How to Effectively Overload operator== in a Class Hierarchy?

Overloading operator== in a Class Hierarchy

In a class hierarchy, overloading operator== to ensure a customizable and accurate comparison of objects becomes crucial. However, determining the correct approach can be challenging.

Free Functions vs. Virtual Member Functions

Free functions overload operator==, allowing for straightforward comparison of leaf nodes in the hierarchy. However, this method prohibits derived classes from inheriting the comparison logic of their base class without casting.

Virtual member functions provide an alternative approach, but they require casting and can be cumbersome for a deeply nested hierarchy.

Effective C Technique

The preferred method, inspired by Scott Meyer's Effective C advice, advocates the following steps:

• Declare non-leaf classes as abstract.
• Provide protected non-virtual operator== helper functions in non-leaf classes.
• Implement public non-virtual operator== in leaf classes.

This approach ensures that comparisons between different types are prevented as the base function is protected. Leaf classes, however, can leverage the parent's comparison logic for specific data members.

Avoiding Abstract Base Class Comparisons

To prevent accidental fallback comparisons, avoid implementing operator== in abstract base classes. Instead, provide a (protected) non-virtual helper function, such as isEqual(), within the base class that can be accessed by derived classes' operator== implementations.

Virtual Comparison with Dynamic Cast

In cases where a dynamic comparison is required, a pure virtual function in the base class can be utilized. The pure virtual function can then be overridden in concrete derived classes, referencing the operator== of the derived class.

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