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Opaque and Boxed Protocol Types | Documentation

docs.swift.org · 92 words · saved by 1 readers

Swift provides two ways to hide details about a value’s type: opaque types and boxed protocol types. Hiding type information is useful at boundaries between a module and code that calls into the module, because the underlying type of the return value can remain private. A function or method that returns an opaque type hides its return value’s type information. Instead of providing a concrete type as the function’s return type, the return value is described in terms of the protocols it supports. Opaque types preserve type identity — the compiler has access to the type information, but clients of the module don’t. A boxed protocol type can store an instance of any type that conforms to the given protocol. Boxed protocol types don’t preserve type identity — the value’s specific type isn’t known until runtime, and it can change over time as different values are stored. For example, suppose you’re writing a module that draws ASCII art shapes. The basic characteristic of an ASCII art shape i

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