MAK MAK - 1 year ago 80
C++ Question

Why is iostream::eof inside a loop condition considered wrong?

I just found a comment in this answer saying that using

in a loop condition is "almost certainly wrong". I generally use something like
- which I guess implicitly checks for EOF, why is checking for eof explicitly using

How is it different from using
in C (which I often use with no problems)?

Xeo Xeo
Answer Source

Because iostream::eof will only return true after reading the end of the stream. It does not indicate, that the next read will be the end of the stream.

Consider this (and assume then next read will be at the end of the stream):

  int data;
  // yay, not end of stream yet, now read ...
  inStream >> data;
  // oh crap, now we read the end and *only* now the eof bit will be set (as well as the fail bit)
  // do stuff with (now uninitialized) data

Against this:

int data;
while(inStream >> data){
  // when we land here, we can be sure that the read was successful.
  // if it wasn't, the returned stream from operator>> would be converted to false
  // and the loop wouldn't even be entered
  // do stuff with correctly initialized data (hopefully)

And on your second question: Because


is the same as

if(!(inStream >> data).eof())

and not the same as

    inFile >> data
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