Beware of `==` on base classes

Reason

It is really hard to write a foolproof and useful == for a hierarchy.

Example, bad

class B {
    string name;
    int number;
public:
    virtual bool operator==(const B& a) const
    {
         return name == a.name && number == a.number;
    }
    // ...
};

B's comparison accepts conversions for its second operand, but not its first.

class D : public B {
    char character;
public:
    virtual bool operator==(const D& a) const
    {
        return B::operator==(a) && character == a.character;
    }
    // ...
};

B b = ...
D d = ...
b == d;    // compares name and number, ignores d's character
d == b;    // compares name and number, ignores d's character
D d2;
d == d2;   // compares name, number, and character
B& b2 = d2;
b2 == d;   // compares name and number, ignores d2's and d's character

Of course there are ways of making == work in a hierarchy, but the naive approaches do not scale.

Note

This rule applies to all the usual comparison operators: !=, <, <=, >, >=, and <=>.

Enforcement

• Flag a virtual operator==(); same for other comparison operators: !=, <, <=, >, >=, and <=>.