Re: [boost] Changes in Boost.Optional

Subject: Re: [boost] Changes in Boost.Optional
From: Dean Michael Berris (mikhailberis_at_[hidden])
Date: 2014-08-24 07:55:11

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
>
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