unordered_map with gregorian dates

Question

I would like to store boost::gregorian::date as key of a boost::unordered_map but I cannot compile the code as it is missing a proper hash function for this class.

1. An easy solution would be converting to std::string and store it. I possibly would like to avoid this solution as using string is quite expensive.
2. I tried to find some function exporting a date to number but I can read only the day() function and I am not sure if this is really suitable.
3. Maybe I can calculate the number of days between my date and a reference date?

Is there any other better way to store date or a function exporting date as number?

Answer 1

Implement the hash function for it:

namespace boost { namespace gregorian {

inline size_t hash_value(date const& date)
{
    return boost::hash_value(date.julian_day());
}

} } // boost::gregorian

julian_day is simply the day index since Julian epoch start (whatever that is).