The description of the is() member in paragraph 4 of [facet.ctype.char.members] is broken: According to this description, the second form of the is() method modifies the masks in the ctype object. The correct semantics if, of course, to obtain an array of masks. The corresponding method in the general case, ie. the do_is() method as described in [locale.ctype.virtuals] paragraph 1 does the right thing.