Re: [boost] [variant] Basic rvalue and C++11 features support

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Vicente Botet (vicente.botet_at_[hidden])
Date: 2013-01-11 08:48:15

Hartmut Kaiser wrote
>> On 1/8/13 4:14 AM, Paul Smith wrote:
>> > Joel de Guzman wrote:
>> >> Hi,
>> >>
>> >> I just looked at the current state of variant and noticed that the
>> >> implementation of recursive_variant's move ctor seems to be not as
>> >> optimal as I hoped.
>> >> The
>> >> current implementation as written is:
>> >>
>> >>    recursive_wrapper
> <T>
> ::recursive_wrapper(recursive_wrapper&& operand)
>> >>      : p_(new T( detail::variant::move(operand.get()) ))
>> >>   {
>> >>   }
> 
>> Do you see something that I don't?
>> 
>> Without this move optimization (as it currently is), it is very
>> inefficient especially with big structures (e.g. tuples and fusion
>> adapted
>> structs).
>> Without this optimization, such temporary copies will end up with two
>> heap
>> allocations and unnecessary copying of the structures, instead of one
>> heap
>> allocation and a simple pointer swap. That would mean the missed
>> optimization in the order of magnitudes with applications that use
>> variant
>> heavily (e.g. Spirit).
> 
> I agree 100% with Joel. Move construction means move construction - i.e.
> the
> source object is by definition left in a zombie state. No harm done.
> What's the point in having a move constructor which essentially is
> equivalent to a copy constructor in the first place?

I have not see the code using recursive_wrapper. Do we really need
recursive_wrapper to be movable? if yes, in which cases this optimization is
a must have?

Best, 
Vicente

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