Why is this Singleton design pattern not linking? [duplicate]

This question already has an answer here:

• Undefined reference to a static member 3 answers
• What is an undefined reference/unresolved external symbol error and how do I fix it? 27 answers

The following code is taken directly from "Design Patterns" by Gamma, Helm, Johnson, and Vlissides (1994), with one exception: I have added a public member function named "printOne()" for testing.

I am simply trying to instantiate the class, create the singleton object, and invoke the printOne() method thereon.

This code will not link, and I don't know why.

Thank you for your help. Sincerely, Keith :^)

SOURCE

#include <iostream>

class Singleton {
public:
    static Singleton* Instance();
    void printOne() { std::cout << 1 << std::endl; }
protected:
    Singleton();
private:
    static Singleton* _instance;
};

Singleton* Singleton::_instance = 0;
Singleton* Singleton::Instance() {
    if(_instance == 0) {
        _instance = new Singleton;
    }
    return _instance;
}

int main() {

    Singleton::Instance()->printOne();

    std::cout << std::endl;
    return 0;
}

ERROR

Ld /Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Products/Debug/singleton normal x86_64
    cd /Users/keith/Documents/2017/working/singleton
    export MACOSX_DEPLOYMENT_TARGET=10.11
    /Applications/http://ift.tt/17KlLIW -arch x86_64 -isysroot /Applications/http://ift.tt/1jCtz97 -L/Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Products/Debug -F/Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Products/Debug -filelist /Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Intermediates/singleton.build/Debug/singleton.build/Objects-normal/x86_64/singleton.LinkFileList -mmacosx-version-min=10.11 -stdlib=libc++ -Xlinker -dependency_info -Xlinker /Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Intermediates/singleton.build/Debug/singleton.build/Objects-normal/x86_64/singleton_dependency_info.dat -o /Users/keith/Library/Developer/Xcode/DerivedData/singleton-absganqluliwfnaibrrlstfosfsc/Build/Products/Debug/singleton

Undefined symbols for architecture x86_64:
  "Singleton::Singleton()", referenced from:
      Singleton::Instance() in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

BOOK EXCERPT
Design Patterns - Gamma, Helm, Johnson, Vlissides (1994) - pg 129