February 2009 - Boost-users

Using SFINAE to detect presence of member function - Unreliable
by Naik, Roshan 03 Feb '09

03 Feb '09

I have taken a look at anything I could find through Boost lists and Google that came up as a solution to the problem of detecting the existence of a member function in a type... using SFINAE. All of them have one problem.. they are unable to detect the existence of the member function if it was defined in a base class. They can only detect if the member function was directly defined in the class being tested. At their heart, all solutions boil down to some variation of the following template class with a template parameter that is a (non-type) pointer to member function: template<class T, void (T::*)(void) > struct Tester; Then we try to SFINAE on it by the usual sizeof trick. However the problem is that if method 'void foo(int)' is defined in a Base class. Then the type of &Derived::foo is actually void (Base::*)(int) ... and not void (Derived::*)(int) Although during argument passing the conversion of the Base member function pointer to Derived member function pointer is acceptable to C++, it will NOT do such conversion when instantiating templates. So trying to use Tester<Derived, &Derived::foo> will always fail us if 'foo' is defined in Base. So .. my question is ....does anyone have solution that works for the case when the method is defined in the base class too ? I am thinking ... if it is possible to extract the class name from the actual type of &Derived::foo, then the problem can be solved... but the type extraction mechanism must not error out on types that have absolutley no member function called foo. I know type erasure (which i prefer to call External polymorhism) can solve this issue but SFINAE is more efficient at runtime. -Roshan

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[asio] How to get the name of the endpoint from the ip address of the endpoint.
by Andrew Maclean 03 Feb '09

03 Feb '09

If I have an endpoint as an ip address (IP V4 or IPv6), how can I do a DNS lookup to get the name of the endpoint? In the example below, I understand that the host_name is coming from the query. But how do I do a lookup to get the host name from the IP? This may help. If I do something like this: ip::tcp::resolver_iterator res = ip::tcp::resolver(ios).resolve(ip::tcp::resolver_query("google.com", "80")); for(;res != ip::tcp::resolver_iterator(); ++res) { std::cout << res->host_name() << ":" << res->service_name() << " "; std::cout << res->endpoint().address().to_string() << std::endl; } The the output is: google.com:80 209.85.171.100 google.com:80 74.125.67.100 google.com:80 74.125.45.100 However if I do: ip::tcp::resolver_iterator res = ip::tcp::resolver(ios).resolve(ip::tcp::resolver_query("209.85.171.100", "80")); I get: 209.85.171.100:80 209.85.171.100 so th ehosr name comes from the query. I really want: google.com:80 209.85.171.100 Thanks in advance for any help. Andrew -- ___________________________________________ Andrew J. P. Maclean Centre for Autonomous Systems The Rose Street Building J04 The University of Sydney 2006 NSW AUSTRALIA Ph: +61 2 9351 3283 Fax: +61 2 9351 7474 URL: http://www.acfr.usyd.edu.au/ ___________________________________________

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Question in usage of boost::bind
by Archie14 03 Feb '09

03 Feb '09

Below is an example that fails to compile. I have a vector of pointers to a templated class and I need to use find_if. My understanding of boost::bind. is that the first argument, when binding member function, is an explicit "this" pointer, that should be A* in my case. Any help in explaining what I am doing wrong will be greatly appreciated. #include <algorithm> #include <string> #include <vector> #include <boost/bind.hpp> #include <algorithm> #include <string> #include <vector> #include <boost/bind.hpp> template<typename T> class A { T _val; public: A(T val) : _val(val){}; T Val() {return _val;} bool CompareByVal(A<T>* a, int iType, T ofval) { return ( (a != NULL) && (iType == 1) && (a->Val() == ofval) ); } }; int _tmain(int argc, _TCHAR* argv[]) { std::vector<A<int>*> a; std::find_if (a.begin(), a.end(), boost::bind(&A<int>::CompareByVal, _1, _2, 1,1)); }

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[Proto] proto::reverse_fold_tree
by Kim Kuen Tang 02 Feb '09

02 Feb '09

Hi all, i want to use the function proto::reverse_fold_tree to build a heterogeneous list from the expression "((a*b+c)*d+e)*f". The resulting sequence only contains the two terminals 'a' and b'. So i am a little bit confused. But using the function proto::fold_tree it produces correctly the sequence [f,e,d,c,b,a]. (Btw the two functions proto::fold_tree and proto::fold produce the same sequence [f,e,d,c,b,a], what is their difference? ) Here is the code: #include <iostream> #include <boost/proto/proto.hpp> #include <boost/proto/proto_typeof.hpp> #include <boost/fusion/container.hpp> #include <boost/fusion/algorithm/iteration.hpp> #include <boost/fusion/include/iteration.hpp> #include <boost/typeof/typeof.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; namespace fusion = boost::fusion; using proto::_; proto::terminal<char>::type const a = {'a'}; proto::terminal<char>::type const b = {'b'}; proto::terminal<char>::type const c = {'c'}; proto::terminal<char>::type const d = {'d'}; proto::terminal<char>::type const e = {'e'}; proto::terminal<char>::type const f = {'f'}; struct FoldToList : proto::or_< proto::when<proto::terminal<_>,fusion::cons<proto::_value, proto::_state>(proto::_value, proto::_state) > ,proto::when<proto::nary_expr<_,proto::vararg<_> >, proto::fold_tree<proto::_,fusion::nil(),FoldToList > > > {}; struct FoldToListReverse : proto::or_< proto::when<proto::terminal<_>,fusion::cons<proto::_value, proto::_state>(proto::_value, proto::_state) > ,proto::when<proto::nary_expr<_,proto::vararg<_> >, proto::reverse_fold_tree<proto::_,fusion::nil(),FoldToListReverse > > > {}; struct print { typedef void result_type; template<class T> result_type operator()(T const& t) const { std::cout<<t<<"\t"; } }; int main() { BOOST_PROTO_AUTO(expr, ((a*b+c)*d+e)*f ); proto::display_expr(expr); BOOST_MPL_ASSERT(( proto::matches<BOOST_TYPEOF(expr), FoldToList>)); BOOST_MPL_ASSERT(( proto::matches<BOOST_TYPEOF(expr), FoldToListReverse>)); BOOST_AUTO(sequence1,FoldToList()(expr)); fusion::for_each(sequence1,print()); std::cout << "\n--------------------\n"; BOOST_AUTO(sequence2,FoldToListReverse()(expr)); fusion::for_each(sequence2,print()); std::cout << "\n--------------------\n"; } Cheers Kim Tang

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[boost.filesystem] query
by Sean Farrow 02 Feb '09

02 Feb '09

Hi: I am using boost.filesystem to hold a path including filename. would the function stem() of the path object give me the path excluding the filename? If i have: c:\program files\boost\welcome.txt The output wanted is: c:\program files\ Any help apreciated. Sean.

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using boost.foreach with boost.array
by Sean Farrow 02 Feb '09

02 Feb '09

Hi: is it possible to use boost.foreach with boost.array? Cheers Sean.

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Generate automatically an enum inside a template regarding to parameter
by Hansi 02 Feb '09

02 Feb '09

Hi, Is there a way to generate automatically an enum inside an template. I have e.g. the following template: template<int nr> class Test { enum { R1, ... Rx // where x is equal to nr }; } Thanks for any hint! Regards Hansjörg

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Preprocessor to generate enum
by Hansi 02 Feb '09

02 Feb '09

Hi, what is the best way to generate the following enum with the preprocessor library (boost 1.34.1)? enum { R1=0, R2=1, ... }; Best regards Hansjörg

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Re: [Boost-users] Compiler Error With boost::bind and boost::signals
by KERSAM 02 Feb '09

02 Feb '09

I forgot: I am using the Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86 and Boost Version 1.35. KERSAM wrote: > > Hello. > > I have a compiler error when I try to use boost::bind and boost::signals. > Here is the code: > > -------------------- > #include<boost/bind.hpp> > #include<boost/signals.hpp> > > class MyBoostTest { > public: > void testFunction(int x, int y) {} > }; > > > void doConnect() { > boost::signal2<void, int, int> signal; > signal.connect(boost::bind<void>(&MyBoostTest::testFunction, new > MyBoostTest, _1)); > } > ----------------------------------- > > The compiler gives me the following error: > c:\3rdparty\boost\boost\bind.hpp(289) : error C2064: term does not > evaluate to a function taking 2 arguments > class does not define an 'operator()' or a user defined conversion > operator to a pointer-to-function or reference-to-function that takes > appropriate number of arguments > > What am I doing wrong? > > Regards > -- View this message in context: http://www.nabble.com/Compiler-Error-With-boost%3A%3Abind-and-boost%3A%3Asi… Sent from the Boost - Users mailing list archive at Nabble.com.

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std::wstring issue with Boost::FileSystem
by Sean Farrow 02 Feb '09

02 Feb '09

Hi: Whilst using boost::filesystem I'm getting the following issue: I've defines: std::wstring Process =ProcessToFind; //ProcessToFind is an input to the function I'm writing. std::wstring test =boost::filesystem::wpath(szModName).filename(); //szModName is obtained from a win32 api function. Process.Compare(test); //This line gives the following Error: 1>c:\work\access for everyone\dragonspeak\dragonspeak.cpp(45) : error C2039: 'Compare' : is not a member of 'std::basic_string<_Elem,_Traits,_Ax>' 1> with 1> [ 1> _Elem=wchar_t, 1> _Traits=std::char_traits<wchar_t>, 1> _Ax=std::allocator<wchar_t> 1> ] I'm using the trunk as of yesterday (feb 1) and visual studio 2005. Anyone any ideas what's going wrong, or what I've forgotten? Cheers Sean.

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