RE: [boost] Re: Singleton

From: Noah Stein (noah_at_[hidden])
Date: 2005-01-10 15:52:17

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]]
> On Behalf Of Jason Hise
> Sent: Saturday, January 08, 2005 11:56 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] Re: Singleton

> Here is a stripped down example of what I am
> trying (destruction would be virtually identical, and thus is omited):
> 
> template < typename T >
> class AllocateUsingNew
> {
> public:
>     static T * Create (  )
>     {
>         T * p = reinterpret_cast < T * > ( new char [ sizeof ( T ) ] );
>         try
>         {
>             return T :: Allocation :: Construct ( p );
>         }
>         catch ( ... )
>         {
>             delete [] reinterpret_cast < char * > ( p );
>             throw;
>         }
>     }
> };
> 
> template < typename T, typename A = AllocateUsingNew < T > >
> class Singleton
> {
> public:
>     class Allocation
>     {
>     private:
>         friend A; // allow the allocator access to Allocation::Create
> 
>         static void Create ( T * p )
>         {
>             T :: Create < T > ( p ); // call the derived specialization?
>         }
>     };
> 
>     friend Allocation; // allow allocation access to Create
> 
> protected:
>     template < typename T >
>     static void Create ( T * p ); // undefined base template that
> derived should inherit
> };
> 
> template < typename T >
> void T :: Create < T > ( T * p ) // specialize derived's Create method?
> {
>     new ( p ) T;
> }
> 
> Any suggestions?

Your Create() function in AllocateUsingNew isn't guaranteed to work for all
classes.  The type of the singleton class might have alignment restrictions.
char has no alignment restrictions; therefore, the program may crash when
trying to initialize the singleton.  I believe boost::aligned_storage is
your friend in this situation.

-- Noah