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

Subject: Re: [boost] [utility/value_init] boost::value_initialized<T> direct-initialized?
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-04-01 14:09:37

Niels Dekker - address until 2010-10-10 wrote:
>>> Do you agree that its support for member data initialization would be
>>> the main reason for having boost::initialized<T>?
> 
> Jeffrey Hellrung wrote:
>> Yes.
> 
> Within the context of member data initialization, the "explicit" keyword 
> doesn't do anything. (For example, when the class has a data member of 
> type boost::initialized<T>.) That's why I though the extra 
> boost::direct_initialized_t tag parameter might be useful. Clearly so 
> far I haven't convinced anyone. But I realize such a tag parameter will 
> be less relevant when we're going to introduce a new initialized<T> 
> template class, instead of modifying the interface of the old 
> boost::value_initialized<T>. Fernando's boost::value_initialized<T> has 
> been around for so long that I think backward-compatibility should be 
> taken into account.

To me, the tag is a syntactic wart, but I understand your concerns, and 
you've clearly considered this problem more than I have...

[...]
> Honestly the MSVC bug just made me more aware that copy-constructors and 
> non-copy-constructors can get confused, in a generic context. But the 
> example I added to the ticket (#3472) would also be ambiguous on other 
> compilers, when value_initialized(T const&) would be added:
> 
>  class my_integer
>  {
>    value_initialized<int> m_data;
>  public:
>    operator value_initialized<int>() const;
>    operator int() const;
>  };
> 
>  int main()
>  {
>    my_integer my;
>    value_initialized<int> val(my);
>  }
> 
> See http://codepad.org/zukxSDbB

Point taken.  See below.

> Remaining question, triggered by the comments by Steven: should the new 
> direct-initializing boost::initialized<T> constructor itself be a 
> template? In that case, the "explicit" keyword certainly would make more 
> sense to me:
> 
>   // Note: untested!
>   template<class T>
>   class initialized
>   {
>      ...
>   public:
>     initialized();
> 
>     template<class U>
>     explicit initialized(U const&);
> 
>     T const & data() const;
>     T& data();
>     void swap(initialized &);
>     operator T const &() const;
>     operator T&();
>   };
> 
> What do you think?

What do you think about using the templated constructor *and* SFINAE out 
via enable_if the binding of U to initialized<T> (or, maybe, to any 
class derived from initialized<T>)?  What problems does that have?

- Jeff