Re: [Boost-users] Is there an elegant way to initialize an instanceof 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: 2008-07-09 13:46:09

Ferdinand Prantl wrote:
> Thanks for your suggestions! Niels, you nailed it!
  [...]
> The current requirements of boost::any ensure that the contained
> object is always valid there (if not used too wildly). I should have
> thought about the copying myself... :-) Your wrapper is an elegant
> solution isolating the construction differences from the rest of
> the holder template.

I'm glad you like it.  :-)  But how do you use it in practice?  The 
added boost::any array support I suggested would require you to always 
specify the number of character, when retrieving C-style string 
(char[N]) from a boost::any. Wouldn't it?

Maybe an extra function, boost::any::size_of_value(), would be helpful, 
returning the number of bytes of the stored object.  What do you think? 
How would you deal with various string lengths, within your application?

BTW, after re-reading, I found two shortcomings to the implementation as 
I originally suggested, caused by the implementation of the wrapper:

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

First of all, it doesn't support storing a multidimentional array (e.g. 
int[42][69]) into a boost::any.  Secondly, it copies its argument by 
means of copy-assignment, instead of copy-initialization.  So when T is 
a class, wrapping an array of T would do default-construction + 
assignment, instead of just copy-construction.  I'm pretty sure that 
both of these issues can be solved, though.  The data would need to be 
stored in a boost::aligned_storage, and the copying should probably be 
done by placement-new, to solve the second issue.  Anyway, these two 
issues are irrelevant to your request for C-style string support...

Good luck,

  Niels