Re: [boost] Adding polymorphic_value to boost

On 23 Nov 2017, at 15:24, Thorsten Ottosen via Boost wrote:

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Den 23-11-2017 kl. 00:31 skrev Phil Endecott via Boost:

There's another way of implementing this, though; rather than pointing to the object in the heap you can store it in-place by allocating enough space for the largest possible derived subclass:

[snip]

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That's just a sketch; many details omitted! There are some obvious advantages and disadvantages of each approach and I'm definitely not saying this is better than the pointer-to-heap design.

That's going to use way too much space, make swap/move slow and possibly noexcept(false), give poor reference stability. For that case of not heap-allocating each object in isolation, boost::poly_collection seems much better. So I would stick to the allocate on the heap approach.

kind regards

-Thorsten

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One can limit the size of the buffer to make it tolerable (like std::function does) and only use the buffer for types than are noexcept-moveable. Reference stability is not a big concern to me (given the intended uses of the api) but is probably worth documenting. My proposed addition does not have a small object optimisation but I would not wish to rule out adding one.