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why still end of file with ifstream::open(..

why still end of file with ifstream::open(..

why still end of file with ifstream::open(..

(OP)
Isn't once we use open(..) function to open the ifstream file will automatically place the file pointer to the beginning?

CODE

ifstream inF("file.dat", ios::in | ios::binary);
    for (;;) {
        inF.read((char *)&num, sizeof(int));
        if (inF.gcount() == 0)
            break;
        cout << num << endl;
    }
    inF.close();

    ifstream inF("file.dat", ios::in | ios::binary);
    inF.clear();  // ???
    for (;;) {
        inF.read((char *)&num, sizeof(int));
        if (inF.gcount() == 0)
            break;
        cout << num << endl;
    }
    inF.close();

Look at the line with question mark commented out, if this clear() function not exist there, the file cannot be readed out, it is definitely eof reported with the open function. why?

RE: why still end of file with ifstream::open(..

Your code won't even compile because you are declaring the same variable name twice.

Normally, clear is used when you are re-using an fstream. So if your code was actually this:

CODE

    ifstream inF("file.dat", ios::in | ios::binary);
    for (;;) {
        inF.read((char *)&num, sizeof(int));
        if (inF.gcount() == 0)
            break;
        cout << num << endl;
    }
    inF.close();

    inF.open("file.dat", ios::in | ios::binary);
    inF.clear();  // ???
    for (;;) {
        inF.read((char *)&num, sizeof(int));
        if (inF.gcount() == 0)
            break;
        cout << num << endl;
    }
    inF.close();

Then the clear() is necessary because the previous bit of code sets a failbit when it can no longer read. Calling close or open doesn't reset the failbit, you have to call clear() explicitly when you re-use a stream.

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