Describes types that have a natural equivalence relation, such as an integer. There are types for which multiple good equivalence relationships exist--consider a modifiable sequence, that can be compared either element-by-element or by reference. On the other hand, for an immutable sequence element-by-element comparison makes more sense.

When obviously best choice doesn't exist, it is preferrable to force the programmer to make decision which equivalence relation to use explicitly. Data structures have been designed to allow the relation to be selected.

For operators == and != to work on values of a type, the type must implement this interface.