From cppreference.com
friendbooloperator==(conststd::flat_multimap&lhs,conststd::flat_multimap&rhs); (1) (since C++23)friendsynth-three-way-result<value_type>operator<=>(conststd::flat_multimap&lhs,conststd::flat_multimap&rhs); (2) (since C++23) Compares the contents of the underlying containers of two container adaptors. The comparison is done by applying the corresponding operator to the underlying containers.
Parameters
lhs, rhs - container adaptors whose contents to compare Return value
1)true if the corresponding comparison yields true, false otherwise.
2) Result of three-way comparison on underlying containers.
Complexity
1) Constant if lhs and rhs are of different size, otherwise linear in the size of the flat_multimap.
2) Linear in the size of the container.
Example