement the variant_is_a<>() feature using a visitor such as: template <typename T> struct is_a_visitor : public boost::static_visitor<bool> { template <typename U> bool operator()(const U &) const { return false; } bool operator()(const T &) const { return true; } }; template <typename T, class Variant> bool variant_is_a(const Variant &v) { return v.apply_visitor(is_a_visitor<T>()); } but this way has a problem: calling variant_is_a<double>() on a variant of type variant<int, string> (no double for this variant) will simply return false, rather than just not compile, which is what I prefer. Making this not compile will require a new feature from variant: a boolean constant 'can_be'. With this, the variant_is_a() function could be written with additional line: BOOST_STATIC_ASSERT(Variant::can_be<T>::value); IMO, the is_a/can_be features are general and useful enough to be included in the library. Wouldn't you agree? Yuval

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RE: [Boost-users] Re: Bug in boost::lambda using operator <<?
by Richard_Cox＠Dell.com 05 Feb '05

05 Feb '05

On Friday, February 04, 2005 5:47 PM Kevin Wheatley wrote: > > Richard_Cox(a)Dell.com wrote: > > > > The program below outputs > > > > String: <First string> Second string> Fourth string> Fifth string> > > > > All but the first "<" is missing. Any ideas? > > std::for_each(coll.begin(), coll.end(), out << gt << _1 > << L"> "); > > > does this work? > > std::for_each(coll.begin(), coll.end(), out << var(gt) << _1 << L"> > "); > > not sure of the fix, but out << gt associates left to right and is > evaluated once and has the type of out, you want it lazy evaluated so > that the whole of the expression is considered as one... see the > delaying constants and variables section of the lambda documents. > > Kevin Of course, "obvious" now :-( out << gt would evaluate returning the wostream which the lambda expression created from the 2nd and 3rd <<'s then operates on. Many thanks. Richard All opinions are mine and do not represent in any way the position of Dell Corporation.

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implementing an iterator
by Chris Goller 04 Feb '05

04 Feb '05

I'm using iterator_facade to implement an iterator over file based data. So, my question is, since deference returns a reference to a value does that reference need to persist when the iterator is incremented or decremented? Because this is file based data I can't always keep a reference around because the data is stored on disk. Thanks, Chris

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boost::function as drop-in replacment for ordinary function pointers?
by Oliver Kowalke 04 Feb '05

04 Feb '05

Hi, is it possible to use boost::function as drop-in replacment for ordinary function pointers - or are there some other solutions? I can't get the code below to work. thx and best regards, Oliver typedef void (*FunctionPointer) ( int); void my_fun( int i) { std::cout << i << std::endl; } class MyFun { public: void operator()( int i) { std::cout << i << std::endl; } }; class Test { public: void f( FunctionPointer fp) { fp( 1); } }; void main() { Test tst; MyFun mf; boost::function< void(int) > f; f = boost::ref( mf); // tst.f( my_fun); // works tst.f( f); // ??? - doesn't compile } -- DSL Komplett von GMX +++ Superg�nstig und stressfrei einsteigen! AKTION "Kein Einrichtungspreis" nutzen: http://www.gmx.net/de/go/dsl

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Bug in boost::lambda using operator <<?
by Richard_Cox＠Dell.com 04 Feb '05

04 Feb '05

The program below outputs String: <First string> Second string> Fourth string> Fifth string> All but the first "<" is missing. Any ideas? (On vc++7.1 using Boost 1.32.0.) Richard All opinions are mine and do not represent in any way the position of Dell Corporation. #include <ostream> #include <iostream> #include <string> #include <vector> #include <algorithm> #include <boost/lambda/lambda.hpp> using std::wstring; using std::wcout; using std::endl; using boost::lambda::_1; typedef std::vector<wstring> StringCollection; wstring gt(L"<"); std::wostream& operator << (std::wostream& out, StringCollection const& coll) { std::for_each(coll.begin(), coll.end(), out << gt << _1 << L"> "); return out; } int main(int, char*[]) { StringCollection s; s.push_back(L"First string"); s.push_back(L"Second string"); s.push_back(L"Fourth string"); s.push_back(L"Fifth string"); wcout << L"String: " << s << endl; return 0; }

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Use of dollar signs in variable names when using boost::archive::xml_iarchive
by lloyd.reed＠leica-microsystems.com 04 Feb '05

04 Feb '05

I have found a bug or undocumented limitation in the boost serialization library. I'm using VC++ 2003, which allows you to use variable names which include dollar signs ($). Variables with names containing dollar signs can be saved to an archive using the boost::archive::xml_oarchive class. When boost::archive::xml_iarchive is used to reload the variable, however, the parsing of the variable name fails, and basic_xml_iarchive<Archive>::load_start(const char *name) calls boost::throw_exception( archive_exception(archive_exception::stream_error) ). For example, std::string filename( "C:\\dollar.txt" ); int $(0); { std::ofstream archive_file( filename.c_str() ); if( archive_file.good() ) { boost::archive::xml_oarchive output_archive(archive_file); output_archive << BOOST_SERIALIZATION_NVP( $ ); } } produces the following file: <?xml version="1.0" encoding="UTF-8" standalone="yes" ?> <!DOCTYPE boost_serialization> <boost_serialization signature="serialization::archive" version="3"> <$>0</$> </boost_serialization> but trying to load the file causes a boost::archive::archive_exception to be thrown: { std::ifstream archive_file( filename.c_str() ); if( archive_file.good() ) { boost::archive::xml_iarchive input_archive(archive_file); input_archive >> BOOST_SERIALIZATION_NVP( $ ); // throws } } I think the spirit definitions for the names of variables are in C:\boost_1_32_0\libs\serialization\src\basic_xml_grammar.ipp and C:\boost_1_32_0\libs\serialization\src\xml_grammar.cpp. Name = (Letter | '_' | ':') >> *(NameChar); and template<> void xml_grammar::init_chset(){ Char = chset_t("\x9\xA\xD\x20-\xFF"); Letter = chset_t("\x41-\x5A\x61-\x7A\xC0-\xD6\xD8-\xF6\xF8-\xFF"); Digit = chset_t("0-9"); Extender = chset_t('\xB7'); Sch = chset_t("\x20\x9\xD\xA"); NameChar = Letter | Digit | chset_p("._:-") | Extender ; } ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________

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shared_ptr on HP-UX - patch
by Jaroslav Gresula 04 Feb '05

04 Feb '05

I tried to compile the smart pointer regression tests on HP-UX and found out that the compiler takes the 'advanced' version of get_deleter() template function (defined in shared_ptr.hpp) which is evidently beyond its limits (we use aCC, version HP ANSI C++ B3910B A.03.35). In case anyone is interested, I attached a 'patch' against shared_ptr.hpp, revision 1.52 - it just redirects the compiler to the already existing workaround version. /Jarda *** shared_ptr.hpp.orig Wed Feb 2 22:12:50 2005 --- shared_ptr.hpp Wed Feb 2 21:49:23 2005 *************** *** 442,448 **** // get_deleter (experimental) ! #if (defined(__GNUC__) && (__GNUC__ < 3)) || (defined(__EDG_VERSION__) && (__EDG_VERSION__ <= 238)) // g++ 2.9x doesn't allow static_cast<X const *>(void *) // apparently EDG 2.38 also doesn't accept it --- 442,450 ---- // get_deleter (experimental) ! #if (defined(__GNUC__) && (__GNUC__ < 3)) ||\ ! (defined(__EDG_VERSION__) && (__EDG_VERSION__ <= 238)) ||\ ! (defined(__HP_aCC) && (__HP_aCC <=33500)) // g++ 2.9x doesn't allow static_cast<X const *>(void *) // apparently EDG 2.38 also doesn't accept it

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weak_ptr_test - non-portable?
by Jaroslav Gresula 04 Feb '05

04 Feb '05

When I run the weak_ptr_test (a part of the regression tests) on HP-UX (aCC version HP ANSI C++ B3910B A.03.35) I'm getting a lot of failures. The reason is that IMHO the weak_ptr_test.cpp relies on a non-portable construct. There is copy_assignment() template function which takes shared_ptr<> by value. All caller sites of this function pass on an unnamed shared_ptr<> temporary. The first line of copy_assignment() checks if shared_ptr<>::unique() is true for the passed object. This test succeeds for all compilers we use in our shop but for the aCC. The reason is that upon entering copy_assignment() shared_ptr::use_count() is equal 2 as the compiler creates a temporary on the caller site and copy-ctor is used to pass on the object to copy_assignment() which seems to be in concert with the C++ standard [12.2]. Is my observation correct? Thanks, Jarda

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Installation log
by Nicolas Busca 04 Feb '05

04 Feb '05

Hi! Is there any way to see the installation log of boost? When I installed it I got a message in the end that ~50 out of ~1000 links failed to be created. How could I know which they were? Thanks, Nicolas Busca.

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BGL + SubGraph + Remove
by Gordon Smith 04 Feb '05

04 Feb '05

Is there anyway to remove a subgraph once created? Gordon.

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