Re: [Boost-users] Is there an elegant way to initialize an instance of boost::any with a C-style string constant?

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-06-30 14:46:54

Peter Foelsche wrote:
> I suggested some time ago to add a special constructor:
>
> template<class T, size_t SIZE>
> any(T _ap[SIZE]);
>
> which should do the same like
> any(T *_p);

Ferdinand Prantl wrote:
> Well, a little shorter as the static_cast version but still a
> "superfluous" cast.
>
> I could not find anything about it but the thread which I referred in
> the first post to. Do you remember why the constructor has not been
> added to the official version?

I don't know, but it makes sense to me: an array is not just a pointer! 
boost::any always stores a /copy/ of its constructor argument or 
assignment argument.  So /if/ it would support built-in arrays, I think 
it should copy their elements internally.  Still, it appears technically 
possible to add array support to boost::any, e.g., by having it 
internally hold a wrapper<ValueType>, instead of a ValueType object (as 
holder<ValueType>::held data member):

  template<typename ValueType>
  struct wrapper
  {
    ValueType data;

    explicit wrapper(const ValueType & value)
      : data(value)
    {
    }
  };

The wrapper would need to be specialized for array types, copying the 
array elements during construction:

  template<typename ElementType, std::size_t NumberOfElements>
  struct wrapper<ElementType[NumberOfElements]>
  {
    typename remove_const<ElementType>::type data[NumberOfElements];

    explicit wrapper(const ElementType (& value)[NumberOfElements])
    {
      std::copy(value, value + NumberOfElements, data);
    }
  };

The data member holder<ValueType>::held should be declared as 
wrapper<ValueType>, and references to "held" should be replaced by 
"held.data".  Would that solve your problem in an elegant way?  If so, 
you could consider making it a feature request  :-) 
https://svn.boost.org/trac/boost/newticket

Kind regards,

--
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center