Re: [boost] [utility/value_init] boost::value_initialized<T> direct-initialized?

Subject: Re: [boost] [utility/value_init] boost::value_initialized<T> direct-initialized?
From: Niels Dekker - address until 2010-10-10 (niels_address_until_2010-10-10_at_[hidden])
Date: 2010-03-31 04:27:06

>> Can anybody please have a look at the following feature request by
>> Edward Diener? https://svn.boost.org/trac/boost/ticket/3472 He
>> proposed to add an explicit constructor, value_initialized(T const&),
>> to boost::value_initialized<T>. This constructor would copy the value
>> of its argument to the object held by value_initialized<T>.
>>
>> When such a constructor would be added, an object held by
>> value_initialized<T> might no longer be value-initialized. Instead it
>> might be direct-initialized. Does anybody have any moral (?)
>> objections against the idea that value_initialized<T> might hold a
>> non-value-initialized object?

Jeffrey Hellrung wrote:
> Is defining a new template class (e.g., value_or_direct_initialized<T>,
> or whatever) out of the question? Just a suggestion, and not confident
> it's a good one...

Thanks for the suggestion, Jeffrey! I like it, actually.  :-) Maybe we 
could simply call the new template class boost::initialized<T>.

I can think of two ways to implement boost::initialized<T> (and adapt 
value_initialized<T> accordingly):

The implementation of boost::initialized could be simply copied from the 
old value_initialized<T>, but having an extra constructor, initialized(T 
const&).  The new value_initialized<T> could then be implemented in 
terms of boost::initialized<T>:

   template<class T>
   class value_initialized
   {
     initialized<T> m_data;
   public:
     // Forwarding to m_data member functions:
     T const & data() const;
     T& data();
     void swap(value_initialized &);
     operator T const &() const;
     operator T&();
   };

Or boost::initialized<T> could be derived from boost::value_initialized<T>:

   template<class T>
   class initialized: public value_initialized<T>
   {
   public:
     initialized(T const& arg):
       value_initialized(arg) {}

     void swap(initialized &);
   };

In that case, value_initialized<T> would still need the extra 
constructor, value_initialized(T const&), but it could be declared 
protected.

What do you think?

Kind regards,

    Niels

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