c++ - Testing stream.good() or !stream.eof() reads last line twice

I have the following piece of code:

ifstream f("x.txt");
string line;
while (f.good()) {
  getline(f, line);
  // Use line here.
}

But this reads the last line twice. Why does this happen and how do I fix it?

Something very similar happens with:

ifstream f("x.txt");
string line;
while (!f.eof()) {
  getline(f, line);

  // Use line here.
}

Answer

You very, very rarely want to check bad, eof, and good. In particular for eof (as !stream.eof() is a common mistake), the stream currently being at EOF does not necessarily mean the last input operation failed; conversely, not being at EOF does not mean the last input was successful.

All of the stream state functions – fail, bad, eof, and good – tell you the current state of the stream rather than predicting the success of a future operation. Check the stream itself (which is equivalent to an inverted fail check) after the desired operation:

if (getline(stream, line)) {
  use(line);

}
else {
  handle_error();
}

if (stream >> foo >> bar) {
  use(foo, bar);
}
else {
  handle_error();

}

if (!(stream >> foo)) {  // operator! is overloaded for streams
  throw SomeException();
}
use(foo);

To read and process all lines:

for (std::string line; getline(stream, line);) {
  process(line);
}

Pointedly, good() is misnamed and is not equivalent to testing the stream itself (which the above examples do).