std::numpunct::grouping,std::numpunct::do_grouping(3)

NAME
std::numpunct::grouping,std::numpunct::do_grouping - std::numpunct::grouping,std::numpunct::do_grouping

Synopsis
   Defined in header <locale>
   public: (1)
   std::string grouping() const;
   protected: (2)
   virtual std::string do_grouping() const;
   1) Public member function, calls the member function do_grouping of the most derived
   class.
   2) Returns an std::string holding, in each char element, the number of digits in
   each group of the numeric output formatted by num_put::put() (and, therefore,
   basic_ostream::operator<<).
   This function returns a string, vec, which is used as a vector of integer values.
   (For example, "\003" specifies groups of 3 digits each, while "3" implies groups of
   51 digits each.). Each element vec[i] represents the number of digits in the ith
   digit group of the integer part of the number, counting from the right: vec[0] holds
   the number of digits in the rightmost group, vec[1] - in the second group from the
   right, etc. The grouping indicated by the last character, vec[vec.size()-1], is
   repeatedly reused to group all remaining digits in the (left part of) the number. If
   vec[i] is non-positive or equals CHAR_MAX the size of the corresponding digit group
   is unlimited.

Return value
   The object of type std::string holding the groups. The standard specializations of
   std::numpunct return an empty string, indicating no grouping. Typical groupings
   (e.g. the en_US locale) return "\003".

Example
// Run this code
 #include <iostream>
 #include <limits>
 #include <locale>
 struct space_out : std::numpunct<char>
 {
     char do_thousands_sep() const { return ' '; } // separate with spaces
     std::string do_grouping() const { return "\1"; } // groups of 1 digit
 };
 struct g123 : std::numpunct<char>
 {
     std::string do_grouping() const { return "\1\2\3"; }
 };
 int main()
 {
     std::cout << "Default locale: " << 12345678 << '\n';
     std::cout.imbue(std::locale(std::cout.getloc(), new space_out));
     std::cout << "Locale with modified numpunct: " << 12345678 << '\n';
     std::cout.imbue(std::locale(std::cout.getloc(), new g123));
     std::cout << "Locale with \\1\\2\\3 grouping: "
               << std::numeric_limits<unsigned long long>::max() << '\n'
               << "Same, for a floating-point number: "
               << std::fixed << 123456789.123456789 << '\n';
 }

Output:
 Default locale: 12345678
 Locale with modified numpunct: 1 2 3 4 5 6 7 8
 Locale with \1\2\3 grouping: 18,446,744,073,709,551,61,5
 Same, for a floating-point number: 123,456,78,9.123457

See also
   do_thousands_sep provides the character to use as thousands separator
   [virtual] (virtual protected member function)