How Can I Avoid Double-Reading the Last Line When Using ifstream in C ?

Stream State Management: Avoiding Double-Reading the Last Line

When reading from a file using ifstream, it's essential to properly manage stream state to avoid unexpected behavior, such as reading the last line twice.

Why Double-Reading Occurs

Using f.good() or !f.eof() in the loop condition can cause the last line to be read twice. This is because:

• f.good(): Checks only if there have been no errors encountered so far, but does not indicate whether the next operation (e.g., getline) will succeed.
• !f.eof(): Checks if the stream has not reached the end of the file yet, but does not guarantee that the next operation will be successful.

At the end of the file, there might be an incomplete or malformed line that causes getline to fail. If f.good() or !f.eof() is used in the loop condition, the loop continues even after getline fails, leading to the rereading of the last valid line.

Correct Stream State Management

To avoid this issue, stream state should be checked after performing the desired input operation. This can be done using the following techniques:

• After getline:

  if (getline(stream, line)) {
      use(line);
  } else {
      handle_error();
  }

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• After operator>>:

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

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For reading and processing all lines, the following loop construction is recommended:

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

This loop automatically terminates when getline fails to read another line.

Note on good()

The function good() can be misleadingly named. It only indicates that there have been no errors encountered so far, but it does not imply that the next operation will be successful. Therefore, it is generally not recommended to use good() to predict the outcome of future input operations.

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