5.8 Object Equality and Hashing

By default, objects that are instances of different classes or that are instances of a non-transparent class are equal? only if they are eq?. Like transparent structures, two objects that are instances of the same transparent class (i.e., every superclass of the class has #f as its inspector) are equal? when their field values are equal?.

To customize the way that a class instance is compared to other instances by equal?, implement the equal<%> interface.

interface
equal<%> : interface?

The equal<%> interface includes three methods, which are analogous to the functions provided for a structure type with prop:equal+hash:

equal-to? — Takes two arguments. The first argument is an object that is an instance of the same class (or a subclass that does not re-declare its implementation of equal<%>) and that is being compared to the target object. The second argument is an equal?-like procedure of two arguments that should be used for recursive equality testing. The result should be a true value if the object and the first argument of the method are equal, #f otherwise.
equal-hash-code-of — Takes one argument, which is a procedure of one argument that should be used for recursive hash-code computation. The result should be an exact integer representing the target object’s hash code.
equal-secondary-hash-code-of — Takes one argument, which is a procedure of one argument that should be used for recursive hash-code computation. The result should be an exact integer representing the target object’s secondary hash code.

The equal<%> interface is unusual in that declaring the implementation of the interface is different from inheriting the interface. Two objects can be equal only if they are instances of classes whose most specific ancestor to explicitly implement equal<%> is the same ancestor.

See prop:equal+hash for more information on equality comparisons and hash codes. The equal<%> interface is implemented with interface* and prop:equal+hash.

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1	Language Model
2	Syntactic Forms
3	Datatypes
4	Structures
5	Classes and Objects
6	Units
7	Contracts
8	Pattern Matching
9	Control Flow
10	Concurrency and Parallelism
11	Macros
12	Input and Output
13	Reflection and Security
14	Operating System
15	Memory Management
16	Unsafe Operations
17	Running Racket
	Bibliography
	Index

5.1	Creating Interfaces
5.2	Creating Classes
5.3	Creating Objects
5.4	Field and Method Access
5.5	Mixins
5.6	Traits
5.7	Object and Class Contracts
5.8	Object Equality and Hashing
5.9	Object Serialization
5.10	Object Printing
5.11	Object, Class, and Interface Utilities
5.12	Surrogates