Comparison operators should not be virtual

Making a comparison operator virtual implies that you want to compare objects of different types by overriding operator==, for instance, in a subclass to compare instances of the base class with instances of the subclass. But polymorphic comparison operators are very difficult to get right, and are actually questionable in concept. After all, can two objects with only a few common members really be equal?

This rule raises issues on virtual comparison operators.

Noncompliant Code Example

struct Foo {
  virtual bool operator==(const Foo &other) const; // Noncompliant
  virtual bool operator!=(const Foo &other) const; // Noncompliant
};

Compliant Solution

struct Foo {
  bool operator==(const Foo &other) const;
  bool operator!=(const Foo &other) const;
};

See

• C++ Core Guidelines C.87 - Beware of == on base classes