cpp/chrono/tzdb functions

These functions provide access to the program-wide time zone database.

@1@ Returns a reference to the global std singleton. If this is the first access to the database, initialize the database. After the initialization, the database will hold a single initialized std object. This function is thread-safe: concurrent calls to this function from multiple threads do not introduce a data race. @2@ Returns a reference to the first std object held by the singleton. Equivalent to. @3@ Returns a string containing the latest remote database version. @4@ If, pushes a new object representing the remote database to the front of the  singleton referenced by. Otherwise there are no effects. No references, pointers or iterators are invalidated. Calling this function concurrently with or   does not introduce a data race.

Exceptions
@1@ std if for any reason a reference to a containing one or more valid  cannot be returned.

Return value
@1@ A reference to the global std singleton. @2@ . @3@ A string containing the latest remote database version. @4@ (after any update made by this function).