How Does C \'s `ifstream::eof()` Function Really Work with `get()` and the Extraction Operator?

Understanding ifstream's eof() Function

The ifstream class's eof() function plays a crucial role in file input operations in C . However, its behavior can be puzzling at times, especially in relation to the get() function.

Consider the example provided:

#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;
}

When the input file "ex.txt" contains "abc," the first while loop reads four characters before terminating. This is because eof() only sets the EOF flag after an attempt to read past the end of the file. The first loop reads character by character until it reaches a failed read, setting the EOF flag. However, get() returns -1 to indicate end-of-file, not considering the EOF flag.

The second loop, using the >> operator, exhibits correct behavior. The >> operator attempts to read a character (in this case, a string) and sets the EOF flag upon a failed read. Thus, the loop ends after reading "abc."

Resolving Confusion

To avoid confusion, it's crucial to note that:

• -1 is get()'s way of indicating end-of-file.
• eof() sets the EOF flag after a failed read attempt beyond the file's end.

• performs both a read and EOF flag update.

Therefore, using >> is recommended over eof() with get() to accurately detect the end of a file.

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