boost::asio序列9: service_register

 主要用於service註冊，其中，註冊的key爲execution_context::service::key類型:

  friend class boost::asio::detail::service_registry;
  struct key
  {
    key() : type_info_(0), id_(0) {}
    const std::type_info* type_info_;
    const execution_context::id* id_;
  } key_;

完整代碼如下: 

class service_registry
  : private noncopyable
{
public:
  // Constructor.
  BOOST_ASIO_DECL service_registry(execution_context& owner);

  // Destructor.
  BOOST_ASIO_DECL ~service_registry();

  // Shutdown all services.
  BOOST_ASIO_DECL void shutdown_services();

  // Destroy all services.
  BOOST_ASIO_DECL void destroy_services();

  // Notify all services of a fork event.
  BOOST_ASIO_DECL void notify_fork(execution_context::fork_event fork_ev);

  // Get the service object corresponding to the specified service type. Will
  // create a new service object automatically if no such object already
  // exists. Ownership of the service object is not transferred to the caller.
  template <typename Service>
  Service& use_service();

  // Get the service object corresponding to the specified service type. Will
  // create a new service object automatically if no such object already
  // exists. Ownership of the service object is not transferred to the caller.
  // This overload is used for backwards compatibility with services that
  // inherit from io_context::service.
  template <typename Service>
  Service& use_service(io_context& owner);

  // Add a service object. Throws on error, in which case ownership of the
  // object is retained by the caller.
  template <typename Service>
  void add_service(Service* new_service);

  // Check whether a service object of the specified type already exists.
  template <typename Service>
  bool has_service() const;

private:
  // Initalise a service's key when the key_type typedef is not available.
  template <typename Service>
  static void init_key(execution_context::service::key& key, ...);

#if !defined(BOOST_ASIO_NO_TYPEID)
  // Initalise a service's key when the key_type typedef is available.
  template <typename Service>
  static void init_key(execution_context::service::key& key,
      typename enable_if<
        is_base_of<typename Service::key_type, Service>::value>::type*);
#endif // !defined(BOOST_ASIO_NO_TYPEID)

  // Initialise a service's key based on its id.
  BOOST_ASIO_DECL static void init_key_from_id(
      execution_context::service::key& key,
      const execution_context::id& id);

#if !defined(BOOST_ASIO_NO_TYPEID)
  // Initialise a service's key based on its id.
  template <typename Service>
  static void init_key_from_id(execution_context::service::key& key,
      const service_id<Service>& /*id*/);
#endif // !defined(BOOST_ASIO_NO_TYPEID)

  // Check if a service matches the given id.
  BOOST_ASIO_DECL static bool keys_match(
      const execution_context::service::key& key1,
      const execution_context::service::key& key2);

  // The type of a factory function used for creating a service instance.
  typedef execution_context::service*(*factory_type)(void*);

  // Factory function for creating a service instance.
  template <typename Service, typename Owner>
  static execution_context::service* create(void* owner);

  // Destroy a service instance.
  BOOST_ASIO_DECL static void destroy(execution_context::service* service);

  // Helper class to manage service pointers.
  struct auto_service_ptr;
  friend struct auto_service_ptr;
  struct auto_service_ptr
  {
    execution_context::service* ptr_;
    ~auto_service_ptr() { destroy(ptr_); }
  };

  // Get the service object corresponding to the specified service key. Will
  // create a new service object automatically if no such object already
  // exists. Ownership of the service object is not transferred to the caller.
  BOOST_ASIO_DECL execution_context::service* do_use_service(
      const execution_context::service::key& key,
      factory_type factory, void* owner);

  // Add a service object. Throws on error, in which case ownership of the
  // object is retained by the caller.
  BOOST_ASIO_DECL void do_add_service(
      const execution_context::service::key& key,
      execution_context::service* new_service);

  // Check whether a service object with the specified key already exists.
  BOOST_ASIO_DECL bool do_has_service(
      const execution_context::service::key& key) const;

  // Mutex to protect access to internal data.
  mutable boost::asio::detail::mutex mutex_;

  // The owner of this service registry and the services it contains.
  execution_context& owner_;

  // The first service in the list of contained services.
  execution_context::service* first_service_;
};

template <typename Service>
Service& service_registry::use_service()
{
  execution_context::service::key key;
  init_key<Service>(key, 0);
  factory_type factory = &service_registry::create<Service, execution_context>;
  return *static_cast<Service*>(do_use_service(key, factory, &owner_));
}

template <typename Service>
Service& service_registry::use_service(io_context& owner)
{
  execution_context::service::key key;
  init_key<Service>(key, 0);
  factory_type factory = &service_registry::create<Service, io_context>;
  return *static_cast<Service*>(do_use_service(key, factory, &owner));
}

template <typename Service>
void service_registry::add_service(Service* new_service)
{
  execution_context::service::key key;
  init_key<Service>(key, 0);
  return do_add_service(key, new_service);
}

template <typename Service>
bool service_registry::has_service() const
{
  execution_context::service::key key;
  init_key<Service>(key, 0);
  return do_has_service(key);
}

template <typename Service>
inline void service_registry::init_key(
    execution_context::service::key& key, ...)
{
  init_key_from_id(key, Service::id);
}

#if !defined(BOOST_ASIO_NO_TYPEID)
template <typename Service>
void service_registry::init_key(execution_context::service::key& key,
    typename enable_if<
      is_base_of<typename Service::key_type, Service>::value>::type*)
{
  key.type_info_ = &typeid(typeid_wrapper<Service>);
  key.id_ = 0;
}

template <typename Service>
void service_registry::init_key_from_id(execution_context::service::key& key,
    const service_id<Service>& /*id*/)
{
  key.type_info_ = &typeid(typeid_wrapper<Service>);
  key.id_ = 0;
}
#endif // !defined(BOOST_ASIO_NO_TYPEID)

template <typename Service, typename Owner>
execution_context::service* service_registry::create(void* owner)
{
  return new Service(*static_cast<Owner*>(owner));
}

service_registry::service_registry(execution_context& owner)
  : owner_(owner),
    first_service_(0)
{
}

service_registry::~service_registry()
{
}

// 關閉services
void service_registry::shutdown_services()
{
  execution_context::service* service = first_service_;
  while (service)
  {
    service->shutdown();
    service = service->next_;
  }
}

// 銷燬services 
void service_registry::destroy_services()
{
  while (first_service_)
  {
    execution_context::service* next_service = first_service_->next_;
    destroy(first_service_);
    first_service_ = next_service;
  }
}

void service_registry::notify_fork(execution_context::fork_event fork_ev)
{
  // 拷貝當前的services 
  // Make a copy of all of the services while holding the lock. We don't want
  // to hold the lock while calling into each service, as it may try to call
  // back into this class.
  std::vector<execution_context::service*> services;
  {
    boost::asio::detail::mutex::scoped_lock lock(mutex_);
    execution_context::service* service = first_service_;
    while (service)
    {
      services.push_back(service);
      service = service->next_;
    }
  }

  // 對於fork_prepare事件，採用正序，否則採用反序
  // If processing the fork_prepare event, we want to go in reverse order of
  // service registration, which happens to be the existing order of the
  // services in the vector. For the other events we want to go in the other
  // direction.
  std::size_t num_services = services.size();
  if (fork_ev == execution_context::fork_prepare)
    for (std::size_t i = 0; i < num_services; ++i)
      services[i]->notify_fork(fork_ev);
  else
    for (std::size_t i = num_services; i > 0; --i)
      services[i - 1]->notify_fork(fork_ev);
}

// 構造service key 
void service_registry::init_key_from_id(execution_context::service::key& key,
    const execution_context::id& id)
{
  key.type_info_ = 0;
  key.id_ = &id;
}

// 比較servic key 
bool service_registry::keys_match(
    const execution_context::service::key& key1,
    const execution_context::service::key& key2)
{
  if (key1.id_ && key2.id_)
    if (key1.id_ == key2.id_)
      return true;
  if (key1.type_info_ && key2.type_info_)
    if (*key1.type_info_ == *key2.type_info_)
      return true;
  return false;
}

// 銷燬(釋放)service 
void service_registry::destroy(execution_context::service* service)
{
  delete service;
}

// 獲取指定key對應的service
execution_context::service* service_registry::do_use_service(
    const execution_context::service::key& key,
    factory_type factory, void* owner)
{
  boost::asio::detail::mutex::scoped_lock lock(mutex_);

  // First see if there is an existing service object with the given key.
  execution_context::service* service = first_service_;
  while (service)
  {
    if (keys_match(service->key_, key))
      return service;
    service = service->next_;
  }

  // Create a new service object. The service registry's mutex is not locked
  // at this time to allow for nested calls into this function from the new
  // service's constructor.
  lock.unlock();
  auto_service_ptr new_service = { factory(owner) };
  new_service.ptr_->key_ = key;
  lock.lock();

  // Check that nobody else created another service object of the same type
  // while the lock was released.
  service = first_service_;
  while (service)
  {
    if (keys_match(service->key_, key))
      return service;
    service = service->next_;
  }

  // Service was successfully initialised, pass ownership to registry.
  new_service.ptr_->next_ = first_service_;
  first_service_ = new_service.ptr_;
  new_service.ptr_ = 0;
  return first_service_;
}

// 插入key和對應的service
void service_registry::do_add_service(
    const execution_context::service::key& key,
    execution_context::service* new_service)
{
  if (&owner_ != &new_service->context())
    boost::asio::detail::throw_exception(invalid_service_owner());

  boost::asio::detail::mutex::scoped_lock lock(mutex_);

  // Check if there is an existing service object with the given key.
  execution_context::service* service = first_service_;
  while (service)
  {
    if (keys_match(service->key_, key))
      boost::asio::detail::throw_exception(service_already_exists());
    service = service->next_;
  }
   
  // Take ownership of the service object.
  new_service->key_ = key;
  new_service->next_ = first_service_;
  first_service_ = new_service;
}

// 判斷是否存在對應key的services
bool service_registry::do_has_service(
    const execution_context::service::key& key) const
{
  boost::asio::detail::mutex::scoped_lock lock(mutex_);

  execution_context::service* service = first_service_;
  while (service)
  {
    if (keys_match(service->key_, key))
      return true;
    service = service->next_;
  }

  return false;
}