zoned_traits struct

Allows you to associate a different default time zone with a zoned_time, and optionally map a custom name to a time zone.

Syntax

1)
template<class T>
struct zoned_traits {}; // C++20

2)
template <>
struct zoned_traits<const time_zone*>; // C++20

Parameters

1) T - A pointer to a type that provides a custom time_zone.
2) When you don't provide the time_zone* template argument, this specialization provides const std::chrono::time_zone*, which defaults to the UTC time zone.

Remarks

The pointer to the type that you supply doesn't have to provide the static functions default_zone() or locate_zone(). But if it doesn't, the zoned_time constructors won't be considered during overload resolution.

Members

Requirements

Header: <chrono>

Namespace: std::chrono

Compiler Option: /std:c++latest

Microsoft C++ supports zoned_traits class starting in Visual Studio 2019 version 16.10.

Time-zone data is only available for Windows 10 version 1903/19H1 and later, and Windows Server 2022 and later.

default_zone

Gets the time_zone for the default time zone. For more information about how this works, see the code example at the end of this topic.

static const time_zone* default_zone();

Return value

If the template argument isn't provided, then the template specialization provides zoned_traits<const time_zone*>, which returns the UTC time zone. Otherwise, it returns the default time zone provided by the template argument T.

locate_zone

Returns the time_zone pointer for the specified time zone. For more information about how this works, see the code example at the end of this topic.

static const time_zone* locate_zone(string_view name);

Template parameters

name
The name of the time zone to locate. For example, "UTC".

Return value

If you constructed a zoned_traits without providing the template argument of a custom time zone pointer, then the return value is std::chrono::locate_zone(name). Otherwise, it returns the value of locate_zone() as defined in the template argument T.

Example: zoned_traits

This following example shows how to use zoned_traits to supply a custom default time zone.

First, CustomTimeZonePtr is defined, which provides the pointer to the custom time zone type via operator->().

Then, a zoned_traits is declared in which default_zone is defined to return the custom default time zone. In this case, the South Pole.

In the example, zoned_traits<CustomTimeZonePtr>::locate_zone() passes the specified time zone name to std::chrono::locate_zone(). This function is where you could map a custom time zone name to another time zone.

Finally, stdZT is defined, which uses the standard time_zone pointer because it doesn't provide a template argument, so the specialization is used that provides const std::chrono::time_zone*.

Run this example to see the zoned_time use the custom, and then the standard, time_zone pointer.

// compile using: /std:c++latest
#include <iostream>
#include <chrono>

using namespace std::chrono;

struct CustomTimeZonePtr
{
    CustomTimeZonePtr() {}
    CustomTimeZonePtr(const time_zone* tz) : tzptr(tz) {}

    const time_zone* operator->() const
    {
        return tzptr;
    }

private:
    const time_zone* tzptr;
};

template <>
struct zoned_traits<CustomTimeZonePtr>
{
    static const CustomTimeZonePtr default_zone()
    {
        return CustomTimeZonePtr{ locate_zone("Antarctica/South_Pole") };
    }

    static const CustomTimeZonePtr locate_zone(std::string_view name)
    {
        // Here you can provide your own mapping between the name
        // parameter and another time zone before passing it to locate_zone()
        return CustomTimeZonePtr{ std::chrono::locate_zone(name) };
    }
};

int main()
{
    std::cout << "-- Custom time zone pointer with specialized zoned_traits and different default zone behavior --\n";
    zoned_time<seconds, CustomTimeZonePtr> myZT;
    std::cout << myZT.get_info() << "\n";

    std::cout << "-- Built-in time zone pointer with standard zoned_traits --\n";
    zoned_time<seconds, const time_zone*> stdZT;
    std::cout << stdZT.get_info() << "\n";

    return 0;
}

-- Custom time zone pointer with specialized zoned_traits and different default zone behavior --
begin: 1945-12-31 12:00:00, end: 1974-11-02 14:00:00, offset: 43200s, save: 0min, abbrev: GMT+12
-- Built-in time zone pointer with standard zoned_traits --
begin: -32767-01-01 00:00:00, end: 32767-12-31 23:59:59, offset: 0s, save: 0min, abbrev: UTC

See also

<chrono>
time_zone
zoned_time class
Header files reference