singleton.h

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#pragma once
 
#include <type_traits>
 
namespace myproject {
 
template <typename T, typename D = T>
class Singleton {
  friend D;
  static_assert(std::is_base_of_v<T, D>, "T should be a base type for D");
 
 public:
  Singleton() = default;
  ~Singleton() = default;
  Singleton(const Singleton&) = delete;
  Singleton(Singleton&&) = delete;
  Singleton& operator=(const Singleton&) = delete;
  Singleton& operator=(Singleton&&) = delete;
  static T& instance();
};
 
template <typename T, typename D>
T& Singleton<T, D>::instance() {
  static D inst;
  return inst;
}
 
}  // namespace myproject