nonmutating

Nonmutating describes an operation that does not modify the state of the object it is invoked on, often referred to as the receiver. In programming, distinguishing mutating from nonmutating behavior helps programmers reason about side effects, concurrency, and value semantics. A nonmutating method or function should leave the receiver unchanged, even if it performs computations, reads data, or alters other objects.

In the Swift programming language, nonmutating is used as an attribute related to protocol requirements and value types. It indicates that a conforming type’s implementation of a given method will not mutate self, allowing value types to fulfill protocol requirements without changing their own state. This contrasts with mutating methods, which are allowed to modify self. The distinction supports safer, more predictable code, especially when protocols are implemented by both classes and value types through value semantics.

Nonmutating behavior is central to emphasis on immutability in functional and value-oriented programming styles. It enables patterns such as pure functions and query-like operations that compute results without changing the input’s state. However, nonmutating does not necessarily prohibit all side effects; a nonmutating method may still modify other objects or global state as long as the receiver remains unchanged.

Across languages, related concepts appear under different terms. C++ uses const member functions to promise no modification of the object, while functional languages enforce immutability more broadly. In Swift and other modern languages, clearly labeling nonmutating operations supports safer APIs and clearer reasoning about code paths and performance implications.