C++ Implementation of Singleton Design Pattern (Help Please) [duplicate]

I'm going through the book "Head First Design Patterns", and I'm on the Chapter Discussing the Singleton Design Pattern. I am attempting to strengthen my OOP skills for a C++ Project, and would like to implement the Singleton Design Pattern in C++. I've appended the compiler error below. I don't understand the meaning of "undefined reference", as from my perspective the Singleton is defined.

Thank you for your help

Compiler Error + Code