Re: libstdc++ PR 57272 Fancy pointer support in Hashtable

["François Dumont" <frs.dumont@gmail.com>]

On 20/10/20 1:04 pm, Jonathan Wakely wrote:
> On 28/09/20 22:37 +0200, François Dumont via Libstdc++ wrote:
>> Following recent changes on _Hashtable I rebase the patch and 
>> completely review it.
>>
>> I managed to integrate the allocator custom pointer type without 
>> touching to _Hashtable base types like _Hash_code_base or 
>> _Hashtable_base. However I cannot see how to use the custom pointer 
>> type without impacting the node types like _Hash_node_base which now 
>> takes a template parameter, the custom pointer type.
>>
>> On an abi point of view node types are different however the data 
>> structure is the same. The only difference is that the 
>> _Hash_node_base _M_nxt is now a _Hash_node<> custom pointer rather 
>> than a simple _Hash_node_base*.
>>
>> Even if this patch can't go in because of the abi breaking change I 
>> am going to adapt some of the code simplifications for master. 
>> Especially the _Hash_code_base and _Local_iterator_base simplifications.
>>
>> Let me know if you can think of a way to integrate the custom pointer 
>> without impacting abi. Unless impacting node types and associated 
>> iterator types is fine even if I already noticed that pretty printer 
>> tests are broken with those changes.
>
> The approach I used for the other containers (which was never
> completed and committed) is something like:
>
> struct _Node_base
> {
>   _Node_base* _M_next;
> };
>
> template<typename _Ptr>
> struct _Fancy_node_base
> {
>   _Ptr _M_next;
> };
>
> template<typename _Ptr>
>   using node_base = conditional_t<is_pointer<_Ptr>::value,
>                                   _Node_base,
> _Fancy_node_base<_Ptr>>;
>
> This way all existing code that has allocators with non-fancy pointers
> continues to use the same type. Code using fancy pointers (which
> doesn't currently work properly anyway) changes to use the new types
> that depend on the pointer type.
>
Ok, thanks for the tips, I'll do something similar.