C++ Tutorial/File Stream/istream exceptions

Deal with istream exceptions

<source lang="cpp">/* The following code example is taken from the book

* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/

1. include <iostream>
2. include <cstdlib>

namespace MyLib {

   double readAndProcessSum (std::istream&);

} int main() {

   using namespace std;
   double sum;
   try {
       sum = MyLib::readAndProcessSum(cin);
   }
   catch (const ios::failure& error) {
       cerr << "I/O exception: " << error.what() << endl;
       return EXIT_FAILURE;
   }
   catch (const exception& error) {
       cerr << "standard exception: " << error.what() << endl;
       return EXIT_FAILURE;
   }
   catch (...) {
       cerr << "unknown exception" << endl;
       return EXIT_FAILURE;
   }
   // print sum
   cout << "sum: " << sum << endl;

}

1. include <istream>

namespace MyLib {

   double readAndProcessSum (std::istream& strm)
   {
       using std::ios;
       double value, sum;
   
       // save current state of exception flags
       ios::iostate oldExceptions = strm.exceptions();
   
       /* let failbit and badbit throw exceptions
        * - NOTE: failbit is also set at end-of-file
        */
       strm.exceptions (ios::failbit | ios::badbit);
   
       try {
           /* while stream is OK
            * - read value and add it to sum
            */
           sum = 0;
           while (strm >> value) {
               sum += value;
           }
       }
       catch (...) {
           /* if exception not caused by end-of-file
            * - restore old state of exception flags
            * - rethrow exception
            */
           if (!strm.eof()) {
               strm.exceptions(oldExceptions);  // restore exception flags
               throw;                           // rethrow
           }
       }
   
       // restore old state of exception flags
       strm.exceptions (oldExceptions);
   
       // return sum
       return sum;
   }

}</source>

a
I/O exception: basic_ios::clear