[boost] Re: Singleton

From: Pavel Vozenilek (pavel_vozenilek_at_[hidden])
Date: 2005-01-16 17:21:32

"Jason Hise" wrote:

> I just wanted to get some opinions on the following static allocation 
> policy, revamped to ensure that there won't be alignment problems.  Does 
> anything need to be revised?
>
>    template < typename Type >
>    class AllocateStatically
>    {
>    public:
>        typedef Type * Pointer;
>
>        static Pointer Create (  )
>        {
>            static Type instance;
>            static bool first = true;
>
>            if ( first )
>            {
>                first = false;
>            }
>            else
>            {
>                new ( reinterpret_cast < Pointer > ( &instance ) ) Type;
>            }
>
>            return &instance;
>        }
>
>        static void Destroy ( Pointer p )
>        {
>            p->~Type (  );
>        }
>    };
>
It is wrong IMHO because:
1. The Type needs default constructor
2. The Type instance gets always created (and possibly
     recreated again)
3. The first Typeinstance may be "destroyed" w/o
    calling destructor.

Alexandresu uses union aligned to fit anything
with sizeof(Type):

    template <class T> struct CreateStatic
    {
        union MaxAlign
        {
            char t_[sizeof(T)];
            short int shortInt_;
            int int_;
            long int longInt_;
            float float_;
            double double_;
            long double longDouble_;
            struct Test;
            int Test::* pMember_;
            int (Test::*pMemberFn_)(int);
        };

        static T* Create()
        {
            static MaxAlign staticMemory_;
            return new(&staticMemory_) T;
        }

        static void Destroy(T* p)
        {
            p->~T();
        }
    };

/Pavel