Question

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Why is iostream::eof inside a loop condition considered wrong?
eof() bad practice?

My teacher said we shouldn’t use EOF to read in text file or binary file information instead we should use (afile>>x). He didn’t explain why, can someone explain to me. Can someone also explain what are the differences in this two different method of reading

//Assuming declaration 
//ifstream foobar



( ! foobar.eof() )
{
    foobar>>x; // This is discouraged by my teacher

}


 while (foobar>>x)
{
  //This is encouraged by my teacher

}

Answer 1

Because the file is not at the end before you try to read from it.

operator>> returns a reference to the stream in the state it is after the read has been attempted and either succeeded or failed, and the stream evaluates to true if it succeeded or false if it failed. Testing for eof() first means that the file can have no useful data in it but not be at EOF yet, then when you read from it, it’s at EOF and the read fails.

Another important detail is that operator>> for streams skips all leading whitespace, not trailing whitespace. This is why a file can not be at EOF before the read and be at EOF after a read.

Additionally, the former works when the next data in the file is data that cannot be read into an integer (for example, the next data is x), not just when it’s at EOF, which is very important.

Example:

Consider the code:

int x, y;

f >> x;

if (!f.eof())
    f >> y;

Assuming f is a file that contains the data 123␣ (the ␣ means space), the first read will succeed, but afterwards the file has no more integers in it and it is not at EOF. The second read will fail and the file will be at EOF, but you don’t know because you tested for EOF before you tried reading. Then your code goes on to cause undefined behaviour because y is uninitialised.