Generics enable developers to write flexible, reusable functions and types that work with any type. Instead of duplicating logic for each type, a generic placeholder like T is used in the function or type signature and is replaced with a concrete type when the code is used. A generic swap function, for example, can operate on any type rather than only on integers or only on strings.
Generic functions and types can be constrained to require certain capabilities. Type constraints use a protocol or class requirement to ensure the generic type provides the methods or properties the implementation needs. For example, func findIndex<T: Equatable> requires T to conform to Equatable so the == operator is available. A where clause adds further constraints, allowing multiple conditions or compound constraints that cannot be expressed inline, and is also used in for-in loops, switch statements, and protocol extensions to refine conditions.
Generic types parameterize a class, struct, or enum over one or more type placeholders, making the type itself reusable. A Stack<Element> struct, for instance, can hold any kind of value while preserving type safety. Protocols can use associated types as placeholders within their declarations, defined with associatedtype, leaving the conforming type to specify the concrete type. This allows protocols to describe behavior without committing to a specific type, providing flexibility while preserving the structure of the requirement.