How do you Handle Operator Overloading in Complex Class Hierarchies?

Operator Overloading in Class Hierarchies: Best Practices

Overloading operator== for a class hierarchy can present challenges, particularly when dealing with non-leaf classes and potential type conversions. Let's delve into the recommended approaches to achieve correct and maintainable operator== implementations.

Free Functions for Concrete Types

For leaf-node classes, implementing operator== as free functions is recommended. This approach provides type safety and ensures that comparisons between different types won't compile. For example:

bool operator==(const B& lhs, const B& rhs) {
    return lhs.isEqual(rhs) && lhs.bar == rhs.bar;
}

Helper Functions in Base Classes

If the base class contains data members, consider providing a (protected) non-virtual isEqual helper function. Derived classes' operator== implementations can leverage this function to compare common data members. This prevents accidental fallbacks where only the base class portions of different objects are compared.

Virtual Functions in Base Classes: Use with Caution

Implementing operator== as a virtual member function with dynamic_cast is possible but should be used cautiously. Consider introducing a pure virtual isEqual function in the base class, which derived classes override and implement using the operator== for the derived class. For example:

bool B::pubIsEqual(const A& rhs) const {
    const B* b = dynamic_cast<const B*>(rhs);
    return b != nullptr && *this == *b;
}

Additional Considerations

• Make non-leaf classes abstract to avoid ambiguity.
• Keep non-virtual compare functions protected to enforce type safety.
• Only implement operator== for concrete leaf-node classes.

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