Make a new locale and attach it to the standard output stream : locale « Development « C++ Tutorial

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2. Development
3. locale

#include <iomanip>
#include <iostream>
#include <locale>

using namespace std;

class Separator_facet: public numpunct<char>
{
   public:
   explicit Separator_facet( size_t refs = 0): numpunct<char>( refs ){}

   protected:
   virtual string do_grouping() const{ return "\3"; }
};

int main()
{
   const int million = 1000000;
   const double number = 1234.56789;
   cout << million << fixed << setprecision( 5 ) << number;

   locale separator_locale( cout.getloc(), new Separator_facet );
   cout.imbue( separator_locale );
   cout << million << " " << number << endl;
}