$ clang main.cpp Undefined symbols for architecture x86_64: "std::ostream::operator<<(std::ostream& (*)(std::ostream&))", referenced from: _main in example-YhevQb.o "std::ios_base::Init::Init()", referenced from: ___cxx_global_var_init in example-YhevQb.o "std::ios_base::Init::~Init()", referenced from: ___cxx_global_var_init in example-YhevQb.o "std::cout", referenced from: _main in example-YhevQ