25 Localization library [localization]

25.4 Standard locale categories [locale.categories]

25.4.3 The numeric punctuation facet [facet.numpunct]

25.4.3.1 Class template numpunct [locale.numpunct]

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
    };
}
1
#
numpunct<> specifies numeric punctuation.
The specializations required in Table 64 ([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.
2
#
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.

25.4.3.1.1 numpunct members [facet.numpunct.members]

char_type decimal_point() const;
1
#
Returns: do_­decimal_­point().
char_type thousands_sep() const;
2
#
Returns: do_­thousands_­sep().
string grouping() const;
3
#
Returns: do_­grouping().
string_type truename() const; string_type falsename() const;
4
#
Returns: do_­truename() or do_­falsename(), respectively.

25.4.3.1.2 numpunct virtual functions [facet.numpunct.virtuals]

char_type do_decimal_point() const;
1
#
Returns: A character for use as the decimal radix separator.
The required specializations return '.' or L'.'.
char_type do_thousands_sep() const;
2
#
Returns: A character for use as the digit group separator.
The required specializations return ',' or L','.
string do_grouping() const;
3
#
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.
4
#
The required specializations return the empty string, indicating no grouping.
string_type do_truename() const; string_type do_falsename() const;
5
#
Returns: A string representing the name of the boolean value true or false, respectively.
6
#
In the base class implementation these names are "true" and "false", or L"true" and L"false".
241)
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".