Re: [boost] [optional] Specializing optional to save space

On 28.09.2015 00:47, Nevin Liber wrote:

On 27 September 2015 at 16:19, Andrey Semashev wrote:

...

I got the impression that the OP intended that the specialized optional<> still controlled the lifetime of the adopted object the way it does now. The proposed change basically offloads engagement checking to a user-specializable trait but really nothing more than that.

His example was to use a specific value of std::string::size() as a sentinel. How does that work with his specialized optional if the specialization is orthogonal to lifetime control?

Excerpt from the OP:

For instance, for common implementations of std::string you cannot have std::string::size() == std::numeric_limitsstd::string::size_type::max() *. We can take advantage of this if our std::string implementation stores the capacity as an integer value, rather than a pointer to the end of the storage.

From this I gather that the intention is to use the storage from the capacity member of std::string to store the discriminator. I assume, the std::string object has to be not constructed for this as in no way you can create such string through its interface. But maybe I'm seeing too much here - I'll let David clarify this himself.