Re: [boost] [optional] Why was optional<T>::reset() deprecated?

Subject: Re: [boost] [optional] Why was optional<T>::reset() deprecated?
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2016-03-03 08:10:44

2016-03-03 13:23 GMT+01:00 Vicente J. Botet Escriba <
vicente.botet_at_[hidden]>:

> Le 02/03/2016 12:30, Andrzej Krzemienski a écrit :
>
>> 2016-03-02 12:21 GMT+01:00 Beman Dawes <bdawes_at_[hidden]>:
>>
>> On Wed, Mar 2, 2016 at 3:20 AM, Andrey Semashev <
>>> andrey.semashev_at_[hidden]
>>> wrote:
>>>
>>> On 2016-03-02 01:51, Andrzej Krzemienski wrote:
>>>>
>>>> At some earlier stages of development, boost::optional did not have the
>>>>> assignment from boost::none_t (and probably also from T); and reset was
>>>>> the
>>>>> only way to efficiently change the optional from the state of having
>>>>> the
>>>>> value to the state of not having the value.
>>>>>
>>>>> After the addition of more fancy syntax (conversion to bool, assignment
>>>>> from none_t), reset() (and is_initialized()) became redundant, and
>>>>> hence
>>>>> the deprecation.
>>>>>
>>>>> I got that from reading Boost mailing archives once.
>>>>>
>>>>> FWIW, I don't think that having support for 'none' is enough reason to
>>>> deprecate 'reset' (same for 'is_initialized'). I know some of my
>>>>
>>> colleagues
>>>
>>>> who prefer these methods to the fancy syntax because it feels more
>>>>
>>> aligned
>>>
>>>> with other Boost and standard library components. Having 'reset' also
>>>> allows not to include boost/none.hpp.
>>>>
>>>> The standards committee's library enhancements working group (LEWG)
>>> agrees
>>> with you. Nevin's question came up in the context of aligning several
>>> Library Fundamentals TS functions with each other and the rest of the
>>> standard library. The proposal is progressing and will likely be part of
>>> C++17.
>>>
>>
>> P0032 <
>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0032r1.pdf>
>> mentions reset() with no arguments. I can un-deprecate this overload when
>> the proposal is accepted. I am not sure about the overload with T: we have
>> a more generalized emplace() for this purpose.
>>
>>
>> Yes, reset() is not an assignment and std::optional::emplace do what
> boost::optional::reset(T) does, isn't it?
>

I was incorrect.

  opt_v.reset(v);

is in fact equivalent to:

  opt_v = v;

I guess that the two reset's were deprecated in favor of two assignments:

  opt_v.reset();
  opt_v.reset(v);

is equivalent to:

    opt_v = none;
    opt_v = v;

Regards,
&rzej