I'm probably not the first one to ask. But why is that behavior?
It is the standard way to implement facets. The locale system keep a reference count for each facet and deletes them when the last locale uses it is destroyed. As with other facets it is possible to override this behaviour by passing 1 as the last parameter "ref" to the constructor. It should works fine for temporary stringstreams but I wouldn't use it for cout or in the global locale. Almost all locale operations allocate heap memory so one more allocation on facet creation makes a very small difference. There is one theoretical memory leak problem if an exception occurs between the allocation and the time when the facet is added to a locale. Note that the examples in the date_time library: // create date_facet and date_input_facet using all defaults date_facet* date_output = new date_facet(); ss.imbue(locale(ss.getloc(), date_output)); // replace names in the output facet date_output->long_month_names(long_months); date_output->long_weekday_names(long_days); isn't "standard" since the facets are meant to be immutable once they are added to a locale. The streams are allowed to cache the values returned by the facets so changes are not guaranteed to be effective until the next imbue call. However, the date_time streaming is implemented without caches so it works.