Re: hashtable local iterator

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 19/12/2011, François Dumont wrote:
>
>      I have however a remark regarding this patch. For the moment, on my
> 64bits platform, sizeof(local_iterator) is 32 while it should normally
> be 24 for the 2 size_t and the node pointer. I thought about using
> inheritance in _Hash_code_base type in replacement of _M_extract, _M_h1,
> _M_h2, _M_hash fields to benefit from EBO, would it be ok ? This way I
> could replace the static assertion:
>
>        static_assert(__or_<integral_constant<bool, __cache_hash_code>,
>                            __and_<is_empty<_H1>, is_empty<_H2>,
>                          is_empty<_Hash>, is_empty<_ExtractKey>>>::value,
>              "Cache the hash code or make _ExtractKey, _H1 and _Hash
> functors"
>              " empty");
>
> with the more accurate:
>
>        static_assert(__or_<integral_constant<bool, __cache_hash_code>,
>                                        is_empty<_HCBase>>::value,
>              "Cache the hash code or make _ExtractKey, _H1 and _Hash
> functors"
>              " empty");
>
> because H1, H2 and Hash are never used all together.
>
> I already noticed that I couldn't inherit from both _Hash and _Equal
> because those 2 types might be identical resulting in an ambiguity. But
> between _ExtractKey, _H1, _H2 and _Hash there is no chance of ambiguity.

If you use std::tuple to store the members it will automatically use
EBO if possible.

If two types might be the same use std::tuple<X,Y,X> where X might be
empty and Y is not empty, that ensures the two X objects will not have
the same address so can have zero size.