Overloading the stream insertion operator?

Question

I am making a three-dimensional vector class (called Vector3) in c++. Right now, I am trying to overload the stream insertion operator (<<) so that I can directly print all of the vector's components all at once. I copied the sintax from MSD, but I get an 3 errors.

error: passing 'const Vector3 ' as 'this' argument of 'float Vector3::getX()' discards qualifiers [-fpermissive]
error: passing 'const Vector3 ' as 'this' argument of 'float Vector3::getY()' discards qualifiers [-fpermissive]
error: passing 'const Vector3 ' as 'this' argument of 'float Vector3::getZ()' discards qualifiers [-fpermissive]

^{(differences are marked in bold)}

In Vector3.h under public, I entered this function declaration:

friend ostream& operator<<(ostream &os, const Vector3 &vector);

In Vector3.cpp, I implemented it:

ostream& operator<<(ostream& os, const Vector3& vector)
{
    os << "(" << vector.getX() << ", " << vector.getY() << ", " << vector.getZ() << ")" << endl;
    return os;
}

It should print out something like (x, y, z) according to the x, y, and z variables.

On a side note, shouldn't the vector.getX() lines use the -> instead of the dot, because the vector object is an address pointer?

Your operator<< doesn't need to be a friend if it only calls public member functions of Vector3. — Blastfurnace
As to your side note, no it should not - the vector parameter is not a pointer (syntactically), it is a reference. Reference variables have the same "dot" syntax as value variables. — Rob I

BoBTFish BoBTFish · Accepted Answer · 2012-08-14T13:46:02

Add const qualifiers to your get... functions.

class Vector3
{
    public:
        float getX() const {return x;};
};

A const-qualified function simply means you can call it on a const instance of the class. Here, the getters won't change anything. However, if you don't specify that, the compiler doesn't know, so calling a function that might change something on a const variable is not allowed.

You can read about const-correctness here.

And the dot notation is correct, vector is not a pointer, but a reference (i.e. another name for some other Vector3. The & notation can be a little confusing for beginners I think. Roughly: when you apply it to a variable, it takes its address, when it's part of a type, it means that type is a reference. E.g. see here or here.

Overloading the stream insertion operator?

3 Answers