/* Code for exercise 21.18. |  | "C++ Solutions--Companion to The C++ Programming Language, Third Edition" | by David Vandevoorde; Addison-Wesley 1998; ISBN 0-201-30965-3. | | Permission to use, copy, modify, distribute and sell this software | and its documentation for any purpose is hereby granted without fee. | The author makes no representations about the suitability of this | software for any purpose.  It is provided "as is" without express or | implied warranty. `----------------------------------------------------------------------*/#include <assert.h>#include <iostream>#include <stdexcept>#include <string>namespace { // unnamed namespace   char ext_digits[] =      "0123456789abcdefghijklmnopqrstuvwxyz";} // end unnamed namespacestruct based {   based(int b, int v): base_(b), value_(v) {      if (b<2 or b>sizeof(ext_digits))         throw std::invalid_argument(std::string("based"));   }   int base() const { return base_; }   int value() const { return value_; }private:   int base_, value_;};std::ostream& operator<<(std::ostream &output, based v) {   int const bufsize = 130; // Enough for 128 base-2 digits   char buffer[bufsize];   int value = v.value(), base = v.base();   bool negative = value<0;   if (negative)      value = -value;   int endpos = bufsize;   buffer[--endpos] = '\0'; // C-string termination   do {      buffer[--endpos] = ext_digits[value%base];      assert(endpos>0);   } while (value /= base);   if (negative)      buffer[--endpos] = '-';   return output << &buffer[endpos];}int main() {   std::cout << "31 in base 16 is " << based(16, 31) << '\n';   return 0;}