Re: [Boost-users] [value_initialized] when T is const

Subject: Re: [Boost-users] [value_initialized] when T is const
From: Edward Diener (eldiener_at_[hidden])
Date: 2009-09-21 13:11:26

Niels Dekker - address until 2010-10-10 wrote:
>>> I guess we'd need to add an extra constructor that would copy from T
>>> to value_initialized<T>, right? I'm not yet entirely sure about its
>>> signature... IMO, all of them have their pro's and cons. For example:
>>>
>>> [1] value_initialized(const T&);
>>> [2] explicit value_initialized(const T&);
>>> [3] value_initialized(const T&, explicit_copy_t);
>>> [4] explicit template <class U> value_initialized(const U&);
>>> [5] template <class U> value_initialized(const U&, explicit_copy_t);
> 
> Edward Diener wrote:
>> I do not really understand the need for the 'explicit_copy_t'
>> in your possibilities.
> 
> If I recall correctly, there might be some rather exotic cases where 
> adding value_initialized(const T&) would cause ambiguity in a program 
> that would work before.  The compiler might not be able to choose 
> between value_initialized(const T&) and value_initialized(const 
> value_initialized&) in some rare cases. But maybe I'm wrong... If I 
> can't find such a case before the end of the week, I'll assume that it's 
> a non-issue  :-)

You can remove the copy constructor if you want by the usual means of 
making it private and providing no implementation. I admit I tend to do 
that whenever I want to forbid copy construction ( as well as assignment 
). But I rarely have to code for the vagaries of many compilers as Boost 
implementations have to do.

> 
>> I favor 2), although 1) is fine also. I see little reason for the
>> templated constructor in practical use, especially since the current
>> usage of value_initialized does not consider conversions to and from
>> the T type in any of its functionality.
> 
> It /might/ be nice to allow conversion from U to value_initialized<T> 
> whenever there is a conversion from U to T.  But if nobody needs it, 
> let's forget option [4] and [5] for the time being.
> 
> It would be helpful if you create a ticket regarding your request: 
> https://svn.boost.org/trac/boost/newticket

I created the ticket. BTW value_initialized is not listed among the 
components one can choose, although it should be, so I chose "utility" 
instead. Hopefully it will be assigned to you or Fernando.