Question

I would like to open a file and read a line from it. There will be only one line in the file so I don’t really need to worry about looping, although for future reference it would be nice to know how to read multiple lines.

int main(int argc, const char* argv[]) {

    // argv[1] holds the file name from the command prompt

    int number = 0; // number must be positive!

    // create input file stream and open file
    ifstream ifs;
    ifs.open(argv[1]);

    if (ifs == NULL) {
        // Unable to open file
        exit(1);
    } else {
        // file opened
        // read file and get number
        ...?
        // done using file, close it
        ifs.close();
    }
}

How would I do this? Also, am I handling the file open correctly in terms of successful open?

Thanks.

Answer 1

A couple of things:

1. You can read a number with the >> stream extraction operator: ifs >> number.

2. The standard library function getline will read a line from a file, if you want a full line of text.

3. To check if the file opened, just write if (ifs) or if (!ifs). Leave out the == NULL.

4. You don’t need to explicitly close the file at the end. That will happen automatically when the ifs variable goes out of scope.

Revised code:

if (!ifs) {
    // Unable to open file.
} else if (ifs >> number) {
    // Read the number.
} else {
    // Failed to read number.
}