重载operator <<作为成员打印

Question

Is there a way to overload the << operator, as a class member, to print values as a text stream. Such as:

class TestClass {
public:
    ostream& operator<<(ostream& os) {
        return os << "I'm in the class, msg=" << msg << endl;
    }

private:
    string msg;
};


int main(int argc, char** argv) {
    TestClass obj = TestClass();
    cout << obj;

    return 0;
}

The only way I could think of was to overload the operator outside of the class:

ostream& operator<<(ostream& os, TestClass& obj) {
    return os << "I'm outside of the class and can't access msg" << endl;
}

But then the only way to access the private parts of the object would be to friend the operator function, and I'd rather avoid friends if possible and thus ask you for alternative solutions.

Any comments or recommendations on how to proceed would be helpful :)

Answer 1

It needs to be a non-member, since the class forms the second argument of the operator, not the first. If the output can be done using only the public interface, then you're done. If it needs access to non-public members, then you'll have to declare it a friend; that's what friends are for.

class TestClass {
public:
    friend ostream& operator<<(ostream& os, TestClass const & tc) {
        return os << "I'm a friend of the class, msg=" << tc.msg << endl;
    }

private:
    string msg;
};

Answer 2

You have stumbled across the canonical way to implement this functionality. What you have is correct.

Answer 3

I believe one popular way to do this is a non-member, non-friend free operator<< that calls a public non-virtual print method within your class. This print method can either do the work or delegate to a protected virtual implementation.

class TestClass {
public:
    ostream& print(ostream& os) const {
        return os << "I'm in the class, msg=" << msg << endl;
    }

private:
    string msg;
};


ostream& operator<<(ostream& os, TestClass& obj) {
    return obj.print(os);
}

int main(int argc, char** argv) {
    TestClass obj;
    cout << obj;

    return 0;
}

Answer 4

You can make it member of the class, that is on the left of << , which is ostream in your case.

What you can do, though, is have a base class with void do_stream(ostream& o); member for all your streamables and non-member operator<< that would call it.

Answer 5

You're right, that's the only way to implement the stream operator - outside the class.

You need to declare the method as friend .

That's how it's done.

Answer 6

You have to make it a non member (as the first parameter is not your class).

But you can write it inside your class definition (as a friend):

class TestClass
{
public:
    // Have a nice friend.
    // This tightly binds this operator to the class.
    // But that is not a problem as in reality it is already tightly bound.
    friend ostream& operator<<(ostream& os, TestClass const& data)
    {
        return os << "I'm in the class, msg=" << data.msg << endl;
    }

private:
    string msg;
};

I see nothing wrong with making this a friend.