namespace std::chrono {
struct tzdb {
string version;
vector<time_zone> zones;
vector<time_zone_link> links;
vector<leap_second> leap_seconds;
const time_zone* locate_zone(string_view tz_name) const;
const time_zone* current_zone() const;
};
}
Each
vector in a
tzdb object
is sorted to enable fast lookup
.const time_zone* locate_zone(string_view tz_name) const;
Returns:
- If zones contains an element tz
for which tz.name() == tz_name,
a pointer to tz;
- otherwise,
if links contains an element tz_l
for which tz_l.name() == tz_name,
then a pointer to the element tz of zones
for which tz.name() == tz_l.target().
[
Note 1:
A
time_zone_link specifies an alternative name for a
time_zone. —
end note]
Throws: If a
const time_zone* cannot be found
as described in the
Returns: element,
throws a
runtime_error. [
Note 2:
On non-exceptional return, the return value is always a pointer to a valid
time_zone. —
end note]
const time_zone* current_zone() const;
Returns: A pointer to the time zone which the computer has set as its local time zone
. namespace std::chrono {
class tzdb_list {
public:
tzdb_list(const tzdb_list&) = delete;
tzdb_list& operator=(const tzdb_list&) = delete;
class const_iterator;
const tzdb& front() const noexcept;
const_iterator erase_after(const_iterator p);
const_iterator begin() const noexcept;
const_iterator end() const noexcept;
const_iterator cbegin() const noexcept;
const_iterator cend() const noexcept;
};
}
The
tzdb_list database is a singleton;
the unique object of type
tzdb_list can be
accessed via the
get_tzdb_list() function
.[
Note 1:
This access is only needed for those applications
that need to have long uptimes and
have a need to update the time zone database while running
.Other applications can implicitly access the
front() of this list
via the read-only namespace scope functions
get_tzdb(),
locate_zone(), and
current_zone(). —
end note]
The
tzdb_list object contains a list of
tzdb objects
.tzdb_list::const_iterator is a constant iterator
which meets the
Cpp17ForwardIterator requirements
and has a value type of
tzdb. const tzdb& front() const noexcept;
Synchronization: This operation is thread-safe with respect to
reload_tzdb(). [
Note 2:
reload_tzdb() pushes a new
tzdb
onto the front of this container
. —
end note]
Returns: A reference to the first
tzdb in the container
. const_iterator erase_after(const_iterator p);
Preconditions: The iterator following
p is dereferenceable
. Effects: Erases the
tzdb referred to by the iterator following
p. Postconditions: No pointers, references, or iterators are invalidated
except those referring to the erased
tzdb. [
Note 3:
It is not possible to erase the
tzdb
referred to by
begin(). —
end note]
Returns: An iterator pointing to the element following the one that was erased,
or
end() if no such element exists
. const_iterator begin() const noexcept;
Returns: An iterator referring to the first
tzdb in the container
. const_iterator end() const noexcept;
Returns: An iterator referring to the position one past the last
tzdb in the container
. const_iterator cbegin() const noexcept;
const_iterator cend() const noexcept;
tzdb_list& get_tzdb_list();
Effects: If this is the first access to the time zone database,
initializes the database
. If this call initializes the database,
the resulting database will be a
tzdb_list
holding a single initialized
tzdb.Synchronization: It is safe to call this function from multiple threads at one time
. Returns: A reference to the database
. Throws:
runtime_error if for any reason
a reference cannot be returned to a valid
tzdb_list
containing one or more valid
tzdbs
. Returns:
get_tzdb_list().front(). const time_zone* locate_zone(string_view tz_name);
Returns:
get_tzdb().locate_zone(tz_name). [
Note 1:
The time zone database will be initialized
if this is the first reference to the database
. —
end note]
const time_zone* current_zone();
Returns:
get_tzdb().current_zone(). The local time zone database
is that supplied by the implementation
when the program first accesses the database,
for example via
current_zone().While the program is running,
the implementation may choose to update the time zone database
.This update shall not impact the program in any way
unless the program calls the functions in this subclause
.const tzdb& reload_tzdb();
Effects: This function first checks
the version of the remote time zone database
. If the versions of the local and remote databases are the same,
there are no effects
.Otherwise the remote database is pushed
to the front of the
tzdb_list
accessed by
get_tzdb_list().Synchronization: This function is thread-safe with respect to
get_tzdb_list().front() and
get_tzdb_list().erase_after(). Postconditions: No pointers, references, or iterators are invalidated
. Returns:
get_tzdb_list().front(). Throws:
runtime_error if for any reason
a reference cannot be returned to a valid
tzdb. Returns: The latest remote database version
. [
Note 1:
This can be compared with
get_tzdb().version
to discover if the local and remote databases are equivalent
. —
end note]