Question

Why am I receiving an error when I do not put const in the function bool operator<(const Node& otherNode) //const?

stl_algo.h:91: error: passing 'const Node' as 'this' argument of 'bool Node::operator<(const Node&)' discards qualifiers

Should all the overloaded operators be constant?

class Node {
public:
    double coordinate;

    bool operator==(const Node& other) const{
        return coordinate == other.coordinate;
    }

    bool operator<(const Node& other) const{
        return coordinate < other.coordinate;
    }
};

Answer 1

Not all operators, but == and < should definitely be made const, yes. They logically don’t modify either of the objects being compared.

The error likely comes from calling a non-const method from a const one, e.g.:

bool isSmaller(const Node& other) const
{
   return *this < other;
}

In this case, since the method isSmaller is const, this is implicitly a const object, therefore operator < also has to be const in order for the call within that context to be valid.

From the error message, it appears that Node::operator < is being called on a const object, from a function in stl_algo.h – sorting/ordering functions, hashing functions, etc.