Re: [c++0x] aligned_union implementation

[CoffeeBuzz <chris dot fairles at gmail dot com>]

CoffeeBuzz wrote:
> 
> Tossed together a aligned_union implementation. I just want to make sure
> I'm understanding what its suppose to do. 
> 
>   template<typename... _Types>
>     struct __largest_alignment;
>   
>   template<typename _Tp, typename... _Types>
>     struct __largest_alignment<_Tp,_Types...> {
>         typedef typename conditional< 
>             (alignment_of<typename
> __largest_alignment<_Types...>::type>::value > alignment_of<_Tp>::value),
>                 integral_constant<size_t,alignment_of<typename
> __largest_alignment<_Types...>::type>::value>,
>                 integral_constant<size_t,alignment_of<_Tp>::value>>::type
> type;
>     };
>     
>   template<typename _Tp>
>     struct __largest_alignment<_Tp> {typedef
> integral_constant<size_t,alignment_of<_Tp>::value> type;};
>   
>   template<std::size_t _Len, typename... _Types>
>   struct aligned_union {   
>     static const size_t alignment_value =
> __largest_alignment<_Types...>::type::value; 
>     typedef typename aligned_storage<_Len, alignment_value>::type type;
>   };
> 
> So 'alignment_value' gives the "largest" alignment of all the types. I'm
> not sure if "largest" means "most stringent" so correct me if I'm wrong.
> Now, for the type I think the above isn't quite correct since it must be
> "at least" _Len but if _Len is smaller than the largest in _Types then it
> should be sizeof(the largest type) correct?
> 
> Chris
> 

Actually, I just realized that you can use the ternary operator. 

  template<typename _Tp, typename... _Types>
    struct __largest_alignment<_Tp,_Types...> {
        typedef _Tp type;
        static const size_t value = 
        alignment_of<typename __largest_alignment<_Types...>::type>::value >
alignment_of<_Tp>::value ?
        alignment_of<typename __largest_alignment<_Types...>::type>::value :
alignment_of<_Tp>::value;     
    };

  template<typename _Tp>
    struct __largest_alignment<_Tp> 
    { typedef _Tp type; static const size_t value =
alignment_of<_Tp>::value; };

And the aligned_union looks something like:

  template<std::size_t _Len, typename... _Types>
  struct aligned_union {   
    static const size_t alignment_value =
__largest_alignment<_Types...>::value; 
    typedef typename aligned_storage<(_Len > sizeof(typename
__largest_type<_Types...>::type) ? _Len : sizeof(typename
__largest_type<_Types...>::type)), alignment_value>::type type;
  };

 
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