Errors encountered during file operation

Errors can occur during file operations, for example when opening a missing file or reading past the end. C++ records them in the stream state flags of ios, which can be tested with member functions.

Common error situations

1. Opening a file for reading that does not exist.
2. Opening a file with an invalid or non-existent name/path, or a write-protected file.
3. Opening a file for writing when the disk is full or write permission is missing.
4. Reading past the end of the file.
5. Using a stream that has not been opened, or attempting an operation not allowed by the mode (for example, reading from an fstream opened only with ios::out).
6. Hardware or disk failures during reading or writing, and unreadable data (reading text where a number is expected).
7. Too many files open at once.

Error-state flags and functions

Example

#include <iostream>
#include <fstream>
using namespace std;
int main() {
    ifstream fin("missing.txt");
    if (fin.fail()) {                       // or: if (!fin)
        cout << "Cannot open file\n";
        return 1;
    }
    int x;
    while (fin >> x) cout << x << endl;
    if (fin.eof()) cout << "End of file reached\n";
    else if (fin.bad()) cout << "Fatal read error\n";
    fin.close();
    return 0;
}

Also check is_open() after opening, and always test the result of each read or write.