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

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2013-01-14 02:09:26

2013/1/13 Vicente J. Botet Escriba <vicente.botet_at_[hidden]>:
> Le 13/01/13 09:58, Antony Polukhin a écrit :
>
>> 2013/1/6 Joel de Guzman <djowel_at_[hidden]>:
>>>
>>> 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:
>>
>> ...
>>
>> I`ve finally got back from vacation, so let me add my 2 kopecks.
>>
>> We can make move constructor fast and noexcept (just like Joel
>> proposed) and also guarantee non-empty behavior.
>> The idea is to delay construction of object:
>>
>> T& get() {
>>      if (!get_pointer()) p_ = new T;
>>      return *get_pointer();
>> }
>>
>> const T& get() const {
>>      if (!get_pointer()) p_ = new T; // mark p_ as mutable
>>      return *get_pointer();
>> }
>>
>> So if move occurred object is empty, and if it is required again - it
>> is default constructed.
>>
>> Any objections?
>>
> This would make less efficient the get operation. I would prefer to force
> the boost::blank default construction.

I see no big difference between "BOOST_ASSERT(p_);" and "if (p_) p_ =
new T;" except default construction requirement for T. Construction
and "new" call will occur only if recursive_wrapper is being reused
after move, which must be a really rare case.

boost::blank is relevant to boost:variant, not to recursive_wrapper:
boost::variant<boost::blank, boost::recursive_wrapper<foo> >
var(std::move(variable));
// Will call recursive_wrapper(recursive_wrapper&&) if `variable` is a
recursive_wrapper type or boost::blank(boost::blank&&) if `variable`
is a boost::blank

--
Best regards,
Antony Polukhin