The following code example is taken from the book
The C++ Standard Library - A Tutorial and Reference, 2nd Edition
by Nicolai M. Josuttis, Addison Wesley Longman, 2012
Copyright © 2012 by Pearson Education, Inc. and Nicolai M. Josuttis
#include <functional>
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html
template <typename T>
inline void hash_combine (std::size_t& seed, const T& val)
{
seed ^= std::hash<T>()(val) + 0x9e3779b9 + (seed<<6) + (seed>>2);
}
// auxiliary generic functions to create a hash value using a seed
template <typename T>
inline void hash_val (std::size_t& seed, const T& val)
{
hash_combine(seed,val);
}
template <typename T, typename... Types>
inline void hash_val (std::size_t& seed,
const T& val, const Types&... args)
{
hash_combine(seed,val);
hash_val(seed,args...);
}
// auxiliary generic function to create a hash value out of a heterogeneous list of arguments
template <typename... Types>
inline std::size_t hash_val (const Types&... args)
{
std::size_t seed = 0;
hash_val (seed, args...);
return seed;
}