30 Localization library [localization]

30.3 Locales [locales]

30.3.1 Class locale [locale] Types [locale.types] Class locale​::​id [locale.id]

namespace std { class locale::id { public: id(); void operator=(const id&) = delete; id(const id&) = delete; }; }
The class locale​::​id provides identification of a locale facet interface, used as an index for lookup and to encapsulate initialization.
[Note 1: 
Because facets are used by iostreams, potentially while static constructors are running, their initialization cannot depend on programmed static initialization.
One initialization strategy is for locale to initialize each facet's id member the first time an instance of the facet is installed into a locale.
This depends only on static storage being zero before constructors run ([basic.start.static]).
— end note]