Question

class Person {
private:
    string firstName;
    string lastName;
public:
    Person() {}

    Person(ifstream &fin) {
       fin >> firstName >> lastName;
    }

    void print() {
       cout << firstName
           << " "
           << lastName
           << endl;
    }
};

int main() {
    vector<Person> v;
    ifstream fin("people.txt");

    while (true) {
        Person p(fin);
        if (fin == NULL) { break; }
        v.push_back(p);
    }

    for (size_t i = 0; i < v.size(); i++) {
       v[i].print();
    }

    fin.close();
    return 0;
}

Please can you explain me, how following code snippet works?
if (fin == NULL) { break; }

fin is a object on stack, not a pointer so it can not become NULL.
I was unable to find overloaded operator== function in ifstream class.
So I can not understand how this snippet works.

Answer 1

The ifstream class has an operator void *() (or operator bool() in C++11). This is what is called when you test (fin == NULL).

Testing fin == NULL should be exactly the same as testing fin.fail().