[PATCH] libstdc++: Compare keys and values separately in flat_map::operator==

[Patrick Palka]

On Thu, 29 May 2025, Tomasz Kaminski wrote:

> 
> 
> On Thu, May 29, 2025 at 4:37 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> 
> 
> On Thu, May 29, 2025 at 3:56 PM Patrick Palka <ppalka@redhat.com> wrote:
>       Tested on x86_64-pc-linux-gnu, does this look OK for trunk/15?
> 
>       -- >8 --
> 
>       Instead of effectively doing a zipped comparison of the keys and values,
>       compare them separately to leverage the underlying containers' optimized
>       equality implementations.
> 
>       libstdc++-v3/ChangeLog:
> 
>               * include/std/flat_map (_Flat_map_impl::operator==): Compare
>               keys and values separately.
>       ---
>        libstdc++-v3/include/std/flat_map | 5 ++++-
>        1 file changed, 4 insertions(+), 1 deletion(-)
> 
>       diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
>       index c0716d12412a..134307324190 100644
>       --- a/libstdc++-v3/include/std/flat_map
>       +++ b/libstdc++-v3/include/std/flat_map
>       @@ -873,7 +873,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>              [[nodiscard]]
>              friend bool
>              operator==(const _Derived& __x, const _Derived& __y)
>       -      { return std::equal(__x.begin(), __x.end(), __y.begin(), __y.end()); }
>       +      {
>       +       return __x._M_cont.keys == __y._M_cont.keys
>       +         && __x._M_cont.values == __y._M_cont.values;
> 
> Previously we supported containers that do not have operator==, by calling equal.
> For the flat_set we also do not compare the containers. I would suggest using in both:
>   ranges::equal(x._M_cont)
> Or using == on containers in both flat_map and flat_set.
> 
> queue and stack uses operator== for the containers, so I think  we should use == on containers in both.

Using operator== in flat_set for consistency with the other container
adaptors makes sense to me.

>             +      }
> 
>                    template<typename _Up = value_type>
>                     [[nodiscard]]
>             --
>             2.50.0.rc0
> 
> 
>