Re: [Boost-users] How to implement a custom hash function for __uint128_t?

Subject: Re: [Boost-users] How to implement a custom hash function for __uint128_t?
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-12-09 22:18:22

On Sun, Dec 9, 2012 at 3:37 PM, Scott Smedley <scott_at_[hidden]>wrote:

> Hi all,
>
> How can I implement a hash function for a __uint128_t type that is
> compatible with Boost?
>
> I tried this:
>
> #include <boost/functional/hash.hpp>
> #include <unordered_set>
>
> #ifdef USE_64
> typedef uint64_t MyId; // works fine
> #else
> typedef __uint128_t MyId;
> #endif
>
> inline std::size_t hash_value (const MyId &x)
> {
>     // not a very good hash function, but I just want to get it working first!
>     return static_cast<std::size_t>(x);
> }
>
> int main (int argc, char *argv[])
> {
>     std::unordered_set<MyId, boost::hash<MyId>> s;
>     s.insert(42);
> }
>
> but I get 144 lines of errors (!) using g++-4.7.2 with Boost v1.51.0 on 64-bit Scientific Linux 6.2:
>
> error: no matching function for call to 'hash_value(const __int128 unsigned&)'
>
> Any pointers/help would be muchly appreciated.
>
> Scott. :)
>
>
Try rearranging and putting the hash inclusion after your custom definition
of hash_value? I'm guessing since your custom hash_value isn't findable by
ADL, you'd need it declared prior to its use with hash.hpp...?

- Jeff