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
| Name | Description |
|---|---|
default_zone |
Gets the time_zone pointer for the default time zone. |
locate_zone |
Gets the time_zone pointer for the specified time zone. |
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