“undefined reference” error in a very very simple c++ program

Question

I have a simple program, which I copied exactly from the example in http://www.learncpp.com/cpp-tutorial/19-header-files/ because I\'m learning how to make c++ programs with

Answer 1

Undefined references may happen when having many .c or .cpp sources and some of them is not compiled.

One good "step-by-step" explanation on how to do it is here