30 Localization library [localization]

30.4 Standard locale categories [locale.categories]

30.4.2 The ctype category [category.ctype]

30.4.2.4 ctype<char> specialization [facet.ctype.special]

30.4.2.4.3 Members [facet.ctype.char.members]

In the following member descriptions, for unsigned char values v where v >= table_size, table()[v] is assumed to have an implementation-specific value (possibly different for each such value v) without performing the array lookup.
explicit ctype(const mask* tbl = nullptr, bool del = false, size_t refs = 0);
Preconditions: Either tbl == nullptr is true or [tbl, tbl+table_size) is a valid range.
Effects: Passes its refs argument to its base class constructor.
bool is(mask m, char c) const; const char* is(const char* low, const char* high, mask* vec) const;
Effects: The second form, for all *p in the range [low, high), assigns into vec[p - low] the value table()[(unsigned char)*p].
Returns: The first form returns table()[(unsigned char)c] & m; the second form returns high.
const char* scan_is(mask m, const char* low, const char* high) const;
Returns: The smallest p in the range [low, high) such that table()[(unsigned char) *p] & m is true.
const char* scan_not(mask m, const char* low, const char* high) const;
Returns: The smallest p in the range [low, high) such that table()[(unsigned char) *p] & m is false.
char toupper(char c) const; const char* toupper(char* low, const char* high) const;
Returns: do_toupper(c) or do_toupper(low, high), respectively.
char tolower(char c) const; const char* tolower(char* low, const char* high) const;
Returns: do_tolower(c) or do_tolower(low, high), respectively.
char widen(char c) const; const char* widen(const char* low, const char* high, char* to) const;
Returns: do_widen(c) or do_widen(low, high, to), respectively.
char narrow(char c, char dfault) const; const char* narrow(const char* low, const char* high, char dfault, char* to) const;
Returns: do_narrow(c, dfault) or do_narrow(low, high, dfault, to), respectively.
const mask* table() const noexcept;
Returns: The first constructor argument, if it was nonzero, otherwise classic_table().