C++ Operator Overload - call function to get value inside overload

Question

Is there a way I can call an operator overload in C++ and call the parameter's function during the comparison?

For example:

class MyClass{
  private:
    int x;
    int y;
  public:
    MyClass(int x, int y);
    int getX();
    int getY();
    bool operator < (const MyClass &other) const {
        return (x < other.getX()); //this does not work!
        // this would work, though, if x was public:
        // return x < other.x;
    }
};

Basically, where I call other.getX(), how can I make it return its own x-value through a function to compare to the local one, instead of having to make x public? Is there a way of doing that?

Thanks for all your help!

It's a bit hard to get to the heart of your question. There is nothing wrong with your posted code, both other.getX() and other.x are valid (but getX() needs to be marked as const). — Martin Gunia
– Martin Gunia, Commented Sep 2, 2011 at 20:38
@RTT: Yes, I noticed the const problem after commenting first. However, both calls were ok, only not const-correct. It's funny how some posts get 4 answers before one is able to write few lines of comments :) — Martin Gunia
– Martin Gunia, Commented Sep 2, 2011 at 20:42
You can just do x < other.x a class is automatically a friend of itself. So you have full access to members of other instances of the same class. — Loki Astari
– Loki Astari, Commented Sep 2, 2011 at 21:08

PlasmaHH · Accepted Answer · 2011-09-02 20:32:41Z

9

You need to make the functions const, since you are using a reference to const MyClass:

int getX() const;

You can read about good use of const (const correctness) at:

Additionally I would recommend that you make the operator< a free function instead.

answered Sep 2, 2011 at 20:32

PlasmaHH

16.1k5 gold badges46 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Peter · Accepted Answer · 2011-09-02 20:33:06Z

6

It doesn't work because getX() is not a const function. Change it to:

int getX() const;

and it will work. You could also remove the const in the operator argument, but that wouldn't normally be considered as good.

answered Sep 2, 2011 at 20:33

Peter

7,3552 gold badges37 silver badges46 bronze badges

Comments

Loki Astari · Accepted Answer · 2011-09-02 21:10:25Z

1

Access is per-class, not per-instance. You can access other.x with no problem.

Other than that, you could return a const reference, but it really wouldn't make any difference performance-wise and just looks weird.

bool operator < (const MyClass &other) const
{
    return x < other.x;
}

edited Sep 2, 2011 at 21:10

Loki Astari

266k87 gold badges344 silver badges575 bronze badges

answered Sep 2, 2011 at 20:30

Cat Plus Plus

131k27 gold badges205 silver badges226 bronze badges

1 Comment

Loki Astari Over a year ago

+1 This is the preferred method. I think its safe to assume you know how your own class works so it is safe to access members of other instances so there is no point in going through useless getter method.

TeaWolf · Accepted Answer · 2011-09-02 20:32:19Z

-1

You can declare the operator as a friend of the class.

Edit: I seem to have tomatoes on my eyes, your operator is class member and already has private access. You could friend it however it was defined outside of class scope.

answered Sep 2, 2011 at 20:32

TeaWolf

7145 silver badges10 bronze badges

3 Comments

Peter Over a year ago

The operator is a member of the class, so already has all the access friend would give to a nonmember function.

TeaWolf Over a year ago

Yes, I realized that after posting as well, hence my earlier edit.

Loki Astari Over a year ago

A class is already a friend of itself. So it can access members of other instances.

Collectives™ on Stack Overflow

C++ Operator Overload - call function to get value inside overload

4 Answers 4

Comments

Comments

1 Comment

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related