How to Safely Overload the Equality Operator in Class Hierarchies?

Operator Overloading for Class Hierarchies: A Comprehensive Approach

In the realm of object-oriented programming, it is often desired to compare objects of related classes for equality. However, when dealing with class hierarchies, determining the correct approach for overloading the equality operator can be a challenge.

Consider the following class hierarchy:

class A
{
    int foo;
    virtual ~A() = 0;
};

A::~A() {}

class B : public A
{
    int bar;
};

class C : public A
{
    int baz;
};

There are several approaches to overloading the equality operator for such a hierarchy.

Free Functions

Overloading operator== as free functions allows for direct comparison of objects without casting. However, this approach prevents leveraging the equality check of the base class (A) for derived classes (B and C).

Virtual Member Functions

Using the virtual member function approach, derived classes can override the equality check. However, this requires dynamic casting to avoid comparing objects of different types, which can feel verbose.

The preferred approach is to follow the principles outlined by Scott Meyer's "Effective C ":

Abstract Base Classes

Avoid declaring concrete base classes and make them abstract if they do not have complete implementations.

Protected Non-Virtual Member Functions

In non-leaf classes, provide protected non-virtual helper functions for equality checking (e.g., isEqual()).

Public Non-Virtual Member Functions

In leaf classes, define public non-virtual equality operator overloads that leverage the helper functions in the base class.

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

This approach prevents accidental fallbacks and ensures type safety when comparing objects of different types.

For cross-hierarchy equality checks, consider using a pure virtual function in the base class that is overridden in derived classes.

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

By adhering to these principles, it is possible to achieve robust and type-safe equality operator overloading for complex class hierarchies.

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