How to Correctly Test for End-of-File When Reading from a File Stream in C ?

Testing Stream Input State vs. Future Success

When reading input using streams, it's essential to understand the difference between testing stream state and predicting the success of future operations.

Issue: Reading Last Line Twice

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

In this code, the last line is read twice because f.good() tests the current stream state, not whether getline() will succeed. Once getline() reaches the EOF, f remains in a "good" state, so the loop continues even after the last line has been read.

Solution: Test Stream After Operation

Instead, check the stream state after performing the input operation:

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

Alternative Using eof()

While it's not recommended to use eof() in loop conditions, here's an equivalent solution using !f.eof():

ifstream f("x.txt");
string line;
while (!f.eof()) {
  if (getline(f, line)) {
    // Use line here.
  } else {
    break;
  }
}

Range-Based Loop for All Lines

To read and process all lines, use a range-based loop:

ifstream f("x.txt");
for (std::string line; getline(f, line);) {
  process(line);
}

Good vs. Stream State

Note that f.good() is misnamed. It does not truly indicate the stream's "goodness" but rather returns true when the stream is not in a "fail" or "bad" state. Always test the stream itself (i.e., check its inverted fail state) after performing input operations to ensure success.

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