Re: Mother of all Variants

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2019-06-27 08:24:56

Am 27.06.19 um 10:13 schrieb degski:
> On Thu, 27 Jun 2019 at 11:01, Alexander Grund via Boost 
> <boost_at_[hidden] <mailto:boost_at_[hidden]>> wrote:
>
>
>     On how this is possible: Imagine `std::list` as a simple linked list
>     (pointer to (element, pointer-to-next)-tuples).
>     On move, you simply move the HEAD pointer to the new list and a
>     NULL to
>     the old list.
>
>     Result: Pointers/References to all elements are still valid. Same for
>     e.g. vectors: Only pointers to array are swapped -> Unchanged element
>     pointers
>
>
> Â How does that gel with: "Some implementations of std::list throw from 
> their move constructors" (David Sankel) ..." [because I would think 
> the same as you]? Yes, swap head around, and if you really want 
> safety, you can use a xor-swap of the [those] pointer[s], i.e. 
> guaranteed no allocation (stack or heap).

He also wrote: "I understand that the implementation is easier if even 
an empty list has a dummy node, which requires allocation."

So a move constructor must construct a dummy node for the moved-from 
list and that allocation might throw. I don't know WHY one would require 
this, but I'll just take his word that it MAY be required.