How to Effectively Overload `operator==` in Class Hierarchies?

Operator Overloading in Class Hierarchies: Best Practices

When dealing with class hierarchies, it's crucial to consider the appropriate approach to overload operator== to ensure proper comparisons and avoid potential issues.

Free Function vs. Virtual Member Function

Overloading operator== as free functions for all classes may lead to issues where derived classes cannot leverage the base class's version without casting. Additionally, this approach prevents deep comparisons with only references to the base class.

Making operator== virtual member functions also has limitations. Derived class versions may require complex casts, as seen in the example provided. This can feel awkward and potentially introduce runtime errors.

Meyers' Effective C Approach

A preferred approach is to follow Scott Meyer's advice in Effective C . It involves:

• Defining non-leaf classes as abstract
• Declaring operator== protected and non-virtual in non-leaf classes
• Declaring operator== public and non-virtual in leaf classes

This approach prevents direct comparisons of objects of different types, as the base function is protected. However, leaf classes can leverage the base class's operator== to compare the common data members.

Additional Considerations

• If the base class contains data members, consider providing a protected non-virtual helper function in the base class that derived classes' operator== can use.
• Avoid implementing operator== that works on abstract base classes.
• Use caution when implementing a virtual compare function with dynamic_cast. If necessary, consider using a pure virtual function in the base class that is overriden in concrete derived classes and calls operator== for the derived class.

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