How Can I Avoid Double-Reading Lines When Using C File Input Streams?

Testing Stream State for File Input: Avoiding Double Reads

When reading data from a file, it's essential to avoid reading the same line multiple times. However, checking stream states like good() or !eof() before reading can lead to unexpected behavior.

Consider the following code:

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

This code is intended to read all lines from the file until the end is reached. However, it reads the last line twice. To understand why, we need to delve into the internals of stream state flags.

Stream State Flags

good(), eof(), fail(), and bad() are stream state flags that indicate the current status of an input stream. The good() flag is typically expected to indicate whether the stream is in a valid state for further reading, but that's not strictly accurate. It's more accurately described as the absence of errors or end-of-file conditions.

The Problem with good() and !eof()

When you check stream.good() or !stream.eof() before reading, you're not asking the stream if the next read operation will be successful. You're only querying the current state.

For example, when the last line is read from a file, the eof() flag is set. However, the good() flag might still be true, indicating no error occurred during the last read.

Solution

To avoid double reads, the correct approach is to test the stream after the read operation:

// Read all lines from a file
for (std::string line; getline(f, line);) {
  process(line);
}

// Read an integer and store it in foo
int foo;
if (f >> foo) {
  use(foo);
} else {
  handle_error();
}

This approach ensures that you only process data that was successfully read from the file.

The above is the detailed content of How Can I Avoid Double-Reading Lines When Using C File Input Streams?. For more information, please follow other related articles on the PHP Chinese website!