Why does while(file >> a) work?

Question

I have to read from a file where are only few numbers. Read it, sum them up and that's it. (Learning C++). If I was using fstream::eof() function it read the last number twice, so I did it like this:

#include <iostream>
#include <fstream>

using namespace std;

int main() {
    fstream file;
    int a = 0, sum = 0;

    file.open("nums.txt",fstream::in);

    if (!file.is_open()) {
        cout << "Unable to open file!\n" << endl;
    }

    while (file >> a) {
        cout << a << endl;
        sum += a;
    }

    cout << "Sum: " << sum << endl;

    file.close();

    getchar();
    return 0;
}

Now it doesn't read last number twice, but I don't understand what is happening in while() brackets, I thought if I read 0 it would stop, but it reads it as well. Is file >> a actually returning something like "managed to read" = 1 and "fail" = 0 ??

Thanks

Possible duplicate of [How is "std::cin>>value" evaluated in a while loop?](https://stackoverflow.com/questions/34395801/how-is-stdcinvalue-evaluated-in-a-while-loop) — Tadeusz Kopec for Ukraine, Oct 03 '17 at 07:49

score 7 · Answer 1 · answered Oct 03 '17 at 07:02

7

Is file >> a actually returning something

Yes. It returns a reference to file itself. And std::fstream is contextually convertible to bool via its operator bool

It essentially returns the logical inverse of the streams fail() member.

answered Oct 03 '17 at 07:02

StoryTeller - Unslander Monica

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score 3 · Answer 2 · answered Oct 03 '17 at 07:04

The >> operator, when applied to a stream, returns that same stream. This is why you can write stuff like file >> a >> b. It also happens that the stream has an operator bool() that is equivalent to !fail(). Thus, once the stream has reached eof, the operator bool() will return false, and since >> returns the stream, while (file >> a) will loop until the stream hits eof (or has an error).

Why does while(file >> a) work?

2 Answers2