ACE中静态实例管理方式

ACE中的很多类使用了单例模式，为了便于管理单例对象，ACE使用了一个组件——ACE_Framework_Component来专门管理。

我们以ACE_Reactor这个单例类的创建和释放为例。

1、Reactor.cpp中，包括了类的创建释放。其中，单例模式的接口有两个instance函数提供——有参和无参。

首先来看无参instance：

ACE_Reactor *
ACE_Reactor::instance (void)
{
  ACE_TRACE ("ACE_Reactor::instance");

  if (ACE_Reactor::reactor_ == 0)
    {
      // Perform Double-Checked Locking Optimization.
      ACE_MT (ACE_GUARD_RETURN (ACE_Recursive_Thread_Mutex, ace_mon,
                                *ACE_Static_Object_Lock::instance (), 0));

      if (ACE_Reactor::reactor_ == 0)
        {
          ACE_NEW_RETURN (ACE_Reactor::reactor_,
                          ACE_Reactor,
                          0);
          ACE_Reactor::delete_reactor_ = true; //由于动态生成了一个ACE_Reactor对象，所以将delete_reactor_字段设为true
          ACE_REGISTER_FRAMEWORK_COMPONENT(ACE_Reactor, ACE_Reactor::reactor_) //将Singleton对象注册到一个统一的管理器中
        }
    }
  return ACE_Reactor::reactor_;
}

这里使用了经典的双检锁的实现方式。ACE_MT宏会根据系统是否启用多线程来采取相应的操作：如果系统没有启用多线程，那么它的定义为空，这样可以避免给单线程系统添加额外的负担；否则它会执行宏定义的操作，实现方式如下：

# if defined (ACE_MT_SAFE) && (ACE_MT_SAFE != 0)
#   define ACE_MT(X) X
# else
#   define ACE_MT(X)
# endif /* ACE_MT_SAFE */

有参instance：

ACE_Reactor *
ACE_Reactor::instance (ACE_Reactor *r, bool delete_reactor)
{
  ACE_TRACE ("ACE_Reactor::instance");

  ACE_MT (ACE_GUARD_RETURN (ACE_Recursive_Thread_Mutex, ace_mon,
                            *ACE_Static_Object_Lock::instance (), 0));
  ACE_Reactor *t = ACE_Reactor::reactor_;
  ACE_Reactor::delete_reactor_ = delete_reactor;

  ACE_Reactor::reactor_ = r;

  // We can't register the Reactor singleton as a framework component twice.
  // Therefore we test to see if we had an existing reactor instance, which
  // if so means it must have already been registered.
  if (t == 0)
    ACE_REGISTER_FRAMEWORK_COMPONENT(ACE_Reactor, ACE_Reactor::reactor_);

  return t;
}

这里是为了保证灵活性，我们可以创建一个需要的定制reactor并传入ACE_Reactor。具体使用方式如下：

Ace_Reactor_Impl *impl=0;
impl = new ACE_Select_Reactor();
ACE_Reactor *reactor=0;
reactor=new ACE_Reactor(impl);
ACE_Reactor::instance(reactor);

2、生成了单例对象后，通过宏ACE_REGISTER_FRAMEWORK_COMPONENT将其注册到ACE_Framework_Component中，其定义如下：

/// This macro should be called in the instance() method
/// of the Concrete class that will be managed.  Along
/// with the appropriate template instantiation.
#define ACE_REGISTER_FRAMEWORK_COMPONENT(CLASS, INSTANCE) \
        ACE_Framework_Repository::instance ()->register_component \
          (new ACE_Framework_Component_T<CLASS> (INSTANCE));

可以看到其将Singleton对象封装进了一个接口类ACE_Framework_Component_T并注册到ACE_Framework_Repository中进行统一管理。

3、下面来看一下ACE_Framework_Component_T类的实现。

ACE_Framework_Component_T.h

ACE_BEGIN_VERSIONED_NAMESPACE_DECL

/**
 * @class ACE_Framework_Component_T
 *
 * @brief This class inherits the interface of the abstract
 * ACE_Framework_Component class and is instantiated with the
 * implementation of the concrete component class @c class Concrete.
 *
 * This design is similar to the Adapter and Decorator patterns
 * from the ``Gang of Four'' book.  Note that @c class Concrete
 * need not inherit from a common class since ACE_Framework_Component
 * provides the uniform virtual interface!  (implementation based on
 * ACE_Dumpable_Adapter in <ace/Dump_T.h>.
 */
template <class Concrete>
class ACE_Framework_Component_T : public ACE_Framework_Component
{
public:
  // = Initialization and termination methods.

  /// Constructor.
  ACE_Framework_Component_T (Concrete *concrete);

  /// Destructor.
  ~ACE_Framework_Component_T (void);

  /// Close the contained singleton.
  void close_singleton (void);

  ACE_ALLOC_HOOK_DECLARE;
};

ACE_END_VERSIONED_NAMESPACE_DECL

ACE_Framework_Component_T.cpp

ACE_BEGIN_VERSIONED_NAMESPACE_DECL

template <class Concrete>
ACE_Framework_Component_T<Concrete>::ACE_Framework_Component_T (Concrete *concrete)
  : ACE_Framework_Component ((void *) concrete, concrete->dll_name (), concrete->name ())
{
  ACE_TRACE ("ACE_Framework_Component_T<Concrete>::ctor");
}

template <class Concrete>
ACE_Framework_Component_T<Concrete>::~ACE_Framework_Component_T (void)
{
  ACE_TRACE ("ACE_Framework_Component_T<Concrete>::~ACE_Framework_Component_T");
  Concrete::close_singleton ();
}

ACE_ALLOC_HOOK_DEFINE_Tt(ACE_Framework_Component_T)

template <class Concrete> void
ACE_Framework_Component_T<Concrete>::close_singleton (void)
{
  ACE_TRACE ("ACE_Framework_Component_T<Concrete>::close_singleton");
  Concrete::close_singleton ();
}

ACE_END_VERSIONED_NAMESPACE_DECL

可以看到该类主要有构造函数、析构函数、和close_singleton。其中close_singleton会调用模板concrete即传入的单例对象的静态close_singleton函数。

void
ACE_Reactor::close_singleton (void)
{
  ACE_TRACE ("ACE_Reactor::close_singleton");

  ACE_MT (ACE_GUARD (ACE_Recursive_Thread_Mutex, ace_mon,
                     *ACE_Static_Object_Lock::instance ()));

  if (ACE_Reactor::delete_reactor_)
    {
      delete ACE_Reactor::reactor_;
      ACE_Reactor::reactor_ = 0;
      ACE_Reactor::delete_reactor_ = false;
    }
}

4、对于ACE_Framework_Repository，register_component函数首先会遍历component查找该singleton对象是否已存在，而后会将其加入列表。

int
ACE_Framework_Repository::register_component (ACE_Framework_Component *fc)
{
  ACE_TRACE ("ACE_Framework_Repository::register_component");
  ACE_GUARD_RETURN (ACE_SYNCH_MUTEX, ace_mon, this->lock_, -1);
  int i;

  // Check to see if it's already registered
  for (i = 0; i < this->current_size_; i++)
    if (this->component_vector_[i] &&
        fc->this_ == this->component_vector_[i]->this_)
      {
        ACELIB_ERROR_RETURN ((LM_ERROR,
          "AFR::register_component: error, compenent already registered\n"),
                          -1);
      }

  if (i < this->total_size_)
    {
      this->component_vector_[i] = fc;
      ++this->current_size_;
      return 0;
    }

  return -1;
}

5、补充：对于ACE_Framework_Component_T有很有意思的地方，在声明中有这样一个宏：ACE_ALLOC_HOOK_DECLARE，其定义如下：

# if defined (ACE_HAS_ALLOC_HOOKS)
#  define ACE_ALLOC_HOOK_DECLARE \
  void *operator new (size_t bytes); \
  void *operator new (size_t bytes, void *ptr); \
  void *operator new (size_t bytes, const std::nothrow_t &) throw (); \
  void operator delete (void *ptr); \
  void operator delete (void *ptr, const std::nothrow_t &); \
  void *operator new[] (size_t size); \
  void operator delete[] (void *ptr); \
  void *operator new[] (size_t size, const std::nothrow_t &) throw (); \
  void operator delete[] (void *ptr, const std::nothrow_t &)

定义中有宏：ACE_ALLOC_HOOK_DEFINE_Tt(ACE_Framework_Component_T)：

# if defined (ACE_HAS_ALLOC_HOOKS)
……
#  define ACE_ALLOC_HOOK_DEFINE_Tt(CLASS) \
  ACE_GENERIC_ALLOCS (ACE_ALLOC_HOOK_HELPER_Tt, CLASS)

#  define ACE_GENERIC_ALLOCS(MAKE_PREFIX, CLASS) \
  MAKE_PREFIX (void *, CLASS)::operator new (size_t bytes)        \
  {                                                               \
    void *const ptr = ACE_Allocator::instance ()->malloc (bytes); \
    if (ptr == 0)                                                 \
      throw std::bad_alloc ();                                    \
    return ptr;                                                   \
  }                                                               \
  MAKE_PREFIX (void *, CLASS)::operator new (size_t, void *ptr) { return ptr; }\
  MAKE_PREFIX (void *, CLASS)::operator new (size_t bytes, \
                                             const std::nothrow_t &) throw () \
  { return ACE_Allocator::instance ()->malloc (bytes); } \
  MAKE_PREFIX (void, CLASS)::operator delete (void *ptr) \
  { if (ptr) ACE_Allocator::instance ()->free (ptr); } \
  MAKE_PREFIX (void, CLASS)::operator delete (void *ptr, \
                                              const std::nothrow_t &) \
  { if (ptr) ACE_Allocator::instance ()->free (ptr); } \
  MAKE_PREFIX (void *, CLASS)::operator new[] (size_t size)      \
  {                                                              \
    void *const ptr = ACE_Allocator::instance ()->malloc (size); \
    if (ptr == 0)                                                \
      throw std::bad_alloc ();                                   \
    return ptr;                                                  \
  }                                                              \
  MAKE_PREFIX (void, CLASS)::operator delete[] (void *ptr) \
  { if (ptr) ACE_Allocator::instance ()->free (ptr); } \
  MAKE_PREFIX (void *, CLASS)::operator new[] (size_t size, \
                                               const std::nothrow_t &) throw ()\
  { return ACE_Allocator::instance ()->malloc (size); } \
  MAKE_PREFIX (void, CLASS)::operator delete[] (void *ptr, \
                                                const std::nothrow_t &) \
  { if (ptr) ACE_Allocator::instance ()->free (ptr); }

而如果宏定义ACE_HAS_ALLOC_HOOKS未定义，则全部宏为空。

# else
#  define ACE_ALLOC_HOOK_DECLARE struct Ace_ {} /* Just need a dummy... */
#  define ACE_ALLOC_HOOK_DEFINE(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tt(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tcc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tccc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tccct(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tc4(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tc5(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tc6(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tc7(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Ty(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tyc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tycc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tcy(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tcyc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tca(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tco(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tcoccc(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tcs(CLASS)
#  define ACE_ALLOC_HOOK_DEFINE_Tmcc(CLASS)
# endif /* ACE_HAS_ALLOC_HOOKS */

这里可以在编译时选择是否添加内存管理钩子hook，通过ACE框架来管理内存，便于对系统进行管理与调试。