printf_s(str, "%d", value);

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

struct my_number_format : std::numpunct<char>
{
   char do_thousands_sep() const    { return ' '; }         // space as a separator
   std::string do_grouping() const  { return "\3"; }        // digits grouping
};



int main()
{
  std::cout.imbue(std::locale(std::locale(), new my_number_format));

  std::cout << std::setprecision(2) << std::fixed << 12345.678 << std::endl; // 12 345.68
}