Re: When operator>> can throw...

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 19 Jan 2008 13:59:14 -0500

Message-ID:

<daniel_t-8ECA2C.13591419012008@earthlink.vsrv-sjc.supernews.net>

Erik WikstrFm <Erik-wikstrom@telia.com> wrote:

On 2008-01-19 16:15, Daniel T. wrote:

This is basically what I have now...

class Foo { /* definition irrelevant */ };

istream& operator>>( istream& is, Foo& foo ); // could throw

int main() {
   ifstream file( "file.txt" );
   Foo foo;
   int i = 0;
   try {
      while ( file >> foo ) {
         //process foo;
         ++i;
      }
   }
   catch ( exception& e ) {
      cout << "error at: " << i << '\n';
   }
}

The problem I'm having is that op>> could throw. If it does, I don't
want to process the foo object that threw, but I want the program to
report an error and continue extracting foo objects.

I could make op>> a no-throw and give Foo some sort of error or invalid
state, but that doesn't feel right. I can't report the error inside the
op>> because I don't have enough information to make a full report.

Does anyone have a better idea, or do I give Foo and error state and
query it after the read?

Something like this perhaps?

int main() {
   ifstream file( "file.txt" );
   Foo foo;
   int i = 0;
   while ( file.eof() == false ) {
      try {
         file >> foo;
      } catch ( exception& e )
         cout << "error at: " << i << '\n';
         continue;
      }
      ++i;
   }
}

That doesn't handle the end of file correctly...

I could do something like:

typedef int Foo;

int main() {
   ifstream file( "file.txt" );
   Foo foo;
   int i = 0;
   while ( file.eof() == false ) {
      try {
         while ( file >> foo ) {
            // process foo
            ++i;
         }
      } catch ( exception& e ) {
         cout << "error at: " << i << '\n';
      }
   }
}

But that seems strange.

Does anyone have a different idea?