robot/include/boost/locale/date_time.hpp

//
// Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt

#ifndef BOOST_LOCALE_DATE_TIME_HPP_INCLUDED
#define BOOST_LOCALE_DATE_TIME_HPP_INCLUDED

#include <boost/locale/date_time_facet.hpp>
#include <boost/locale/formatting.hpp>
#include <boost/locale/hold_ptr.hpp>
#include <boost/locale/time_zone.hpp>
#include <array>
#include <locale>
#include <stdexcept>
#include <vector>

#ifdef BOOST_MSVC
#    pragma warning(push)
#    pragma warning(disable : 4275 4251 4231 4660)
#endif

namespace boost { namespace locale {
    /// \defgroup date_time Date, Time, Timezone and Calendar manipulations
    ///
    /// This module provides various calendar, timezone and date time services
    /// @{

    /// \brief This error is thrown in case of invalid state that occurred
    class BOOST_SYMBOL_VISIBLE date_time_error : public std::runtime_error {
    public:
        /// Constructor of date_time_error class
        date_time_error(const std::string& e) : std::runtime_error(e) {}
    };

    /// \brief This class represents a pair of period_type and the integer
    /// values that describes its amount. For example 3 days or 4 years.
    ///
    /// Usually obtained as product of period_type and integer or
    /// my calling a representative functions
    /// For example day()*3 == date_time_period(day(),3) == day(3)
    struct date_time_period {
        period::period_type type; ///< The type of period, i.e. era, year, day etc.
        int value;                ///< The value the actual number of \a periods
        /// Operator + returns copy of itself
        date_time_period operator+() const { return *this; }
        /// Operator -, switches the sign of period
        date_time_period operator-() const { return date_time_period(type, -value); }

        /// Constructor that creates date_time_period from period_type \a f and a value \a v -- default 1.
        date_time_period(period::period_type f = period::period_type(), int v = 1) : type(f), value(v) {}
    };

    namespace period {
        ///  Get period_type for: special invalid value, should not be used directly
        inline period_type invalid()
        {
            return period_type(marks::invalid);
        }
        ///  Get period_type for: Era i.e. AC, BC in Gregorian and Julian calendar, range [0,1]
        inline period_type era()
        {
            return period_type(marks::era);
        }
        ///  Get period_type for: Year, it is calendar specific, for example 2011 in Gregorian calendar.
        inline period_type year()
        {
            return period_type(marks::year);
        }
        ///  Get period_type for: Extended year for Gregorian/Julian calendars, where 1 BC == 0, 2 BC == -1.
        inline period_type extended_year()
        {
            return period_type(marks::extended_year);
        }
        ///  Get period_type for: The month of year, calendar specific, in Gregorian [0..11]
        inline period_type month()
        {
            return period_type(marks::month);
        }
        ///  Get period_type for: The day of month, calendar specific, in Gregorian [1..31]
        inline period_type day()
        {
            return period_type(marks::day);
        }
        ///  Get period_type for: The number of day in year, starting from 1, in Gregorian  [1..366]
        inline period_type day_of_year()
        {
            return period_type(marks::day_of_year);
        }
        /// Get period_type for: Day of week, Sunday=1, Monday=2,..., Saturday=7.
        ///
        /// Note that updating this value respects local day of week, so for example,
        /// If first day of week is Monday and the current day is Tuesday then setting
        /// the value to Sunday (1) would forward the date by 5 days forward and not backward
        /// by two days as it could be expected if the numbers were taken as is.
        inline period_type day_of_week()
        {
            return period_type(marks::day_of_week);
        }
        ///  Get period_type for: Original number of the day of the week in month. For example 1st Sunday,
        /// 2nd Sunday, etc. in Gregorian [1..5]
        inline period_type day_of_week_in_month()
        {
            return period_type(marks::day_of_week_in_month);
        }
        ///  Get period_type for: Local day of week, for example in France Monday is 1, in US Sunday is 1, [1..7]
        inline period_type day_of_week_local()
        {
            return period_type(marks::day_of_week_local);
        }
        ///  Get period_type for: 24 clock hour [0..23]
        inline period_type hour()
        {
            return period_type(marks::hour);
        }
        ///  Get period_type for: 12 clock hour [0..11]
        inline period_type hour_12()
        {
            return period_type(marks::hour_12);
        }
        ///  Get period_type for: am or pm marker [0..1]
        inline period_type am_pm()
        {
            return period_type(marks::am_pm);
        }
        ///  Get period_type for: minute [0..59]
        inline period_type minute()
        {
            return period_type(marks::minute);
        }
        ///  Get period_type for: second [0..59]
        inline period_type second()
        {
            return period_type(marks::second);
        }
        ///  Get period_type for: The week number in the year
        inline period_type week_of_year()
        {
            return period_type(marks::week_of_year);
        }
        ///  Get period_type for: The week number within current month
        inline period_type week_of_month()
        {
            return period_type(marks::week_of_month);
        }
        ///  Get period_type for: First day of week, constant, for example Sunday in US = 1, Monday in France = 2
        inline period_type first_day_of_week()
        {
            return period_type(marks::first_day_of_week);
        }

        ///  Get date_time_period for: Era i.e. AC, BC in Gregorian and Julian calendar, range [0,1]
        inline date_time_period era(int v)
        {
            return date_time_period(era(), v);
        }
        ///  Get date_time_period for: Year, it is calendar specific, for example 2011 in Gregorian calendar.
        inline date_time_period year(int v)
        {
            return date_time_period(year(), v);
        }
        ///  Get date_time_period for: Extended year for Gregorian/Julian calendars, where 1 BC == 0, 2 BC == -1.
        inline date_time_period extended_year(int v)
        {
            return date_time_period(extended_year(), v);
        }
        ///  Get date_time_period for: The month of year, calendar specific, in Gregorian [0..11]
        inline date_time_period month(int v)
        {
            return date_time_period(month(), v);
        }
        ///  Get date_time_period for: The day of month, calendar specific, in Gregorian [1..31]
        inline date_time_period day(int v)
        {
            return date_time_period(day(), v);
        }
        ///  Get date_time_period for: The number of day in year, starting from 1, in Gregorian  [1..366]
        inline date_time_period day_of_year(int v)
        {
            return date_time_period(day_of_year(), v);
        }
        ///  Get date_time_period for: Day of week, Sunday=1, Monday=2,..., Saturday=7.
        ///
        /// Note that updating this value respects local day of week, so for example,
        /// If first day of week is Monday and the current day is Tuesday then setting
        /// the value to Sunday (1) would forward the date by 5 days forward and not backward
        /// by two days as it could be expected if the numbers were taken as is.
        inline date_time_period day_of_week(int v)
        {
            return date_time_period(day_of_week(), v);
        }
        ///  Get date_time_period for: Original number of the day of the week in month. For example 1st Sunday,
        /// 2nd Sunday, etc. in Gregorian [1..5]
        inline date_time_period day_of_week_in_month(int v)
        {
            return date_time_period(day_of_week_in_month(), v);
        }
        ///  Get date_time_period for: Local day of week, for example in France Monday is 1, in US Sunday is 1, [1..7]
        inline date_time_period day_of_week_local(int v)
        {
            return date_time_period(day_of_week_local(), v);
        }
        ///  Get date_time_period for: 24 clock hour [0..23]
        inline date_time_period hour(int v)
        {
            return date_time_period(hour(), v);
        }
        ///  Get date_time_period for: 12 clock hour [0..11]
        inline date_time_period hour_12(int v)
        {
            return date_time_period(hour_12(), v);
        }
        ///  Get date_time_period for: am or pm marker [0..1]
        inline date_time_period am_pm(int v)
        {
            return date_time_period(am_pm(), v);
        }
        ///  Get date_time_period for: minute [0..59]
        inline date_time_period minute(int v)
        {
            return date_time_period(minute(), v);
        }
        ///  Get date_time_period for: second [0..59]
        inline date_time_period second(int v)
        {
            return date_time_period(second(), v);
        }
        ///  Get date_time_period for: The week number in the year
        inline date_time_period week_of_year(int v)
        {
            return date_time_period(week_of_year(), v);
        }
        ///  Get date_time_period for: The week number within current month
        inline date_time_period week_of_month(int v)
        {
            return date_time_period(week_of_month(), v);
        }
        ///  Get date_time_period for: First day of week, constant, for example Sunday in US = 1, Monday in France = 2
        inline date_time_period first_day_of_week(int v)
        {
            return date_time_period(first_day_of_week(), v);
        }

        /// Get predefined constant for January
        inline date_time_period january()
        {
            return date_time_period(month(), 0);
        }
        /// Get predefined constant for February
        inline date_time_period february()
        {
            return date_time_period(month(), 1);
        }
        /// Get predefined constant for March
        inline date_time_period march()
        {
            return date_time_period(month(), 2);
        }
        /// Get predefined constant for April
        inline date_time_period april()
        {
            return date_time_period(month(), 3);
        }
        /// Get predefined constant for May
        inline date_time_period may()
        {
            return date_time_period(month(), 4);
        }
        /// Get predefined constant for June
        inline date_time_period june()
        {
            return date_time_period(month(), 5);
        }
        /// Get predefined constant for July
        inline date_time_period july()
        {
            return date_time_period(month(), 6);
        }
        /// Get predefined constant for August
        inline date_time_period august()
        {
            return date_time_period(month(), 7);
        }
        /// Get predefined constant for September
        inline date_time_period september()
        {
            return date_time_period(month(), 8);
        }
        /// Get predefined constant for October
        inline date_time_period october()
        {
            return date_time_period(month(), 9);
        }
        /// Get predefined constant for November
        inline date_time_period november()
        {
            return date_time_period(month(), 10);
        }
        /// Get predefined constant for December
        inline date_time_period december()
        {
            return date_time_period(month(), 11);
        }

        /// Get predefined constant for Sunday
        inline date_time_period sunday()
        {
            return date_time_period(day_of_week(), 1);
        }
        /// Get predefined constant for Monday
        inline date_time_period monday()
        {
            return date_time_period(day_of_week(), 2);
        }
        /// Get predefined constant for Tuesday
        inline date_time_period tuesday()
        {
            return date_time_period(day_of_week(), 3);
        }
        /// Get predefined constant for Wednesday
        inline date_time_period wednesday()
        {
            return date_time_period(day_of_week(), 4);
        }
        /// Get predefined constant for Thursday
        inline date_time_period thursday()
        {
            return date_time_period(day_of_week(), 5);
        }
        /// Get predefined constant for Friday
        inline date_time_period friday()
        {
            return date_time_period(day_of_week(), 6);
        }
        /// Get predefined constant for Saturday
        inline date_time_period saturday()
        {
            return date_time_period(day_of_week(), 7);
        }
        /// Get predefined constant for AM (Ante Meridiem)
        inline date_time_period am()
        {
            return date_time_period(am_pm(), 0);
        }
        /// Get predefined constant for PM (Post Meridiem)
        inline date_time_period pm()
        {
            return date_time_period(am_pm(), 1);
        }

        /// convert period_type to date_time_period(f,1)
        inline date_time_period operator+(period::period_type f)
        {
            return date_time_period(f);
        }
        /// convert period_type to date_time_period(f,-1)
        inline date_time_period operator-(period::period_type f)
        {
            return date_time_period(f, -1);
        }

        /// Create date_time_period of type \a f with value \a v.
        template<typename T>
        date_time_period operator*(period::period_type f, T v)
        {
            return date_time_period(f, v);
        }

        /// Create date_time_period of type \a f with value \a v.
        template<typename T>
        date_time_period operator*(T v, period::period_type f)
        {
            return date_time_period(f, v);
        }
        /// Create date_time_period of type \a f with value \a v.
        template<typename T>
        date_time_period operator*(T v, date_time_period f)
        {
            return date_time_period(f.type, f.value * v);
        }

        /// Create date_time_period of type \a f with value \a v.
        template<typename T>
        date_time_period operator*(date_time_period f, T v)
        {
            return date_time_period(f.type, f.value * v);
        }

    } // namespace period

    /// \brief This class represents a set of periods.
    ///
    /// It is generally created by operations on periods:
    /// 1995*year + 3*month + 1*day. Note: operations are not commutative.
    class date_time_period_set {
    public:
        /// Default constructor - empty set
        date_time_period_set() = default;

        /// Create a set of single period with value 1
        date_time_period_set(period::period_type f) { basic_[0] = date_time_period(f); }

        /// Create a set of single period \a fl
        date_time_period_set(const date_time_period& fl) { basic_[0] = fl; }

        /// Append date_time_period \a f to the set
        void add(date_time_period f)
        {
            const size_t n = size();
            if(n < basic_.size())
                basic_[n] = f;
            else
                periods_.push_back(f);
        }

        /// Get number of items in list
        size_t size() const
        {
            for(size_t i = 0; i < basic_.size(); ++i) {
                if(basic_[i].type == period::period_type())
                    return i;
            }
            return basic_.size() + periods_.size();
        }

        /// Get item at position \a n the set, n should be in range [0,size)
        const date_time_period& operator[](size_t n) const
        {
            if(n >= size())
                throw std::out_of_range("Invalid index to date_time_period");
            if(n < basic_.size())
                return basic_[n];
            else
                return periods_[n - basic_.size()];
        }

    private:
        std::array<date_time_period, 4> basic_;
        std::vector<date_time_period> periods_;
    };

    /// Append two periods sets. Note this operator is not commutative
    inline date_time_period_set operator+(const date_time_period_set& a, const date_time_period_set& b)
    {
        date_time_period_set s(a);
        for(unsigned i = 0; i < b.size(); i++)
            s.add(b[i]);
        return s;
    }

    /// Append two period sets when all periods of set \b change their sign
    inline date_time_period_set operator-(const date_time_period_set& a, const date_time_period_set& b)
    {
        date_time_period_set s(a);
        for(unsigned i = 0; i < b.size(); i++)
            s.add(-b[i]);
        return s;
    }

    /// \brief this class provides an access to general calendar information.
    ///
    /// This information is not connected to specific date but generic to locale, and timezone.
    /// It is used in obtaining general information about calendar and is essential for creation of
    /// date_time objects.
    class BOOST_LOCALE_DECL calendar {
    public:
        /// Create calendar taking locale and timezone information from ios_base instance.
        ///
        /// \throws std::bad_cast: \a ios does not have a locale with installed \ref calendar_facet
        /// facet installed
        calendar(std::ios_base& ios);

        /// Create calendar with locale \a l and time_zone \a zone
        ///
        /// \throws std::bad_cast: \a l does not have \ref calendar_facet facet installed
        calendar(const std::locale& l, const std::string& zone);

        /// Create calendar with locale \a l and default timezone
        ///
        /// \throws std::bad_cast: \a l does not have \ref calendar_facet facet installed
        calendar(const std::locale& l);

        /// Create calendar with default locale and timezone \a zone
        ///
        /// \throws std::bad_cast: global locale does not have \ref calendar_facet facet installed

        calendar(const std::string& zone);

        /// Create calendar with default locale and timezone
        ///
        /// \throws std::bad_cast: global locale does not have \ref calendar_facet facet installed
        calendar();
        ~calendar();

        /// copy calendar
        calendar(const calendar& other);
        /// assign calendar
        calendar& operator=(const calendar& other);

        /// Get minimum value for period f, For example for period::day it is 1.
        int minimum(period::period_type f) const;
        /// Get greatest possible minimum value for period f, For example for period::day it is 1, but may be different
        /// for other calendars.
        int greatest_minimum(period::period_type f) const;
        /// Get maximum value for period f, For example for Gregorian calendar's maximum period::day it is 31.
        int maximum(period::period_type f) const;
        /// Get least maximum value for period f, For example for Gregorian calendar's maximum period::day it is 28.
        int least_maximum(period::period_type f) const;

        /// Get first day of week for specific calendar, for example for US it is 1 - Sunday for France it is 2 - Monday
        int first_day_of_week() const;

        /// get calendar's locale
        const std::locale& get_locale() const;
        /// get calendar's time zone
        const std::string& get_time_zone() const;

        /// Check if the calendar is Gregorian
        bool is_gregorian() const;

        /// Compare calendars for equivalence: i.e. calendar types, time zones etc.
        bool operator==(const calendar& other) const;
        /// Opposite of ==
        bool operator!=(const calendar& other) const;

    private:
        friend class date_time;
        std::locale locale_;
        std::string tz_;
        hold_ptr<abstract_calendar> impl_;
    };

    /// \brief this class represents a date time and allows to perform various operation according to the
    /// locale settings.
    ///
    /// This class allows to manipulate various aspects of dates and times easily using arithmetic operations with
    /// periods.
    ///
    /// General arithmetic functions:
    ///
    /// - date_time + date_time_period_set = date_time: move time point forward by specific periods like date_time +
    /// month;
    /// - date_time - date_time_period_set = date_time: move time point backward by specific periods like date_time -
    /// month;
    /// - date_time << date_time_period_set  = date_time: roll time point forward by specific periods with rolling to
    /// begin if overflows: like "2010-01-31" << 2* day == "2010-01-02" instead of "2010-02-02"
    /// - date_time >> date_time_period_set  = date_time: roll time point backward by specific periods with rolling to
    /// end if overflows: like "2010-01-02" >> 2* day == "2010-01-31" instead of "2009-12-30"
    /// - date_time / period_type = int - current period value: like "2010-12-21" / month == 12. "2010-12-21" / year =
    /// 2010
    /// - (date_time - date_time) / period_type = int: distance between dates in period_type. Like ("2010-12-01" -
    /// "2008-12-01") / month = 24.
    ///
    /// You can also assign specific periods using assignment operator like:
    /// some_time = year * 1995 that sets the year to 1995.

    class BOOST_LOCALE_DECL date_time {
    public:
        /// Default constructor, uses default calendar initialized date_time object to current time.
        ///
        /// \throws std::bad_cast: Global locale does not have \ref calendar_facet facet installed
        date_time();

        /// Copy a date_time
        date_time(const date_time& other);
        // Move construct a date_time
        date_time(date_time&&) noexcept = default;

        /// copy date_time and change some fields according to the \a set
        date_time(const date_time& other, const date_time_period_set& set);

        /// assign the date_time
        date_time& operator=(const date_time& other);
        // Move assign a date_time
        date_time& operator=(date_time&&) noexcept = default;

        /// Create a date_time object using POSIX time \a time and default calendar
        ///
        /// \throws std::bad_cast: Global locale does not have \ref calendar_facet facet installed
        date_time(double time);

        /// Create a date_time object using POSIX time \a time and calendar \a cal
        date_time(double time, const calendar& cal);

        /// Create a date_time object using calendar \a cal and initializes it to current time.
        date_time(const calendar& cal);

        /// Create a date_time object using default calendar and define values given in \a set
        ///
        /// \throws std::bad_cast: Global locale does not have \ref calendar_facet facet installed
        date_time(const date_time_period_set& set);

        /// Create a date_time object using calendar \a cal and define values given in \a set
        date_time(const date_time_period_set& set, const calendar& cal);

        /// assign values to various periods in set \a f
        date_time& operator=(const date_time_period_set& f);

        /// set specific period \a f value to \a v
        void set(period::period_type f, int v);

        /// get specific period \a f value
        int get(period::period_type f) const;
        /// syntactic sugar for get(f)
        int operator/(period::period_type f) const { return get(f); }

        /// add single period f to the current date_time
        date_time operator+(period::period_type f) const { return *this + date_time_period(f); }
        /// subtract single period f from the current date_time
        date_time operator-(period::period_type f) const { return *this - date_time_period(f); }
        /// add single period f to the current date_time
        date_time& operator+=(period::period_type f) { return *this += date_time_period(f); }
        /// subtract single period f from the current date_time
        date_time& operator-=(period::period_type f) { return *this -= date_time_period(f); }

        /// roll forward a date by single period f.
        date_time operator<<(period::period_type f) const { return *this << date_time_period(f); }
        /// roll backward a date by single period f.
        date_time operator>>(period::period_type f) const { return *this >> date_time_period(f); }
        /// roll forward a date by single period f.
        date_time& operator<<=(period::period_type f) { return *this <<= date_time_period(f); }
        /// roll backward a date by single period f.
        date_time& operator>>=(period::period_type f) { return *this >>= date_time_period(f); }

        /// add date_time_period to the current date_time
        date_time operator+(const date_time_period& v) const;
        /// subtract date_time_period from the current date_time
        date_time operator-(const date_time_period& v) const;
        /// add date_time_period to the current date_time
        date_time& operator+=(const date_time_period& v);
        /// subtract date_time_period from the current date_time
        date_time& operator-=(const date_time_period& v);

        /// roll current date_time forward by date_time_period v
        date_time operator<<(const date_time_period& v) const;
        /// roll current date_time backward by date_time_period v
        date_time operator>>(const date_time_period& v) const;
        /// roll current date_time forward by date_time_period v
        date_time& operator<<=(const date_time_period& v);
        /// roll current date_time backward by date_time_period v
        date_time& operator>>=(const date_time_period& v);

        /// add date_time_period_set v to the current date_time
        date_time operator+(const date_time_period_set& v) const;
        /// subtract date_time_period_set v from the current date_time
        date_time operator-(const date_time_period_set& v) const;
        /// add date_time_period_set v to the current date_time
        date_time& operator+=(const date_time_period_set& v);
        /// subtract date_time_period_set v from the current date_time
        date_time& operator-=(const date_time_period_set& v);

        /// roll current date_time forward by date_time_period_set v
        date_time operator<<(const date_time_period_set& v) const;
        /// roll current date_time backward by date_time_period_set v
        date_time operator>>(const date_time_period_set& v) const;
        /// roll current date_time forward by date_time_period_set v
        date_time& operator<<=(const date_time_period_set& v);
        /// roll current date_time backward by date_time_period_set v
        date_time& operator>>=(const date_time_period_set& v);

        /// Get POSIX time
        ///
        /// The POSIX time is number of seconds since January 1st, 1970 00:00 UTC, ignoring leap seconds.
        double time() const;
        /// Set POSIX time
        ///
        /// The POSIX time is number of seconds since January 1st, 1970 00:00 UTC, ignoring leap seconds.
        /// This time can be fetched from Operating system clock using C function time, gettimeofday and others.
        void time(double v);

        /// Get the name of the associated timezone
        std::string timezone() const;

        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator==(const date_time& other) const;
        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator!=(const date_time& other) const;
        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator<(const date_time& other) const;
        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator>(const date_time& other) const;
        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator<=(const date_time& other) const;
        /// compare date_time in the timeline (ignores difference in calendar, timezone etc)
        bool operator>=(const date_time& other) const;

        /// swaps two dates - efficient, does not throw
        void swap(date_time& other) noexcept;

        /// calculate the distance from this date_time to \a other in terms of periods \a f
        int difference(const date_time& other, period::period_type f) const;

        /// Get minimal possible value for *this time point for a period \a f.
        int minimum(period::period_type f) const;
        /// Get minimal possible value for *this time point for a period \a f. For example
        /// in February maximum(day) may be 28 or 29, in January maximum(day)==31
        int maximum(period::period_type f) const;

        /// Check if *this time point is in daylight saving time
        bool is_in_daylight_saving_time() const;

    private:
        hold_ptr<abstract_calendar> impl_;
    };

    inline void swap(date_time& left, date_time& right) noexcept
    {
        left.swap(right);
    }

    /// Writes date_time \a t to output stream \a out.
    ///
    /// This function uses locale, calendar and time zone of the target stream \a in.
    ///
    /// For example:
    /// \code
    ///  date_time now(time(nullptr),hebrew_calendar)
    ///  std::cout << "Year: " << period::year(now) << " Full Date:" << now;
    /// \endcode
    ///
    /// The output may be Year:5770 Full Date:Jan 1, 2010
    template<typename CharType>
    std::basic_ostream<CharType>& operator<<(std::basic_ostream<CharType>& out, const date_time& t)
    {
        const double time_point = t.time();
        ios_info& info = ios_info::get(out);
        const uint64_t display_flags = info.display_flags();
        if(as::detail::is_datetime_display_flags(display_flags)) {
            out << time_point;
        } else {
            info.display_flags(flags::datetime);
            out << time_point;
            info.display_flags(display_flags);
        }
        return out;
    }

    /// Reads date_time \a t from output stream \a in
    ///
    /// This function uses locale, calendar and time zone of the source stream \a in.
    template<typename CharType>
    std::basic_istream<CharType>& operator>>(std::basic_istream<CharType>& in, date_time& t)
    {
        double v;
        const uint64_t display_flags = ios_info::get(in).display_flags();
        if(as::detail::is_datetime_display_flags(display_flags)) {
            in >> v;
        } else {
            ios_info::get(in).display_flags(flags::datetime);
            in >> v;
            ios_info::get(in).display_flags(display_flags);
        }
        if(!in.fail())
            t.time(v);
        return in;
    }

#ifdef BOOST_MSVC
#    pragma warning(push)
#    pragma warning(disable : 4512) // assignment operator could not be generated
#endif

    /// \brief This class represents a period: a pair of two date_time objects.
    ///
    /// It is generally used as syntactic sugar to calculate difference between two dates.
    ///
    /// Note: it stores references to the original objects, so it is not recommended to be used
    /// outside of the equation you calculate the difference in.
    class date_time_duration {
    public:
        /// Create an object were \a first represents earlier point on time line and \a second is later
        /// point.
        date_time_duration(const date_time& first, const date_time& second) : s_(first), e_(second) {}

        /// find a difference in terms of period_type \a f
        int get(period::period_type f) const { return start().difference(end(), f); }
        /// Syntactic sugar for get(f)
        int operator/(period::period_type f) const { return start().difference(end(), f); }

        /// Get starting point
        const date_time& start() const { return s_; }
        /// Get ending point
        const date_time& end() const { return e_; }

    private:
        const date_time& s_;
        const date_time& e_;
    };
#ifdef BOOST_MSVC
#    pragma warning(pop)
#endif

    /// Calculates the difference between two dates, the left operand is a later point on time line.
    /// Returns date_time_duration object.
    inline date_time_duration operator-(const date_time& later, const date_time& earlier)
    {
        return date_time_duration(earlier, later);
    }

    namespace period {
        ///  Extract from date_time numerical value of Era i.e. AC, BC in Gregorian and Julian calendar, range [0,1]
        inline int era(const date_time& dt)
        {
            return dt.get(era());
        }
        ///  Extract from date_time numerical value of Year, it is calendar specific, for example 2011 in Gregorian
        ///  calendar.
        inline int year(const date_time& dt)
        {
            return dt.get(year());
        }
        ///  Extract from date_time numerical value of Extended year for Gregorian/Julian calendars, where 1 BC == 0, 2
        ///  BC == -1.
        inline int extended_year(const date_time& dt)
        {
            return dt.get(extended_year());
        }
        ///  Extract from date_time numerical value of The month of year, calendar specific, in Gregorian [0..11]
        inline int month(const date_time& dt)
        {
            return dt.get(month());
        }
        ///  Extract from date_time numerical value of The day of month, calendar specific, in Gregorian [1..31]
        inline int day(const date_time& dt)
        {
            return dt.get(day());
        }
        ///  Extract from date_time numerical value of The number of day in year, starting from 1, in Gregorian [1..366]
        inline int day_of_year(const date_time& dt)
        {
            return dt.get(day_of_year());
        }
        ///  Extract from date_time numerical value of Day of week, Sunday=1, Monday=2,..., Saturday=7.
        ///
        /// Note that updating this value respects local day of week, so for example,
        /// If first day of week is Monday and the current day is Tuesday then setting
        /// the value to Sunday (1) would forward the date by 5 days forward and not backward
        /// by two days as it could be expected if the numbers were taken as is.
        inline int day_of_week(const date_time& dt)
        {
            return dt.get(day_of_week());
        }
        ///  Extract from date_time numerical value of Original number of the day of the week in month. For example 1st
        ///  Sunday,
        /// 2nd Sunday, etc. in Gregorian [1..5]
        inline int day_of_week_in_month(const date_time& dt)
        {
            return dt.get(day_of_week_in_month());
        }
        ///  Extract from date_time numerical value of Local day of week, for example in France Monday is 1, in US
        ///  Sunday is 1, [1..7]
        inline int day_of_week_local(const date_time& dt)
        {
            return dt.get(day_of_week_local());
        }
        ///  Extract from date_time numerical value of 24 clock hour [0..23]
        inline int hour(const date_time& dt)
        {
            return dt.get(hour());
        }
        ///  Extract from date_time numerical value of 12 clock hour [0..11]
        inline int hour_12(const date_time& dt)
        {
            return dt.get(hour_12());
        }
        ///  Extract from date_time numerical value of am or pm marker [0..1]
        inline int am_pm(const date_time& dt)
        {
            return dt.get(am_pm());
        }
        ///  Extract from date_time numerical value of minute [0..59]
        inline int minute(const date_time& dt)
        {
            return dt.get(minute());
        }
        ///  Extract from date_time numerical value of second [0..59]
        inline int second(const date_time& dt)
        {
            return dt.get(second());
        }
        ///  Extract from date_time numerical value of The week number in the year
        inline int week_of_year(const date_time& dt)
        {
            return dt.get(week_of_year());
        }
        ///  Extract from date_time numerical value of The week number within current month
        inline int week_of_month(const date_time& dt)
        {
            return dt.get(week_of_month());
        }
        ///  Extract from date_time numerical value of First day of week, constant, for example Sunday in US = 1, Monday
        ///  in France = 2
        inline int first_day_of_week(const date_time& dt)
        {
            return dt.get(first_day_of_week());
        }

        ///  Extract from date_time_duration numerical value of duration in  Era i.e. AC, BC in Gregorian and Julian
        ///  calendar, range [0,1]
        inline int era(const date_time_duration& dt)
        {
            return dt.get(era());
        }
        ///  Extract from date_time_duration numerical value of duration in years
        inline int year(const date_time_duration& dt)
        {
            return dt.get(year());
        }
        ///  Extract from date_time_duration numerical value of duration in extended years (for Gregorian/Julian
        ///  calendars, where 1 BC == 0, 2 BC == -1).
        inline int extended_year(const date_time_duration& dt)
        {
            return dt.get(extended_year());
        }
        ///  Extract from date_time_duration numerical value of duration in months
        inline int month(const date_time_duration& dt)
        {
            return dt.get(month());
        }
        ///  Extract from date_time_duration numerical value of duration in days of month
        inline int day(const date_time_duration& dt)
        {
            return dt.get(day());
        }
        ///  Extract from date_time_duration numerical value of duration in days of year
        inline int day_of_year(const date_time_duration& dt)
        {
            return dt.get(day_of_year());
        }
        ///  Extract from date_time_duration numerical value of duration in days of week
        inline int day_of_week(const date_time_duration& dt)
        {
            return dt.get(day_of_week());
        }
        ///  Extract from date_time_duration numerical value of duration in original number of the day of the week in
        ///  month
        inline int day_of_week_in_month(const date_time_duration& dt)
        {
            return dt.get(day_of_week_in_month());
        }
        ///  Extract from date_time_duration numerical value of duration in local day of week
        inline int day_of_week_local(const date_time_duration& dt)
        {
            return dt.get(day_of_week_local());
        }
        ///  Extract from date_time_duration numerical value of duration in hours
        inline int hour(const date_time_duration& dt)
        {
            return dt.get(hour());
        }
        ///  Extract from date_time_duration numerical value of duration in  12 clock hours
        inline int hour_12(const date_time_duration& dt)
        {
            return dt.get(hour_12());
        }
        ///  Extract from date_time_duration numerical value of duration in  am or pm markers
        inline int am_pm(const date_time_duration& dt)
        {
            return dt.get(am_pm());
        }
        ///  Extract from date_time_duration numerical value of duration in  minutes
        inline int minute(const date_time_duration& dt)
        {
            return dt.get(minute());
        }
        ///  Extract from date_time_duration numerical value of duration in  seconds
        inline int second(const date_time_duration& dt)
        {
            return dt.get(second());
        }
        ///  Extract from date_time_duration numerical value of duration in the week number in the year
        inline int week_of_year(const date_time_duration& dt)
        {
            return dt.get(week_of_year());
        }
        ///  Extract from date_time_duration numerical value of duration in  The week number within current month
        inline int week_of_month(const date_time_duration& dt)
        {
            return dt.get(week_of_month());
        }
        ///  Extract from date_time_duration numerical value of duration in the first day of week
        inline int first_day_of_week(const date_time_duration& dt)
        {
            return dt.get(first_day_of_week());
        }

    } // namespace period

    /// @}

}} // namespace boost::locale

#ifdef BOOST_MSVC
#    pragma warning(pop)
#endif

///
/// \example calendar.cpp
///
/// Example of using date_time functions for generating calendar for current year.
///

#endif