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.

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.
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.
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?

For performance reason of course..map is a binary tree and unordered_map access is O(1) — Abruzzo Forte e Gentile, Jun 06 '14 at 14:58
you can get the julian day from a boost date, and there are plenty of ways to get a string representation. IS that what you are asking for? — sam, Jun 06 '14 at 15:11
.mm I was asking to converting it into a number instead of a string as using string is more expensive — Abruzzo Forte e Gentile, Jun 06 '14 at 15:21

Maxim Egorushkin · Accepted Answer · 2014-06-06T15:47:33.500

9

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).

edited Jun 06 '14 at 15:47

answered Jun 06 '14 at 15:19

Maxim Egorushkin

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Is there a way to construct the boost::gregorian::date from juilian_day? I don't see it in the boost reference. – Behrooz Karjoo Jun 21 '17 at 14:56

unordered_map with gregorian dates

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