std::numpunct::thousands_sep, std::numpunct::do_thousands_sep

From cppreference.com
Defined in header <locale>
public:
char_type thousands_sep() const;
(1)
protected:
char_type virtual do_thousands_sep() const;
(2)

1) public member function, calls the member function do_thousands_sep of the most derived class.

2) returns the character to be used as the separator between digit groups when parsing or formatting integers and integral parts of floating-point values.

[edit] Return value

The object of type char_type to use as the thousands separator. The standard specializations of std::numpunct return ’,’ and L’,’.

[edit] Example

#include <iostream>
#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
};
 
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';
}

Output:

default locale: 12345678
locale with modified numpunct: 1 2 3 4 5 6 7 8

[edit] See also

[virtual]
provides the numbers of digits between each pair of thousands

separators
(virtual protected member function)