Equality Test for Derived Classes in C++ [duplicate]

Answer 1

I remember reading a succinct description of the public-non-virtual/non-public-virtual idiom and its advantages, but not where. This wikibook has an okay description.

Here is how you apply it to op==:

struct A {
  virtual ~A() {}

  int a;

  friend
  bool operator==(A const& lhs, A const& rhs) {
    return lhs.equal_to(rhs);
  }
  // http://en.wikipedia.org/wiki/Barton-Nackman_trick
  // used in a simplified form here

protected:
  virtual bool equal_to(A const& other) const {
    return a == other.a;
  }
};

struct B : A {
  int b;

protected:
  virtual bool equal_to(A const& other) const {
    if (B const* p = dynamic_cast(&other)) {
      return A::equal_to(other) && b == p->b;
    }
    else {
      return false;
    }
  }
};

struct C : A {
  int c;

protected:
  virtual bool equal_to(A const& other) const {
    if (C const* p = dynamic_cast(&other)) {
      return A::equal_to(other) && c == p->c;
    }
    else {
      return false;
    }
  }
};