Re: [boost] [optional] little addition... optional_move_to?

Subject: Re: [boost] [optional] little addition... optional_move_to?
From: Fernando Cacciola (fernando.cacciola_at_[hidden])
Date: 2009-03-06 08:27:30

Hi Niels,

>>> Well, we can achieve almost the same in old C++03, by declaring the
>>> function as a /member/ of optional<T>.  I'd suggest naming it
>>> "optional_move_to", as follows:
> 
> Fernando Cacciola wrote:
>> Nice one! :)
> 
> Thanks  :-)
> 
>> I keep wondering whether being member functions, these move_to() and
>> swap() need the "optional_" name prefix?
> 
> In the case of "optional_move_to", "move_to" is okay to me as well, as 
> "move_to" isn't so much "overloaded".  (Although I still think that 
> "optional_move_to", or even "conditional_move_to", is more clear.)
> 
>> On further thinking, if the optional is an lvalue this would do the
>> right thing but will have the wrong name o)
> 
> If "move_to" does either swap or assign-to, IMO the name is right, even 
> if it moves from an lvalue.

What I really meant is that, with an lvalue, most likely you don't 
*want* move but copy.

> 
>> Or, probably better, a single [optional_]assign_to() that magically
>> does one or the other depending on "this" being and lvalue or rvalue
>> reference
>>
>> Something which, btw, in C++0x can actually be properly overloaded:
>>
>>   template<class U> bool optional<T>::assign_to( U& ) & ;
>>   template<class U> bool optional<T>::assign_to( U& ) && ;
>>
>> This is getting unsurprisingly closer to the initial proposal from
>> Arno :)
> 
> What about adding a "const" overload as well?  ;-)
> 
>  template<class U> bool optional<T>::assign_to( U& ) const & ;
> 
;) indeed

> Maybe Ion Gaztañaga's Boost.Move can be of help to emulate move 
> semantics in C++03? 
> http://svn.boost.org/svn/boost/sandbox/libs/move_semantics/doc/html/index.html 
> 

I'm sure it could help, but I wouldn't like adding such a depedency into 
  Boost.Optional... but maybe I'm over reacting o)

> 
> Anyway, for the time being I think that adding optional_move_to(T&) and 
> optional_swap(T&) members (as I presented), and optional_assign (by 
> Arno) should be sufficient.
 >

Let's see:

With swap(), move sermantics are aways welcome for both l-and-rvalues, 
so an optional_swap(T&) member makes sense.

When assigning from an rvalue, move-assign is aways welcome, while when 
assigning from an lvalue you want move only by explicit request.  So, an 
assign_to() API that won't ever move from an lvalue but will always try 
to with an rvalue also makes sense.. and it must be a member function 
nowadays to potentially move from rvalues.

A member move_to() also makes sense when you explicitely want to move 
even from an rvalue.

So.. I have the same list as you except for optional_assign(), which 
should be a member to make moving from rvalues possible.

That is:

1)

o.assign_to(v)

is the counterpart of:

v = o ;

which would move IFF 'o' is an rvalue.

2)

o.move_to(v)

is the counterpart of:

v = std::move(o)

which always moves.

Best

--
Fernando Cacciola
SciSoft Consulting, Founder
http://www.scisoft-consulting.com