> The outcome is that libstdc++’s unordered_set has performance characteristics that subtly depend on the true name and noexceptness of the hash function.
> - A user-defined struct H : std::hash<std::string> {} will see smaller allocations and more calls to the hash function, than if you had just used std::hash<std::string> directly. (Because std::hash<std::string> is on the blacklist and H is not.)
> - A user-defined hasher with size_t operator() const noexcept will see smaller allocations and more calls to the hash function (especially during rehashing). One with size_t operator() const will see larger allocations and fewer calls to the hash function.
> The outcome is that libstdc++’s unordered_set has performance characteristics that subtly depend on the true name and noexceptness of the hash function.
> - A user-defined struct H : std::hash<std::string> {} will see smaller allocations and more calls to the hash function, than if you had just used std::hash<std::string> directly. (Because std::hash<std::string> is on the blacklist and H is not.)
> - A user-defined hasher with size_t operator() const noexcept will see smaller allocations and more calls to the hash function (especially during rehashing). One with size_t operator() const will see larger allocations and fewer calls to the hash function.