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

Subject: [Boost-users] How to implement a custom hash function for __uint128_t?
From: Scott Smedley (scott_at_[hidden])
Date: 2012-12-09 18:37:59

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. :)