/* Code for exercise 21.12. |  | "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 <fstream>#include <iostream>#include <stdlib.h>struct MappedFile {   MappedFile(std::ifstream &file): file_(file) {}   char operator[](std::ifstream::off_type i) const {      file_.seekg(i);      return char(file_.get());   }private:   std::ifstream &file_;};void error(char const *prog, char const *entity, char const *msg) {   std::cerr << prog << ": " << entity << msg << '\n';   exit(1);}int main(int argc, char *argv[]) {   if (argc!=2)      error(argv[0], "", ": unexpected number of arguments.");   std::ifstream pass0(argv[1]);   if (!pass0)      error(argv[0], argv[1], ": cannot open file for reading.");   std::ifstream::off_type n = 0;   int chksum0 = 0;   while (pass0) {      chksum0 += char(pass0.get());      ++n;   }   std::cerr << "Reading " << n << " characters.\n";   std::cerr << chksum0 << '\n';   int chksum1 = 0;   std::ifstream pass1(argv[1]);   if (!pass1)      error(argv[0], argv[1], ": cannot open file for reading.");   MappedFile accessor(pass1);   for (size_t k = 0; k!=n; ++k) {      chksum1 += accessor[k];   }   std::cerr << chksum1 << '\n';   return 0;}