pb_ds gp_hash_table resize strategy

pb_ds gp_hash_table resize strategy

[CH HC]

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Hi there,
In libstdc++/ext/pb_ds , I noticed that the (automatic) resize function for gp_hash_table does copy-and-destruct operations on old key-value pair (from the line resize_no_store_hash_fn_imps.hpp:58). Meanwhile, cc_hash_table simply moves the old pair as expected.
So for gp_hash_table, this strategy makes it impossible to have non-copyable values, e.g. __gnu_pbds::gp_hash_table<int, std::unique_ptr<int>>. And there exists potential bugs for scenarios like, keeping a gp_hash_table<int, std::list<int>> and saving some iterators of those lists elsewhere for quick access to specific elements in lists.
There could be some workarounds like using std::shared_ptr<T> as value type, but not so perfect.
I wonder if this is caused by some implementation difficulty or just a small bug?
Yours,
chhc

Re: pb_ds gp_hash_table resize strategy

[Jonathan Wakely]

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On Fri, 21 Jun 2024, 02:28 CH HC via Gcc, <gcc@gcc.gnu.org> wrote:

> Hi there,
> In libstdc++/ext/pb_ds , I noticed that the (automatic) resize function
> for gp_hash_table does copy-and-destruct operations on old key-value pair
> (from the line resize_no_store_hash_fn_imps.hpp:58). Meanwhile,
> cc_hash_table simply moves the old pair as expected.
> So for gp_hash_table, this strategy makes it impossible to have
> non-copyable values, e.g. __gnu_pbds::gp_hash_table<int,
> std::unique_ptr<int>>. And there exists potential bugs for scenarios like,
> keeping a gp_hash_table<int, std::list<int>> and saving some iterators of
> those lists elsewhere for quick access to specific elements in lists.
> There could be some workarounds like using std::shared_ptr<T> as value
> type, but not so perfect.
> I wonder if this is caused by some implementation difficulty or just a
> small bug?
> Yours,
> chhc
>

Hi, this question would be more appropriate on the libstdc++@gcc.gnu.org
making list. I've Cc'd that list. Please take the gcc list out of any
future replies, thanks.

The pb_ds code was written pre-C++11 and not updated for move semantics.
All that code is completely unmaintained and my preference would be to
remove it completely.