unordered_map with valarray as key

[David Fang]

>> However, nothing stops one from providing a specialization of
>> std::hash<std::valarray<> >.  You could look at hash<std::string>  ro
>> hash<const char*> as a starting point.
>
> That's not true, it's undefined behaviour to specialize templates from
> namespace std unless the specialization depends on one or more
> user-defined types, so it is not allowed to specialize
> std::hash<std::valarray<long>>.  The alternative is to define your own
> hash function (not a specialization of std::hash) and use that with
> the unordered container.

Yes, passing a custom Hash to unordered_map is much 
cleaner,safer,more-legal than polluting std::.

Fang