overloading operator== complaining of 'must take exactly one argument'

Question

I am trying to overload the operator==, but the compiler is throwing the following error:

‘bool Rationalnumber::operator==(Rationalnumber, Rationalnumber)’ must take exactly one argument

My short piece of code is as follows:

bool Rationalnumber::operator==(Rationalnumber l, Rationalnumber r) {
  return l.numerator() * r.denominator() == l.denominator() * r.numerator();
}

Declaration:

bool operator==( Rationalnumber l, Rationalnumber r );

Does anyone have any ideas why it's throwing the error?

Answer 1

If operator== is a non static data member, is should take only one parameter, as the comparison will be to the implicit this parameter:

class Foo {
  bool operator==(const Foo& rhs) const { return true;}
};

If you want to use a free operator (i.e. not a member of a class), then you can specify two arguments:

class Bar { };
bool operator==(const Bar& lhs, const Bar& rhs) { return true;}