Subject: Re: [boost] Adding polymorphic_value to boost
From: Peter Dimov (lists_at_[hidden])
Date: 2017-11-21 15:56:49
Mathias Gaunard wrote:
> On 21 November 2017 at 10:50, Jonathan Coe <jonathanbcoe_at_[hidden]> wrote:
> > Thanks Matthias,
> > Is there a way we can search the archives for the old discussion?
> 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:
It's "grin_ptr" because Pimpl was known as Chesire's Cat in those days for
My own impl_ptr from 2001 can be found here:
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
(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<shared_ptr> 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 performance-sensitive uses, instead of vector<poly_value<T>> we now have
base_collection<T> in poly_collection:
which is significantly more efficient.
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