axmol/thirdparty/yasio/singleton.hpp

//////////////////////////////////////////////////////////////////////////////////////////
// A multi-platform support c++11 library with focus on asynchronous socket I/O for any
// client application.
//////////////////////////////////////////////////////////////////////////////////////////
/*
The MIT License (MIT)

Copyright (c) 2012-2023 HALX99

Permission is hereby granted, free of charge, to any person obtaining a copy
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*/
// core/singleton.hpp: A common use thread safe singleton class template, support non-delayed or delayed init with variadic args.
//
// refer to:
//   https://www.youtube.com/watch?v=c1gO9aB9nbs&t=1120s
//
// remark:
// Singletons make it hard to determine the lifetime of an object, which can
// lead to buggy code and spurious crashes.
//
// Instead of adding another singleton into the mix, try to identify either:
//   a) An existing singleton that can manage your object's lifetime
//   b) Locations where you can deterministically create the object and pass
//      into other objects
//
// If you absolutely need a singleton, please keep them as trivial as possible
// and ideally a leaf dependency. Singletons get problematic when they attempt
// to do too much in their destructor or have circular dependencies.
#ifndef YASIO__SINGLETON_HPP
#define YASIO__SINGLETON_HPP
#include <new>
#include <memory>
#include <functional>
#include <mutex>
#include <atomic>

namespace yasio
{
template <typename _Ty, bool delay = false>
class singleton_constructor {
public:
  template <typename... _Types>
  static _Ty* construct(_Types&&... args)
  {
    return new _Ty(std::forward<_Types>(args)...);
  }
};

template <typename _Ty>
class singleton_constructor<_Ty, true> {
public:
  template <typename... _Types>
  static _Ty* construct(_Types&&... args)
  {
    auto inst = new _Ty();
    delay_init(inst, std::forward<_Types>(args)...);
    return inst;
  }

private:
  template <typename _Fty, typename... _Types>
  static void delay_init(_Ty* inst, _Fty&& memf, _Types&&... args)
  { // init use specific member func with more than 1 args
    std::mem_fn(memf)(inst, std::forward<_Types>(args)...);
  }

  template <typename _Fty, typename _Arg>
  static void delay_init(_Ty* inst, _Fty&& memf, _Arg&& arg)
  { // init use specific member func with 1 arg
    std::mem_fn(memf)(inst, std::forward<_Arg>(arg));
  }

  template <typename _Fty>
  static void delay_init(_Ty* inst, _Fty&& memf)
  { // init use specific member func without arg
    std::mem_fn(memf)(inst);
  }

  static void delay_init(_Ty* inst)
  { // dummy init without delay init member func, same as no delay constructor
  }
};

template <typename _Ty>
class singleton {
  typedef singleton<_Ty> _Myt;
  typedef _Ty* pointer;

public:
  // Return the singleton instance
  template <typename... _Types>
  static pointer instance(_Types&&... args)
  {
    return get_instance<false>(std::forward<_Types>(args)...);
  }

  template <typename... _Types>
  static pointer instance1(_Types&&... args)
  {
    return get_instance<true>(std::forward<_Types>(args)...);
  }

  static void destroy(void)
  {
    if (auto inst = _Myt::__single__.exchange(nullptr, std::memory_order_acq_rel))
      delete inst;
  }

  // Peek the singleton instance
  static pointer peek() { return _Myt::__single__; }

private:
  template <bool delay, typename... _Types>
  static pointer get_instance(_Types&&... args)
  {
    auto& inst = _Myt::__single__;
    if (inst.load(std::memory_order_acquire))
      return inst;

    {
      std::lock_guard<std::mutex> lck(__mutex__);
      if (!inst.load(std::memory_order_relaxed))
        inst.store(singleton_constructor<_Ty, delay>::construct(std::forward<_Types>(args)...), std::memory_order_release);
    }
    return inst;
  }

  static std::atomic<_Ty*> __single__;
  static std::mutex __mutex__;

private:
  // disable construct, assign operation, copy construct also not allowed.
  singleton(void) = delete;
};

template <typename _Ty>
std::atomic<_Ty*> singleton<_Ty>::__single__;
template <typename _Ty>
std::mutex singleton<_Ty>::__mutex__;

} // namespace yasio

#endif