Overloading << clarification

[daviddoria]

I posted a while back about how to overload <<

It seems all the examples online have the second argument as const, ie

ostream & operator << (ostream &output, const Point &p);

but if I have a member function that simply returns a double

double Point::getX()
{
return x_;
}

In the << function it complains about the const

error: the object has cv-qualifiers that are not compatible with the member function
            object type is: const Point

So I've fixed it two ways:
1) make the getX() function return a const double
2) change the << function to not accept a const, ie

ostream & operator << (ostream &output, const Point &p);

So should accessor functions always return const? Or is it ok to remove const from the << definition?

Thanks!

Dave

[Radical Edward]

> So should accessor functions always return const?
Accessors should be const methods at the very least, because the object state isn't changed in the body:

double Point::getX() const
{
  return x_;
}

That should clear up the error without forcing you to make the return value const. Edward tries to make every method that doesn't affect the object state a const method.

> Or is it ok to remove const from the << definition?
Ed's const rule applies to parameters too. If the parameter can be const, it should be const, so the overloaded operator<< is:

ostream & operator << (ostream &output, const Point &p);

Because the Point object should be treated as read-only for that operator.

[daviddoria]

ok, i'll start making that a habbit

here's another question though:

Vector3 Plane::Normal()
{
        double norm = sqrt(pow(A,2) + pow(B,2) + pow(C,2));
	geom_Vector3 V(A/norm, B/norm, C/norm);
	return V;
}

then in the << function, I can't do

... << P.Normal();

because Normal() isn't a const function. However, I should be able to do

geom_Vector3 N = P.Normal();
  output << "Plane Normal: " << N << endl;

but it's telling me

error: the object has cv-qualifiers that are not compatible with the member function
            object type is: const Plane
    Vector3 N = P.Normal();

Why can't I do that??

[Radical Edward]

> because Normal() isn't a const function
Why not? Edward doesn't see any obvious changes to the object state, so it can be a const method. If you get to the point where you need a const method but the state has to change, consider making the state mutable:

class Test {
  mutable int _state;
public:
  void ChangeState(int newState) const {
    _state = newState;
  }
};

int main()
{
  const Test t;

  t.ChangeState(10);
}

It's pretty rare that you'd need to do that though.

> Why can't I do that??
Because you're using a const object in the context of a non-const object. If the object is const, you can't even call a non-const method with it.

[smcguffee]

Ok, I came across this post because of an error I had that was because of a const that was introduced through netbeans automatically generating a getter function for me. When I removed all the const 's from my code, I was fine again. However, Radical Edward seems to be an expert and suggests as was automated for me that const is something that should be used. Does anyone have a reason for using a const? Does anyone have an explanation for what a const is? All I've ever seen about const is that they get in the way of compilations, adding headaches of const_cast 'ing and stuff. Is there a benefit to const 's? And what is this _state thing?

[daviddoria]

Const just means that the function is unable to modify the value. Of course if you don't want to change the value, then just don't change it! Const just provides a double check in case you made a mistake.