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

       if (ACE_Reactor::reactor_ == )
         {
           ACE_NEW_RETURN (ACE_Reactor::reactor_,
                           ACE_Reactor,
                           );
           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 != )
 #   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 (), ));
   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 == )
     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=;
 impl = new ACE_Select_Reactor();
 ACE_Reactor *reactor=;
 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_ = ;
       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_, -);
   int i;

   // Check to see if it's already registered
   for (i = ; 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"),
                           -);
       }

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

   return -;
 }

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 == )                                                 \
       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 == )                                                \
       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。