How can I overload a 'less than' operator in my C++ class in order to compare consts?

Question

I am trying to overload the less than operator '<' in my class as follows:

//header file

class HuffmanNode{
  private:
    ...
    ...
    int frequency;

  public:
    ...
    ...
    bool operator<(const HuffmanNode &rhs); //overload less than operator
};

---

//cpp file

bool HuffmanNode::operator<(const HuffmanNode &rhs){
  return frequency < rhs.frequency;
}

---

I want to be able to compare the nodes as follows:

bool HuffmanTree::compareNode(const HuffmanNode &a, const HuffmanNode &b){
  if (a < b){
    return true;
  }
  else{
    return false;
  }
}

---

The problem I'm having is finding a way to compare the two nodes as consts. I get an error saying that my operator overloading method needs to be marked as const but changing the code in the header to

const bool operator<(const HuffmanNode &rhs);

and the cpp file code to

const bool HuffmanNode::operator<(const HuffmanNode &rhs){
  return frequency < rhs.frequency;
}

doesn't seem to remove the error.

I have checked out this solution but using the friend keyword didn't seem to work either.

Thanks for any help!

Answer 1

You misunderstand the concept of usage and positioning of the keyword const .

Just to clarify:

const T functionName(something);

means your functionName returns something of the type T and that is a constant.

Now this:

T functionName(something) const;

means your functionName returns something of the type T and that method is not changing anything in the instance and it is safe to use it even with the objects declared as const .

The 2nd option is the one you are looking for.