以前曾经讨论过Singleton的实现，这次在对照ACE和Boost代码的时候，又重新审视了一下二者对Singleton不同的实现。其间的差别也体现了不同的编程哲学：ACE的实现更加偏重多线程中的安全和效率问题；Boost的实现则偏重于使用语言自身的特性满足Singleton模式的基本需求。
o ACE的实现
Douglas C. Schmidt在Double-Checked Locking: An Optimization Pattern for Efficiently Initializing and Accessing Thread-safe Objects一文中对double-check lock（一般译为双检锁）进行了详细的阐述。
ACE的Singleton使用Adapter模式实现对其他类的适配，使之具有全局唯一的实例。由于C++标准并非明确指定全局静态对象的初始化顺序，ACE使用double-check lock保证线程安全，并使之不受全局静态对象初始化顺序的影响，同时也避免了全局静态实现方式的初始化后不使用的开销。
如果你能够准确的区分以下三种实现的弊端和隐患，对double-check lock也就有了足够的了解。
// -------------------------------------------
class Singleton
{
public:
static Singleton *instance (void)
{
// Constructor of guard acquires
// lock_ automatically.
Guard<Mutex> guard (lock_);
// Only one thread in the
// critical section at a time.
if (instance_ == 0)
instance_ = new Singleton;
return instance_;
// Destructor of guard releases
// lock_ automatically.
}
private:
static Mutex lock_;
static Singleton *instance_;
};// ---------------------------------------------
static Singleton *instance (void)
{
if (instance_ == 0) {
Guard<Mutex> guard (lock_);
// Only come here if instance_
// hasn’t been initialized yet.
instance_ = new Singleton;
}
return instance_;
}// ---------------------------------------------
class Singleton
{
public:
static Singleton *instance (void)
{
// First check
if (instance_ == 0)
{
// Ensure serialization (guard
// constructor acquires lock_).
Guard<Mutex> guard (lock_);
// Double check.
if (instance_ == 0)
instance_ = new Singleton;
}
return instance_;
// guard destructor releases lock_.
}
private:
static Mutex lock_;
static Singleton *instance_;
};
更多详情，见Schmidt老师的原文和ACE_Singleton实现。
o Boost的实现
Boost的Singleton也是线程安全的，而且没有使用锁机制。当然，Boost的Singleton有以下限制（遵从这些限制，可以提高效率）：
o The classes below support usage of singletons, including use in program startup/shutdown code, AS LONG AS there is only one thread running before main() begins, and only one thread running after main() exits.
o This class is also limited in that it can only provide singleton usage for classes with default constructors.
// T must be: no-throw default constructible and no-throw destructible
template <typename T>
struct singleton_default
{
private:
struct object_creator
{
// This constructor does nothing more than ensure that instance()
// is called before main() begins, thus creating the static
// T object before multithreading race issues can come up.
object_creator() { singleton_default<T>::instance(); }
inline void do_nothing() const { }
};
static object_creator create_object;singleton_default();
public:
typedef T object_type;// If, at any point (in user code), singleton_default<T>::instance()
// is called, then the following function is instantiated.
static object_type & instance()
{
// This is the object that we return a reference to.
// It is guaranteed to be created before main() begins because of
// the next line.
static object_type obj;// The following line does nothing else than force the instantiation
// of singleton_default<T>::create_object, whose constructor is
// called before main() begins.
create_object.do_nothing();return obj;
}
};
template <typename T>
typename singleton_default<T>::object_creator
singleton_default<T>::create_object;
对于多数Singleton使用，Boost提供的版本完全能够满足需求。为了效率，我们有必要对其使用作出一定的限制。
而在多线程编程中，则有必要使用double-check lock降低频繁加锁带来的开销。