Singleton definition, just to remember

singleton.hpp

class Singleton {
 public:
  static Singleton& instance() {
    static Singleton instance_;
    return instance_;
  }

private: // use protected: if it should be inherited from
 Singleton() = default;
 ~Singleton() = default;
  
 Singleton(const Singleton&) = delete;
 Singleton& operator=(const Singleton&) = delete;
};

Remember the -fno-threadsafe-statics flag for g++ to not have all the guard code instantiated

Make any class into a singleton:

// Singleton generator
template <class C>
struct singleton
{
  static C& instance()
  {
    static C inst;
    return inst;
  }
};

// Singleton helper
template <class C>
constexpr inline C& get_singleton()
{
  return singleton<C>::instance();
}