... already defined in ...lib

when you build visual c project, if you get some error message like Error 4 error LNK2005: "bool __cdecl std::operator==,class std::allocator >(class std::basic_string,class std::allocator > const &,char const *)" (??$?8DU?$char_traits@D@std@@V?$allocator@D@1@@std@@YA_NABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@0@PBD@Z) already defined in websocketpp.lib(uri.obj) msvcprt.lib (this error I get when try to compile examples of websocketpp) just change Runtime Library in project's properties to Multithreaded (/MT) or Multi-threaded Debug (/MD)