Singletons: (1) canonical GoF Singleton Pattern for C++ (2) Scott Meyer's (C++11) singleton

cpp_singleton.h

// http://www.gofpatterns.com/design-patterns/module3/cplus-plus-singleton.php

class Singleton {  
 private:    
   static Singleton* theInstance;  
 protected:
   Singleton();  
 public:    
   static Singleton* getInstance(){
     if (theInstance == 0 ) {    
       theInstance = new Singleton();  
     }
     return theInstance;
   };
};

Singleton* Singleton::theInstance = 0;

meyer-singleton.cpp

class Singleton
{
  
public:
  static Singleton& instance()
  {
    static Singleton s; // initialized the first time control passes to this declaration. Ignored on subsequent calls.
										    // Since C++11, If multiple threads attempt to initialize the same static local variable concurrently,
										    // the initialization is still guaranteed to occur exactly once.
										    // see https://en.cppreference.com/w/cpp/language/storage_duration#Static_local_variables
    return s;
  }
  
private:
  Singleton() {}
  Singleton(const Singleton &) = delete;
  Singleton& operator= (const Singleton &) = delete;
  ~Singleton() {}

};