namespace std {
template <class charT>
class numpunct : public locale::facet {
public:
using char_type = charT;
using string_type = basic_string<charT>;
explicit numpunct(size_t refs = 0);
char_type decimal_point() const;
char_type thousands_sep() const;
string grouping() const;
string_type truename() const;
string_type falsename() const;
static locale::id id;
protected:
~numpunct(); // virtual
virtual char_type do_decimal_point() const;
virtual char_type do_thousands_sep() const;
virtual string do_grouping() const;
virtual string_type do_truename() const; // for bool
virtual string_type do_falsename() const; // for bool
};
}numpunct<> specifies numeric punctuation. The specializations required in Table 69 ([locale.category]), namely numpunct<wchar_t> and numpunct<char>, provide classic "C" numeric formats, i.e., they contain information equivalent to that contained in the "C" locale or their wide character counterparts as if obtained by a call to widen.
The syntax for number formats is as follows, where digit represents the radix set specified by the fmtflags argument value, and thousands-sep and decimal-point are the results of corresponding numpunct<charT> members. Integer values have the format:
integer ::= [sign] units sign ::= plusminus plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
and floating-point values have:
floatval ::= [sign] units [decimal-point [digits]] [e [sign] digits] |
[sign] decimal-point digits [e [sign] digits]
e ::= 'e' | 'E'where the number of digits between thousands-seps is as specified by do_grouping(). For parsing, if the digits portion contains no thousands-separators, no grouping constraint is applied.
char_type decimal_point() const;
char_type thousands_sep() const;
string grouping() const;
string_type truename() const;
string_type falsename() const;
char_type do_decimal_point() const;
Returns: A character for use as the decimal radix separator. The required specializations return '.' or L'.'.
char_type do_thousands_sep() const;
Returns: A character for use as the digit group separator. The required specializations return ',' or L','.
string do_grouping() const;
Returns: A basic_string<char> vec used as a vector of integer values, in which each element vec[i] represents the number of digits241 in the group at position i, starting with position 0 as the rightmost group. If vec.size() <= i, the number is the same as group (i - 1); if (i < 0 || vec[i] <= 0 || vec[i] == CHAR_MAX), the size of the digit group is unlimited.
string_type do_truename() const;
string_type do_falsename() const;
Thus, the string "\003" specifies groups of 3 digits each, and "3" probably indicates groups of 51 (!) digits each, because 51 is the ASCII value of "3".