Re: Am I or Alexandrescu wrong about singletons?

Paul Bibbings <>
Tue, 16 Mar 2010 23:57:16 CST
DeMarcus <> writes:


I do the same as him. This is what he does.

<snip>Following code, at breaks in line numbering</snip>

Here's Singleton.h

00717 template
00718 <
00719 typename T,
00720 template <class> class CreationPolicy = CreateUsingNew,
00721 template <class> class LifetimePolicy = DefaultLifetime,
00722 template <class, class> class ThreadingModel =
00723 class MutexPolicy = LOKI_DEFAULT_MUTEX
00724 >
00725 class SingletonHolder
00726 {
00727 public:

00733 static T& Instance();

00744 typedef typename
00744b ThreadingModel<T*,MutexPolicy>::VolatileType
00744c PtrInstanceType;
00745 static PtrInstanceType pInstance_;

00747 };


00775 // SingletonHolder::Instance
00778 template
00779 <
00780 class T,
00781 template <class> class CreationPolicy,
00782 template <class> class LifetimePolicy,
00783 template <class, class> class ThreadingModel,
00784 class MutexPolicy
00785 >
00786 inline T& SingletonHolder<T, CreationPolicy,
00787 LifetimePolicy, ThreadingModel, MutexPolicy>::Instance()
00788 {
00789 if (!pInstance_)
00790 {
00791 MakeInstance();
00792 }
00793 return *pInstance_;
00794 }

Here's Threads.h containing ThreadingModel.

00252 template < class Host, class MutexPolicy =
00253 class ObjectLevelLockable
00254 {

00299 typedef volatile Host VolatileType;

00305 };

If you look at row 744b you see that he passes T* to ThreadingModel. If
you then look at row 299 you see that his VolatileType becomes a
volatile T*.

Where Host at line 299 is a T*, then VolatileType is not volatile T* but
rather T* volatile; that is, a volatile pointer to T.

He and me then do the exact same thing on row 793,
initializing a T& with a volatile T.

Rather, Alexandrescu is initializing a T& with an l-value of type T. It
is his T* that is volatile, not what it points to.


Paul Bibbings

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