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• Why an auto_ptr can "seal" a container

  - by icephere

  auto_ptr on wikipedia said that "an auto_ptr containing an STL container may be used to prevent further modification of the container.". It used the following example: auto_ptr<vector<ContainedType> > open_vec(new vector<ContainedType>); open_vec->push_back(5); open_vec->push_back(3); // Transfers control, but now the vector cannot be changed: auto_ptr<const vector<ContainedType> > closed_vec(open_vec); // closed_vec->push_back(8); // Can no longer modify If I uncomment the last line, g++ will report an error as t05.cpp:24: error: passing ‘const std::vector<int, std::allocator<int> >’ as ‘this’ argument of ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = int, _Alloc = std::allocator<int>]’ discards qualifiers I am curious why after transferring the ownership of this vector, it can no longer be modified? Thanks a lot!

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• Multithreaded IOCP Client Issue

  - by Carl

  I am writing a multithreaded client that uses an IO Completion Port. I create and connect the socket that has the WSA_FLAG_OVERLAPPED attribute set. if ((m_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == INVALID_SOCKET) { throw std::exception("Failed to create socket."); } if (WSAConnectByName(m_socket, L"server.com", L"80", &localAddressLength, reinterpret_cast<sockaddr*>(&localAddress), &remoteAddressLength, &remoteAddress, NULL, NULL) == FALSE) { throw std::exception("Failed to connect."); } I associate the IO Completion Port with the socket. if ((m_hIOCP = CreateIoCompletionPort(reinterpret_cast<HANDLE>(m_socket), m_hIOCP, NULL, 8)) == NULL) { throw std::exception("Failed to create IOCP object."); } All appears to go well until I try to send some data over the socket. SocketData* socketData = new SocketData; socketData->hEvent = 0; DWORD bytesSent = 0; if (WSASend(m_socket, socketData->SetBuffer(socketData->GenerateLoginRequestHeader()), 1, &bytesSent, NULL, reinterpret_cast<OVERLAPPED*>(socketData), NULL) == SOCKET_ERROR && WSAGetLastError() != WSA_IO_PENDING) { throw std::exception("Failed to send data."); } Instead of returning SOCKET_ERROR with the last error set to WSA_IO_PENDING, WSASend returns immediately. I need the IO to pend and for it's completion to be handled in my thread function which is also my worker thread. unsigned int __stdcall MyClass::WorkerThread(void* lpThis) { } I've done this before but I don't know what is going wrong in this case, I'd greatly appreciate any efforts in helping me fix this problem.

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• How to determine the data type of a CvMat

  - by Chris

  When using the CvMat type, the type of data is crucial to keeping your program running. For example, depending on whether your data is type float or unsigned char, you would choose one of these two commands: cvmGet(mat, row, col); cvGetReal2D(mat, row, col); Is there a universal approach to this? If the wrong data type matrix is passed to these calls, they crash at runtime. This is becoming an issue, since a function I have defined is getting passed several different types of matrices. How do you determine the data type of a matrix so you can always access its data? I tried using the "type()" function as such. CvMat* tmp_ptr = cvCreateMat(t_height,t_width,CV_8U); std::cout << "type = " << tmp_ptr->type() << std::endl; This does not compile, saying "term does not evaluate to a function taking 0 arguments". If I remove the brackets after the word type, I get a type of 1111638032 EDIT minimal application that reproduces this... int main( int argc, char** argv ) { CvMat *tmp2 = cvCreateMat(10,10, CV_32FC1); std::cout << "tmp2 type = " << tmp2->type << " and CV_32FC1 = " << CV_32FC1 << " and " << (tmp2->type == CV_32FC1) << std::endl; } Output: tmp2 type = 1111638021 and CV_32FC1 = 5 and 0

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• C++ -- Is there an implicit cast here from Fred* to auto_ptr<Fred>?

  - by q0987

  Hello all, I saw the following code, #include <new> #include <memory> using namespace std; class Fred; // Forward declaration typedef auto_ptr<Fred> FredPtr; class Fred { public: static FredPtr create(int i) { return new Fred(i); // Is there an implicit casting here? If not, how can we return // a Fred* with return value as FredPtr? } private: Fred(int i=10) : i_(i) { } Fred(const Fred& x) : i_(x.i_) { } int i_; }; Please see the question listed in function create. Thank you // Updated based on comments Yes, the code cannot pass the VC8.0 error C2664: 'std::auto_ptr<_Ty::auto_ptr(std::auto_ptr<_Ty &) throw()' : cannot convert parameter 1 from 'Fred *' to 'std::auto_ptr<_Ty &' The code was copied from the C++ FAQ 12.15. However, after making the following changes, replace return new Fred(i); with return auto_ptr<Fred>(new Fred(i)); This code can pass the VC8.0 compiler. But I am not sure whether or not this is a correct fix.

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• strange redefined symbols

  - by Chris H

  I included this header into one of my own: http://codepad.org/lgJ6KM6b When I compiled I started getting errors like this: CMakeFiles/bin.dir/SoundProjection.cc.o: In function `Gnuplot::reset_plot()': /usr/lib/gcc/x86_64-pc-linux-gnu/4.3.4/include/g++-v4/new:105: multiple definition of `Gnuplot::reset_plot()' CMakeFiles/bin.dir/main.cc.o:project/gnuplot-cpp/gnuplot_i.hpp:962: first defined here CMakeFiles/bin.dir/SoundProjection.cc.o: In function `Gnuplot::set_smooth(std::basic_string, std::allocator const&)': project/gnuplot-cpp/gnuplot_i.hpp:1041: multiple definition of `Gnuplot::set_smooth(std::basic_string, std::allocator const&)' CMakeFiles/bin.dir/main.cc.o:project/gnuplot-cpp/gnuplot_i.hpp:1041: first defined here CMakeFiles/bin.dir/SoundProjection.cc.o:/usr/include/eigen2/Eigen/src/Core/arch/SSE/PacketMath.h:41: multiple definition of `Gnuplot::m_sGNUPlotFileName' I know it's hard to see in this mess, but look at where the redefinitions are taking place. They take place in files like /usr/lib/gcc/x86_64-pc-linux-gnu/4.3.4/include/g++-v4/new:105. How is the new operator getting information about a gnuplot header? I can't even edit that file. How could that ever even be possible? I'm not even sure how to start debugging this. I hope I've provided enough information. I wasn't able to reproduce this in a small project. I mostly just looking for tips on how to find out why this is happening, and how to track it down. Thanks.

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• Setting Position of source and listener has no effect

  - by Ben E

  Hi Guys, First time i've worked with OpenAL, and for the life of my i can't figure out why setting the position of the source doesn't have any effect on the sound. The sounds are in stero format, i've made sure i set the listener position, the sound is not realtive to the listener and OpenAL isn't giving out any error. Can anyone shed some light? Create Audio device ALenum result; mDevice = alcOpenDevice(NULL); if((result = alGetError()) != AL_NO_ERROR) { std::cerr << "Failed to create Device. " << GetALError(result) << std::endl; return; } mContext = alcCreateContext(mDevice, NULL); if((result = alGetError()) != AL_NO_ERROR) { std::cerr << "Failed to create Context. " << GetALError(result) << std::endl; return; } alcMakeContextCurrent(mContext); SoundListener::SetListenerPosition(0.0f, 0.0f, 0.0f); SoundListener::SetListenerOrientation(0.0f, 0.0f, -1.0f); The two listener functions call alListener3f(AL_POSITION, x, y, z); Real vec[6] = {x, y, z, 0.0f, 1.0f, 0.0f}; alListenerfv(AL_ORIENTATION, vec); I set the sources position to 1,0,0 which should be to the right of the listener but it has no effect alSource3f(mSourceHandle, AL_POSITION, x, y, z); Any guidance would be much appreciated

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• strict aliasing and alignment

  - by cooky451

  I need a safe way to alias between arbitrary POD types, conforming to ISO-C++11 explicitly considering 3.10/10 and 3.11 of n3242 or later. There are a lot of questions about strict aliasing here, most of them regarding C and not C++. I found a "solution" for C which uses unions, probably using this section union type that includes one of the aforementioned types among its elements or nonstatic data members From that I built this. #include <iostream> template <typename T, typename U> T& access_as(U* p) { union dummy_union { U dummy; T destination; }; dummy_union* u = (dummy_union*)p; return u->destination; } struct test { short s; int i; }; int main() { int buf[2]; static_assert(sizeof(buf) >= sizeof(double), ""); static_assert(sizeof(buf) >= sizeof(test), ""); access_as<double>(buf) = 42.1337; std::cout << access_as<double>(buf) << '\n'; access_as<test>(buf).s = 42; access_as<test>(buf).i = 1234; std::cout << access_as<test>(buf).s << '\n'; std::cout << access_as<test>(buf).i << '\n'; } My question is, just to be sure, is this program legal according to the standard?* It doesn't give any warnings whatsoever and works fine when compiling with MinGW/GCC 4.6.2 using: g++ -std=c++0x -Wall -Wextra -O3 -fstrict-aliasing -o alias.exe alias.cpp * Edit: And if not, how could one modify this to be legal?

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• Applying policy based design question

  - by Arthur

  I've not read the Modern C++ Design book but have found the idea of behavior injection through templates interesting. I am now trying to apply it myself. I have a class that has a logger that I thought could be injected as a policy. The logger has a log() method which takes an std::string or std::wstring depending on its policy: // basic_logger.hpp template<class String> class basic_logger { public: typedef String string_type; void log(const string_type & s) { ... } }; typedef basic_logger<std::string> logger; typedef basic_logger<std::wstring> wlogger; // reader.hpp template<class Logger = logger> class reader { public: typedef Logger logger_type; void read() { _logger.log("Reading..."); } private: logger_type _logger; }; Now the questing is, should the reader take a Logger as an argument, like above, or should it take a String and then instantiate a basic_logger as an instance variable? Like so: template<class String> class reader { public: typedef String string_type; typedef basic_logger<string_type> logger_type; // ... private: logger_type _logger; }; What is the right way to go?

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• Why boost property tree write_json saves everything as string? Is it possible to change that?

  - by pprzemek

  I'm trying to serialize using boost property tree write_json, it saves everything as strings, it's not that data are wrong, but I need to cast them explicitly every time and I want to use them somewhere else. (like in python or other C++ json (non boost) library) here is some sample code and what I get depending on locale: boost::property_tree::ptree root, arr, elem1, elem2; elem1.put<int>("key0", 0); elem1.put<bool>("key1", true); elem2.put<float>("key2", 2.2f); elem2.put<double>("key3", 3.3); arr.push_back( std::make_pair("", elem1) ); arr.push_back( std::make_pair("", elem2) ); root.put_child("path1.path2", arr); std::stringstream ss; write_json(ss, root); std::string my_string_to_send_somewhare_else = ss.str(); and my_string_to_send_somewhere_else is sth. like this: { "path1" : { "path2" : [ { "key0" : "0", "key1" : "true" }, { "key2" : "2.2", "key3" : "3.3" } ] } } Is there anyway to save them as the values, like: "key1" : true or "key2" : 2.2 ?

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• How to get at contents of placeholder::_1

  - by sheepsimulator

  I currently have the following code: using boost::bind; typedef boost::signal<void(EventDataItem&)> EventDataItemSignal; class EventDataItem { ... EventDataItemSignal OnTrigger; ... } typedef std::list< shared_ptr<EventDataItem> > DataItemList; typedef std::list<boost::signals::connection> ConnectionList; class MyClass { void OnStart() { DataItemList dilItems; ConnectionList clConns; DataItemList::iterator iterDataItems; for(iterDataItems = dilItems.begin(); iterDataItems != dilItems.end(); iterDataItems++) { // Create Connections from Triggers clConns.push_back((*iterDataItems)->OnTrigger.connect( bind(&MyClass::OnEventTrigger, this))); } } void OnEventTrigger() { // ... Do stuff on Trigger... } } I'd like to change MyClass::OnStart to use std::transform to achieve the same thing: void MyClass::OnStart() { DataItemList dilItems; ConnectionList clConns; // Resize connection list to match number of data items clConns.resize(dilItems.size()); // Build connection list from Items // note: errors on the placeholder _1->OnTrigger std::transform(dilItems.begin(), dilItems.end(), clConns.begin(), bind(&EventDataItemSignal::connect, _1->OnTrigger, bind(&MyClass::Stuff, this))); } However, my hiccup is _1-OnTrigger. How can I reference OnTrigger from placeholder _1?

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• Why do pure virtual base classes get direct access to static data members while derived instances do

  - by Shamster

  I've created a simple pair of classes. One is pure virtual with a static data member, and the other is derived from the base, as follows: #include <iostream> template <class T> class Base { public: Base (const T _member) { member = _member; } static T member; virtual void Print () const = 0; }; template <class T> T Base<T>::member; template <class T> void Base<T>::Print () const { std::cout << "Base: " << member << std::endl; } template <class T> class Derived : public Base<T> { public: Derived (const T _member) : Base<T>(_member) { } virtual void Print () const { std::cout << "Derived: " << this->member << std::endl; } }; I've found from this relationship that when I need access to the static data member in the base class, I can call it with direct access as if it were a regular, non-static class member. i.e. - the Base::Print() method does not require a this- modifier. However, the derived class does require the this-member indirect access syntax. I don't understand why this is. Both class methods are accessing the same static data, so why does the derived class need further specification? A simple call to test it is: int main () { Derived<double> dd (7.0); dd.Print(); return 0; } which prints the expected "Derived: 7"

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• Temporary non-const istream reference in constructor (C++)

  - by Christopher Bruns

  It seems that a constructor that takes a non-const reference to an istream cannot be constructed with a temporary value in C++. #include <iostream> #include <sstream> using namespace std; class Bar { public: explicit Bar(std::istream& is) {} }; int main() { istringstream stream1("bar1"); Bar bar1(stream1); // OK on all platforms // compile error on linux, Mac gcc; OK on Windows MSVC Bar bar2(istringstream("bar2")); return 0; } This compiles fine with MSVC, but not with gcc. Using gcc I get a compile error: g++ test.cpp -o test test.cpp: In function ‘int main()’: test.cpp:18: error: no matching function for call to ‘Bar::Bar(std::istringstream)’ test.cpp:9: note: candidates are: Bar::Bar(std::istream&) test.cpp:7: note: Bar::Bar(const Bar&) Is there something philosophically wrong with the second way (bar2) of constructing a Bar object? It looks nicer to me, and does not require that stream1 variable that is only needed for a moment.

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• Is extending a base class with non-virtual destructor dangerous in C++

  - by Akusete

  Take the following code class A { }; class B : public A { }; class C : public A { int x; }; int main (int argc, char** argv) { A* b = new B(); A* c = new C(); //in both cases, only ~A() is called, not ~B() or ~C() delete b; //is this ok? delete c; //does this line leak memory? return 0; } when calling delete on a class with a non-virtual destructor with member functions (like class C), can the memory allocator tell what the proper size of the object is? If not, is memory leaked? Secondly, if the class has no member functions, and no explicit destructor behaviour (like class B), is everything ok? I ask this because I wanted to create a class to extend std::string, (which I know is not recommended, but for the sake of the discussion just bear with it), and overload the +=,+ operator. -Weffc++ gives me a warning because std::string has a non virtual destructor, but does it matter if the sub-class has no members and does not need to do anything in its destructor? -- FYI the += overload was to do proper file path formatting, so the path class could be used like class path : public std::string { //... overload, +=, + //... add last_path_component, remove_path_component, ext, etc... }; path foo = "/some/file/path"; foo = foo + "filename.txt"; //and so on... I just wanted to make sure someone doing this path* foo = new path(); std::string* bar = foo; delete bar; would not cause any problems with memory allocation

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• In C++, what is the scope resolution ("order of precedence") for shadowed variable names?

  - by Emile Cormier

  In C++, what is the scope resolution ("order of precedence") for shadowed variable names? I can't seem to find a concise answer online. For example: #include <iostream> int shadowed = 1; struct Foo { Foo() : shadowed(2) {} void bar(int shadowed = 3) { std::cout << shadowed << std::endl; // What does this output? { int shadowed = 4; std::cout << shadowed << std::endl; // What does this output? } } int shadowed; }; int main() { Foo().bar(); } I can't think of any other scopes where a variable might conflict. Please let me know if I missed one. What is the order of priority for all four shadow variables when inside the bar member function?

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• Crash generated during destruction of hash_map

  - by Alien01

  I am using hash_map in application as typedef hash_map<DWORD,CComPtr<IInterfaceXX>> MapDword2Interface; In main application I am using static instance of this map static MapDword2Interface m_mapDword2Interface; I have got one crash dump from one of the client machines which point to the crash in clearing this map I opened that crash dump and here is assembly during debugging > call std::list<std::pair<unsigned long const ,ATL::CComPtr<IInterfaceXX> >,std::allocator<std::pair<unsigned long const ,ATL::CComPtr<IInterfaceXX> > > >::clear > mov eax,dword ptr [CMainApp::m_mapDword2Interface+8 (49XXXXX)] Here is code where crash dump is pointing. Below code is from stl:list file void clear() { // erase all #if _HAS_ITERATOR_DEBUGGING this->_Orphan_ptr(*this, 0); #endif /* _HAS_ITERATOR_DEBUGGING */ _Nodeptr _Pnext; _Nodeptr _Pnode = _Nextnode(_Myhead); _Nextnode(_Myhead) = _Myhead; _Prevnode(_Myhead) = _Myhead; _Mysize = 0; for (; _Pnode != _Myhead; _Pnode = _Pnext) { // delete an element _Pnext = _Nextnode(_Pnode); this->_Alnod.destroy(_Pnode); this->_Alnod.deallocate(_Pnode, 1); } } Crash is pointing to the this->_Alnod.destroy(_Pnode); statement in above code. I am not able to guess it, what could be reason. Any ideas??? How can I make sure, even is there is something wrong with the map , it should not crash?

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• c++ Mixing printf with wprintf (or cout with wcout)

  - by Bo Jensen

  I know you should not mix printing with printf,cout and wprintf,wcout, but have a hard time finding a good answer why and if it is possible to get round it. The problem is I use a external library that prints with printf and my own uses wcout. If I do a simple example it works fine, but from my full application it simply does not print the printf statements. If this is really a limitation, then there would be many libraries out there which can not work together with wide printing applications. Any insight on this is more than welcome. Update : I boiled it down to : #include <stdio.h> #include <stdlib.h> #include <iostream> #include <readline/readline.h> #include <readline/history.h> int main() { char *buf; std::wcout << std::endl; /* ADDING THIS LINE MAKES PRINTF VANISH!!! */ rl_bind_key('\t',rl_abort);//disable auto-complete while((buf = readline("my-command : "))!=NULL) { if (strcmp(buf,"quit")==0) break; std::wcout<<buf<< std::endl; if (buf[0]!=0) add_history(buf); } free(buf); return 0; } So I guess it might be a flushing problem, but it still looks strange to me, I have to check up on it.

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• Your favourite C++ Standard Library wrapper functions?

  - by Neil Butterworth

  This question, asked this morning, made me wonder which features you think are missing from the C++ Standard Library, and how you have gone about filling the gaps with wrapper functions. For example, my own utility library has this function for vector append: template <class T> std::vector<T> & operator += ( std::vector<T> & v1, const std::vector <T> v2 ) { v1.insert( v1.end(), v2.begin(), v2.end() ); return v1; } and this one for clearing (more or less) any type - particularly useful for things like std::stack: template <class C> void Clear( C & c ) { c = C(); } I have a few more, but I'm interested in which ones you use? Please limit answers to wrapper functions - i.e. no more than a couple of lines of code.

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• C++0x rvalue references and temporaries

  - by Doug

  (I asked a variation of this question on comp.std.c++ but didn't get an answer.) Why does the call to f(arg) in this code call the const ref overload of f? void f(const std::string &); //less efficient void f(std::string &&); //more efficient void g(const char * arg) { f(arg); } My intuition says that the f(string &&) overload should be chosen, because arg needs to be converted to a temporary no matter what, and the temporary matches the rvalue reference better than the lvalue reference. This is not what happens in GCC and MSVC. In at least G++ and MSVC, any lvalue does not bind to an rvalue reference argument, even if there is an intermediate temporary created. Indeed, if the const ref overload isn't present, the compilers diagnose an error. However, writing f(arg + 0) or f(std::string(arg)) does choose the rvalue reference overload as you would expect. From my reading of the C++0x standard, it seems like the implicit conversion of a const char * to a string should be considered when considering if f(string &&) is viable, just as when passing a const lvalue ref arguments. Section 13.3 (overload resolution) doesn't differentiate between rvalue refs and const references in too many places. Also, it seems that the rule that prevents lvalues from binding to rvalue references (13.3.3.1.4/3) shouldn't apply if there's an intermediate temporary - after all, it's perfectly safe to move from the temporary. Is this: Me misreading/misunderstand the standard, where the implemented behavior is the intended behavior, and there's some good reason why my example should behave the way it does? A mistake that the compiler vendors have somehow all made? Or a mistake based on common implementation strategies? Or a mistake in e.g. GCC (where this lvalue/rvalue reference binding rule was first implemented), that was copied by other vendors? A defect in the standard, or an unintended consequence, or something that should be clarified?

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• What is different about C++ math.h abs() compared to my abs()

  - by moka

  I am currently writing some glsl like vector math classes in c++, and I just implemented an abs() function like this: template<class T> static inline T abs(T _a) { return _a < 0 ? -_a : _a; } I compared its speed to the default c++ abs from math.h like this: clock_t begin = clock(); for(int i=0; i<10000000; ++i) { float a = abs(-1.25); }; clock_t end = clock(); unsigned long time1 = (unsigned long)((float)(end-begin) / ((float)CLOCKS_PER_SEC/1000.0)); begin = clock(); for(int i=0; i<10000000; ++i) { float a = myMath::abs(-1.25); }; end = clock(); unsigned long time2 = (unsigned long)((float)(end-begin) / ((float)CLOCKS_PER_SEC/1000.0)); std::cout<<time1<<std::endl; std::cout<<time2<<std::endl; Now the default abs takes about 25ms while mine takes 60. I guess there is some low level optimisation going on. Does anybody know how math.h abs works internally? The performance difference is nothing dramatic, but I am just curious!

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• Problem in Building mplsh-run in lshkit

  - by Yijinsei

  Hi guy, been trying out this for quite some time but I'm still unable to built mplsh-run from lshkit Not sure if this would help to explain my situation during the building process /tmp/cc17kth4.o: In function `lshkit::MultiProbeLshRecallTable::reset(lshkit::MultiProbeLshModel, unsigned int, double, double)': mplsh-run.cpp:(.text._ZN6lshkit24MultiProbeLshRecallTable5resetENS_18MultiProbeLshModelEjdd[lshkit::MultiProbeLshRecallTable::reset(lshkit::MultiProbeLshModel, unsigned int, double, double)]+0x230): undefined reference to `lshkit::MultiProbeLshModel::recall(double) const' /tmp/cc17kth4.o: In function `void lshkit::MultiProbeLshIndex<unsigned int>::query_recall<lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> > >(float const*, float, lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> >&) const': mplsh-run.cpp:(.text._ZNK6lshkit18MultiProbeLshIndexIjE12query_recallINS_11TopkScannerINS_6MatrixIfE8AccessorENS_6metric5l2sqrIfEEEEEEvPKffRT_[void lshkit::MultiProbeLshIndex<unsigned int>::query_recall<lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> > >(float const*, float, lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> >&) const]+0x2c4): undefined reference to `lshkit::MultiProbeLsh::genProbeSequence(float const*, std::vector<unsigned int, std::allocator<unsigned int> >&, unsigned int) const' /tmp/cc17kth4.o: In function `void lshkit::MultiProbeLshIndex<unsigned int>::query<lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> > >(float const*, unsigned int, lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> >&)': mplsh-run.cpp:(.text._ZN6lshkit18MultiProbeLshIndexIjE5queryINS_11TopkScannerINS_6MatrixIfE8AccessorENS_6metric5l2sqrIfEEEEEEvPKfjRT_[void lshkit::MultiProbeLshIndex<unsigned int>::query<lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> > >(float const*, unsigned int, lshkit::TopkScanner<lshkit::Matrix<float>::Accessor, lshkit::metric::l2sqr<float> >&)]+0x4a): undefined reference to `lshkit::MultiProbeLsh::genProbeSequence(float const*, std::vector<unsigned int, std::allocator<unsigned int> >&, unsigned int) const' collect2: ld returned 1 exit status the command that i used to built mplsh-run is g++ -I./lshkit/include -L/usr/lib -lm -lgsl -lgslcblas -lboost_program_options-mt mplsh-run.cpp Do you guys have any clue on how I could solve this?

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• is back_insert_iterator<> safe to be passed by value?

  - by afriza

  I have a code that looks something like: struct Data { int value; }; class A { public: typedef std::deque<boost::shared_ptr<Data> > TList; std::back_insert_iterator<TList> GetInserter() { return std::back_inserter(m_List); } private: TList m_List; }; class AA { boost::scoped_ptr<A> m_a; public: AA() : m_a(new A()) {} std::back_insert_iterator<A::TList> GetDataInserter() { return m_a->GetInserter(); } }; class B { template<class OutIt> CopyInterestingDataTo(OutIt outIt) { // loop and check conditions for interesting data // for every `it` in a Container<Data*> // create a copy and store it for( ... it = ..; .. ; ..) if (...) { *outIt = OutIt::container_type::value_type(new Data(**it)); outIt++; // dummy } } void func() { AA aa; CopyInterestingDataTo(aa.GetInserter()); // aa.m_a->m_List is empty! } }; The problem is that A::m_List is always empty even after CopyInterestingDataTo() is called. However, if I debug and step into CopyInterestingDataTo(), the iterator does store the supposedly inserted data!

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• How to deal with Unicode strings in C/C++ in a cross-platform friendly way?

  - by Sorin Sbarnea

  On platforms different than Windows you could easily use char * strings and treat them as UTF-8. The problem is that on Windows you are required to accept and send messages using wchar* strings (W). If you'll use the ANSI functions (A) you will not support Unicode. So if you want to write truly portable application you need to compile it as Unicode on Windows. Now, In order to keep the code clean I would like to see what is the recommended way of dealing with strings, a way that minimize ugliness in the code. Type of strings you may need: std::string, std::wstring, std::tstring,char *,wchat_t *, TCHAR*, CString (ATL one). Issues you may encounter: cout/cerr/cin and their Unicode variants wcout,wcerr,wcin all renamed wide string functions and their TCHAR macros - like strcmp, wcscmp and _tcscmp. constant strings inside code, with TCHAR you will have to fill your code with _T() macros. What approach do you see as being best? (examples are welcome) Personally I would go for a std::tstring approach but I would like to see how would do to the conversions where they are necessary.

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• Returning c_str from a function

  - by user199421

  This is from a small library that I found online: const char* GetHandStateBrief(const PostFlopState* state) { static std::ostringstream out; ... rest of the function ... return out.str().c_str() Now in my code I am doing this: const char *d = GetHandStateBrief(&post); std::cout<< d << std::endl; Now, at first d contained garbage. I then realized that the c string I am getting from the function is destroyed when the function returns because std::ostringstream is allocated on the stack. So I added: return strdup( out.str().c_str()); And now I can get the text I need from the function. I have two questions: 1) Am I understanding this correctly? 2) I later noticed that the ostringstream was was allocated with static storage. Doesn't that mean that the object is supposed to stay in memory until the program terminates? and if so , then why can't I access the string?

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• Problem in printing array of char pointer passing from Python

  - by Yan Cheng CHEOK

  My following C code works quite well, till my Python code trying to pass an array of char pointer to it. The output I obtain is The file_name is python-file Another 3 string is not being printed out. Anything I had missed out? C Code #include <iostream> #include "c_interface.h" int foo(const char* file_name, const char** names) { std::cout << "The file_name is " << file_name << std::endl; while (*names) { std::cout << "The name is " << *names << std::endl; names++; } return 0; } /* int main() { const char *c[] = {"123gh", "456443432", "789", 0}; foo("hello", c); getchar(); } */ Python Code #!c:/Python27/python.exe -u from ctypes import * name0 = "NAME0" name1 = "NAME1" name2 = "NAME2" names = ((c_char_p * 1024) * 4)() names[0].value = name0 names[1].value = name1 names[2].value = name2 names[3].value = 0 libc = CDLL("foo.dll") libc.foo("python-file", names)

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• Artifacts when trying to draw background grid without anti-aliasing in a QGraphicsScene

  - by estan

  Hi folks, I'm trying to draw a background grid in the drawBackground() function of my QGraphicsScene subclass: void Scene::drawBackground(QPainter *painter, const QRectF &rect) { const int gridSize = 50; const int realLeft = static_cast<int>(std::floor(rect.left())); const int realRight = static_cast<int>(std::ceil(rect.right())); const int realTop = static_cast<int>(std::floor(rect.top())); const int realBottom = static_cast<int>(std::ceil(rect.bottom())); // Draw grid. const int firstLeftGridLine = realLeft - (realLeft % gridSize); const int firstTopGridLine = realTop - (realTop % gridSize); QVarLengthArray<QLine, 100> lines; for (qreal x = firstLeftGridLine; x <= realRight; x += gridSize) lines.append(QLine(x, realTop, x, realBottom)); for (qreal y = firstTopGridLine; y <= realBottom; y += gridSize) lines.append(QLine(realLeft, y, realRight, y)); //painter->setRenderHint(QPainter::Antialiasing); painter->setPen(QPen(QColor(220, 220, 220), 0.0)); painter->drawLines(lines.data(), lines.size()); // Draw axes. painter->setPen(QPen(Qt::lightGray, 0.0)); painter->drawLine(0, realTop, 0, realBottom); painter->drawLine(realLeft, 0, realRight, 0); } However, unless I turn on anti-aliasing, moving items around will sometimes leave artifacts in the grid (areas where it's not drawn). It seems it mostly happens at low zoom levels, when the view is zoomed out a bit. Any ideas what I might be doing wrong here? I'd really don't want to turn anti-aliasing on since the lines are strictly horizontal and vertical, and I'd like them to be as crisp as possible. Any help is much appriciated, Regards, Elvis

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