c++ - Testing stream.good() or !stream.eof() reads last line twice

I
have the following piece of
code:

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

 // Use line
            here.
}

But
this reads the last line twice. Why does this happen and how do I fix
it?

Something very similar happens
with:

ifstream
            f("x.txt");
string line;

while (!f.eof()) {

            getline(f, line);
 // Use line
            here.
}

class="post-text" itemprop="text">

class="normal">Answer

You very,
very rarely want to check bad, eof, and good. In particular for eof (as !stream.eof() is
a common mistake), the stream currently being at EOF does not necessarily mean the last
input operation failed; conversely, not being at EOF does not mean the last input was
successful.

All of the stream state functions –
fail, bad, eof, and good – tell you the current state of the stream rather than
predicting the success of a future operation. Check the stream itself (which is
equivalent to an inverted fail check) after the desired
operation:

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

            handle_error();
}

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

else {

            handle_error();
}

if (!(stream >> foo)) { //
            operator! is overloaded for streams
 throw
            SomeException();
}
use(foo);

To
read and process all lines:

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

            process(line);
}

Pointedly,
good() is misnamed and is not equivalent to testing the stream itself (which the above
examples do).