Language Design: Equality & Identity

Part 1: Overview

Published on 2017-10-31.

Most languages have a notion of equality comparisons based on value equality. Many of them also provide a more restricted equality comparison that works only on references, often called reference equality. Here are a few examples:

Java

  • == implements reference equality on reference types.
  • Object.equals and Objects.equals implement reference equality be default, but can be overridden to implement value equality on reference types.
  • Arrays.deepEquals implements value equality on arrays (equals does reference equality on arrays).
  • == implements value equality on primitive types.
    • Primitive types can be implicitly converted to special wrapper classes, which implement equals slightly differently.
    • As these wrapper classes are reference types, == checks for reference equality of the wrapper classes.

Scala

  • == (defined on Any) implements value equality.
    • An extended version of value equality is used in which different numeric types can be equal if they represent the same value.
    • This does not extend to arrays, which are plain Java arrays underneath.
  • eq (defined on AnyRef) implements reference equality.
  • The artifacts of Java’s wrapper classes for primitive types are significantly reduced.

Rust

  • eq (defined on PartialEq) implements value equality. It does not require that x eq x is true.
  • == (defined on Eq) implements value equality and requires that x eq x is true. Eq implies PartialEq.
  • Reference equality can only be implemented by casting references to pointers and comparing them.

C#

  • Object.Equals implements reference equality be default, but can be overridden to implement value equality on reference types.
  • ValueType.Equals calls Equals on each field contained within a value types.
  • Object.ReferenceEquality implements reference equality. The method always returns false for value types.
  • == implements reference equality on reference types, but can be overloaded to implement value equality (as done on System.String).
  • == can be overloaded to implement value equality for value types.
  • Numeric types use an extended version of value equality in which different numeric types are equal if they represent the same value.
  • IEquatable.Equals can be implemented to reduce boxing for value types, but should return the same results as overridden Equals methods on that type.