eof() bad practice?

c++ eof
Sam picture Sam · Apr 29, 2011 · Viewed 20.1k times · Source

Possible Duplicate:
Why is iostream::eof inside a loop condition considered wrong?

So I've been using the eof() function in a lot of my programs that require file input, and my professor said that it is fine to use but a few people on SO have said that I shouldn't use it without really specifying the reason. So I was wondering, is there a good reason?

Answer

Erik picture Erik · Apr 29, 2011

You can use eof to test for the exact condition it reports - whether you have attempted to read past end of file. You cannot use it to test whether there's more input to read, or whether reading succeeded, which are more common tests.

Wrong:

while (!cin.eof()) {
  cin >> foo;
}

Correct: 

if (!(cin >> foo)) {
  if (cin.eof()) {
    cout << "read failed due to EOF\n";
  } else {
    cout << "read failed due to something other than EOF\n";
  }
}

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