Re: [boost] Changes in Boost.Optional

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 wrote:

Suppose I have the following type:

struct container { boost::optional optional() const { return value_; } private: boost::optional value_; };

struct value_wrapper { value_wrapper(const container& c) : c_(c) {} operator uint16_t () { return 1; } operator boost::optional() { 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 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 some_value(value(c)); assert(!some_value); // some_value's constructor will use the uint16_t conversion operator, therefore some_value == 1

boost::optional some_value = static_cast >(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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