how do I open and read a file using ifstream in C++?

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.

Solution 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.
}

Solution 2

For what you're doing here, simply:

ifs >> number;

Will extract a number from the stream and store it in 'number'.

Looping, depends on the content. If it was all numbers, something like:

int x = 0;
while (ifs >> numbers[x] && x < MAX_NUMBERS)
{
 ifs >> number[x];
 x++;
}

Would work to store a series of numbers in an array. This works because extraction operator's side effect is true if the extraction succeeds, or false if it fails (due to end of file or disk errors, etc.)

Comments

[Iain]

(If anyone saw this in the first 2 minutes, I left out the actual extraction in my first pass)

[John Kugelman]

Heheh, next time just ninja edit it right in and say nothing. That is the Stack Overflow Way.

[Hristo]

@Feaelin... how would I handle whitespace? If the line is something like ` 15 , I just want to get 15` out of there.

[John Kugelman]

@Hristo Whitespace is skipped over by >>. Try it and see!

[Hristo]

.. how would I handle whitespace?

[Martin York]

By default white space is silently dropped by the operator>>