[c++] How does ifstream's eof() work?

#include <iostream>
#include <fstream>

int main() {
    std::fstream inf( "ex.txt", std::ios::in );
    while( !inf.eof() ) {
        std::cout << inf.get() << "\n";
    }
    inf.close();
    inf.clear();
    inf.open( "ex.txt", std::ios::in );
    char c;
    while( inf >> c ) {
        std::cout << c << "\n";
    }
    return 0;
}

I'm really confused about eof() function. Suppose that my ex.txt's content was:

abc

It always reads an extra character and shows -1 when reading using eof(). But the inf >> c gave the correct output which was 'abc'? Can anyone help me explain this?

The EOF flag is only set after a read operation attempts to read past the end of the file. get() is returning the symbolic constant traits::eof() (which just happens to equal -1) because it reached the end of the file and could not read any more data, and only at that point will eof() be true. If you want to check for this condition, you can do something like the following:

int ch;
while ((ch = inf.get()) != EOF) {
    std::cout << static_cast<char>(ch) << "\n";
}

eof() checks the eofbit in the stream state.

On each read operation, if the position is at the end of stream and more data has to be read, eofbit is set to true. Therefore you're going to get an extra character before you get eofbit=1.

The correct way is to check whether the eof was reached (or, whether the read operation succeeded) after the reading operation. This is what your second version does - you do a read operation, and then use the resulting stream object reference (which >> returns) as a boolean value, which results in check for fail().

iostream doesn't know it's at the end of the file until it tries to read that first character past the end of the file.

The sample code at cplusplus.com says to do it like this: (But you shouldn't actually do it this way)

  while (is.good())     // loop while extraction from file is possible
  {
    c = is.get();       // get character from file
    if (is.good())
      cout << c;
  }

A better idiom is to move the read into the loop condition, like so: (You can do this with all istream read operations that return *this, including the >> operator)

  char c;
  while(is.get(c))
    cout << c;