Re: [boost] Adding polymorphic_value to boost

Mathias Gaunard wrote:

On 21 November 2017 at 10:50, Jonathan Coe wrote:

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Thanks Matthias,

Is there a way we can search the archives for the old discussion?

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Here are a few I've found

https://lists.boost.org/Archives/boost/2000/03/2696.php (2000) https://lists.boost.org/Archives/boost/2007/09/126982.php (2007) https://lists.boost.org/Archives/boost/2010/05/166647.php (2010)

The earliest deep-copy pointer is Alan Griffiths's grin_ptr: http://web.archive.org/web/20110907102434/http://www.octopull.demon.co.uk/ar... It's "grin_ptr" because Pimpl was known as Chesire's Cat in those days for some reason. My own impl_ptr from 2001 can be found here: http://pdimov.com/cpp2/impl_ptr.zip Again, impl_ptr because its primary purpose is to implement the Pimpl idiom, but in that same archive there's a variant<T> class which is a polymorphic value. (Both Pimpl and polymorphic values need const propagation.) A copying pointer has been re-proposed repeatedly, but it didn't take. Polymorphic values also didn't become very popular, perhaps because in C++03 when you push_back one into a vector you pay dearly in reallocations. This pushes one towards implementing copy-on-write, for which shared_ptr is better suited, or often you're even fine with vector and Javaesque code that only deep copies on an explicit copy() call. Note that for such a type to be usable for Pimpl, it has to support incomplete types. Meaning, you have to be able to copy and destroy polymorphic_value<T> for incomplete T. If you search for impl_ptr or grin_ptr, you'll find a number of old threads. Or, if you find the `any` formal review thread, you'll see me going on and on about something called ref<T>, which is like polymorphic_value<T>, bit with == and <. (And making the point that `any` is a special case of that for T=void.) For performance-sensitive uses, instead of vector we now have base_collection<T> in poly_collection: http://www.boost.org/doc/libs/1_65_1/doc/html/poly_collection.html which is significantly more efficient.