Do not understand reason for compile error

edit

Consider the following partial application in my new Hydro installation

#include <ros/ros.h>
int main(int argc, char** argv) {
//...
ros::start();
//...
}

when compiled with

g++  -I/opt/ros/hydro/include

I get the error: "undefined reference to 'ros::start()'"

There is no error about a missing include and I even included <ros init.h=""> and got the same error. I can see ROSCPP_DECL void start();

Does anybody know why I get this compile error?