Re: [boost] [optional] Changes in Boost.Optional

Subject: Re: [boost] [optional] Changes in Boost.Optional
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2014-08-24 14:20:22

[Corrected subject to target optional.]

On August 24, 2014 7:55:11 AM EDT, Dean Michael Berris <mikhailberis_at_[hidden]> wrote:
>I realize that this message may have not passed the spam filters.
>
>Can someone comment on the changes in Boost.Optional in 1.56.0 breaking
>existing code?
>On Mon Aug 18 2014 at 8:10:52 PM Dean Michael Berris
><dberris_at_[hidden]>
>wrote:
>
>> Suppose I have the following type:
>>
>> struct container {
>>   boost::optional<uint16_t> optional() const {
>>     return value_;
>>   }
>>  private:
>>   boost::optional<uint16_t> value_;
>> };
>>
>> struct value_wrapper {
>>   value_wrapper(const container& c) : c_(c) {}
>>   operator uint16_t () { return 1; }
>>   operator boost::optional<uint16_t>() { return c_.optional(); }
>>  private:
>>   const container& c_;
>> };
>>
>> And that I have the following function:
>>
>> value_wrapper value(const container& c) {
>>   return value_wrapper(c);
>> }
>>
>> The following used to work in MSVC 2010:
>>
>> container c;
>> // leave c.value_ alone.
>> boost::optional<uint16_t> some_value =
>>   value(c);
>> assert(!some_value);
>>
>> Now with Boost 1.56, this breaks because the constructor for
>> some_value from a temporary optional is now marked explicit.
>>
>> What is the work-around for the previous behavior to be regained? All
>> the following changes cause the assertion to fail:
>>
>> boost::optional<uint16_t> some_value(value(c));
>> assert(!some_value);  // some_value's constructor will use the
>> uint16_t conversion operator, therefore some_value == 1
>>
>> boost::optional<uint16_t> some_value =
>> static_cast<boost::optional<uint16_t> >(value(c));
>> assert(!some_value);  // same problem as above, the temporary created
>> will favor the uint16_t conversion operator
>>
>> Help?
>>
>> PS. For better context, see
>> https://github.com/cpp-netlib/cpp-netlib/blob/0.11-devel/
>> boost/network/protocol/http/algorithms/linearize.hpp#L145

___
Rob

(Sent from my portable computation engine)