On Sun, 14 Nov 2021 at 13:31, François Dumont via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> libstdc++: Unordered containers merge re-use hash code.
>
> When merging between 2 unordered containers with same hasher we can
> re-use
> the cached hash code if any.
Instead of introducing the _ReuseOrComputeHash type, wouldn't it be
simpler to just overload _M_hash_code?
// Same hash function, use the cached hash code.
__hash_code
_M_hash_code(const _Hash&,
const _Hash_node_value<_Value, true>& __n) const
{ return __n._M_hash_code; }
// Compute hash code using a different hash function, _H2
template<typename _H2>
__hash_code
_M_hash_code(const _H2&,
const _Hash_node_value<_Value, __cache_hash_code>& __n) const
{ return this->_M_hash_code(_ExtractKey{}(__n._M_v()); }
The first overload is more specialized, so will be chosen when the
first argument is the same type as _Hash and the cache_has_code
boolean is true.
