How does ifstream's eof() work?

Question

#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?

Answer 1

-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.