-1 is get
's way of saying you've reached the end of file. Compare it using the std::char_traits<char>::eof()
(or std::istream::traits_type::eof()
) - avoid -1, it's a magic number. (Although the other one is a bit verbose - you can always just call istream::eof
)
The EOF flag is only set once a read tries to read past the end of the file. If I have a 3 byte file, and I only read 3 bytes, EOF is false
, because I've not tried to read past the end of the file yet. While this seems confusing for files, which typically know their size, EOF is not known until a read is attempted on some devices, such as pipes and network sockets.
The second example works as inf >> foo
will always return inf
, with the side effect of attempt to read something and store it in foo
. inf
, in an if
or while
, will evaluate to true
if the file is "good": no errors, no EOF. Thus, when a read fails, inf
evaulates to false
, and your loop properly aborts. However, take this common error:
while(!inf.eof()) // EOF is false here
{
inf >> x; // read fails, EOF becomes true, x is not set
// use x // we use x, despite our read failing.
}
However, this:
while(inf >> x) // Attempt read into x, return false if it fails
{
// will only be entered if read succeeded.
}
Which is what we want.