“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

The code is almost perfect.

Add a line #include "add.h" inadd.cpp`.

Compile the files together as g++ main.cpp add.cpp and it will produce an executablea.out

You can run the executable as ./a.out and it will produce the output "The sum of 3 and 4 is 7" (without the quotes)

Answer 2

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