Re: [boost] Adding polymorphic_value to boost

On 20 Nov 2017, at 16:57, Peter Dimov via Boost wrote:

Jonathan Coe wrote:

...

The pointer constructor is needed in migration paths from class heirarchies with a `Clone()` method. It also allows one to support objects coming from C_APIs.

Supporting

X* create(...); X* copy( X const* ); void destroy( X* );

is appropriate for a cloning pointer, but hasn't really much to do with polymorphic values.

struct Shape { virtual ~Shape() = default; Shape* clone() const = 0; void draw() = 0; };

Shape* createCircle( int r );

is admittedly less clear-cut.

Looking at my old code, what I did was supply both a cloning pointer (U*) and a polymorphic value (U&&) types, the latter a simple wrapper over the former.

This was my original design too which was strongly opposed by LEWG.

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