File Modes & EOF

Overview

File modes define how a file is opened (e.g., appending data instead of overwriting, or reading in binary). `.eof()` checks if the end of the file has been reached.

Syntax

cpp

// std::ios::app  = Append mode
// std::ios::trunc = Truncate (overwrite, default for ofstream)
// std::ios::binary = Binary mode

ofstream log("log.txt", std::ios::app);
log << "New entry added to the bottom.\n";

// Checking EOF
ifstream in("data.txt");
while (!in.eof()) {
    // Process file
}

Common Pitfalls

Using `while(!file.eof())` and unconditionally reading inside the loop. It almost always results in the last line of the file being processed twice.

Interview Tips

The `eof()` flag is only set AFTER you try to read past the end of the file. Because of this, `while(!file.eof())` often processes the last line twice if written poorly.

Real-World Example

Properly checking for EOF while reading.

example

cpp

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream file("numbers.txt");
    int num;
    
    // The correct way: extraction operator returns true if successful
    // This perfectly avoids the duplicate-last-line EOF bug
    while (file >> num) {
        cout << "Read: " << num << endl;
    }
    
    return 0;
}

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