I am currently a real beginner in Objective C, and I am developing an app using the framework OpenCV. I am following a tutorial, but while builing the project, I got several errors : Undefined symbols for architecture i386: "std::__1::basic_ostream<char, std::__1::char_traits<char> >::flush()", referenced from: cv::gpu::error(char const*, char const*, int, char const*) in opencv2(gpumat.o) "std::_