Opaque and Boxed Protocol Types

Opaque and Boxed Protocol Types

Hide implementation details about a value's type.

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.

The Problem that Opaque Types Solve

For example, suppose you're writing a module that draws ASCII art shapes. The basic characteristic of an ASCII art shape is a draw() function that returns the string representation of that shape, which you can use as the requirement for the Shape protocol:

protocol Shape {
    func draw() -> String
}

struct Triangle: Shape {
    var size: Int
    func draw() -> String {
       var result: [String] = []
       for length in 1...size {
           result.append(String(repeating: "*", count: length))
       }
       return result.joined(separator: "\n")
    }
}
let smallTriangle = Triangle(size: 3)
print(smallTriangle.draw())
// *
// **
// ***

You could use generics to implement operations like flipping a shape vertically, as shown in the code below. However, there's an important limitation to this approach: The flipped result exposes the exact generic types that were used to create it.

struct FlippedShape<T: Shape>: Shape {
    var shape: T
    func draw() -> String {
        let lines = shape.draw().split(separator: "\n")
        return lines.reversed().joined(separator: "\n")
    }
}
let flippedTriangle = FlippedShape(shape: smallTriangle)
print(flippedTriangle.draw())
// ***
// **
// *

This approach to defining a JoinedShape<T: Shape, U: Shape> structure that joins two shapes together vertically, like the code below shows, results in types like JoinedShape<Triangle, FlippedShape<Triangle>> from joining a triangle with a flipped triangle.

struct JoinedShape<T: Shape, U: Shape>: Shape {
    var top: T
    var bottom: U
    func draw() -> String {
       return top.draw() + "\n" + bottom.draw()
    }
}
let joinedTriangles = JoinedShape(top: smallTriangle, bottom: flippedTriangle)
print(joinedTriangles.draw())
// *
// **
// ***
// ***
// **
// *

Exposing detailed information about the creation of a shape allows types that aren't meant to be part of the ASCII art module's public interface to leak out because of the need to state the full return type. The code inside the module could build up the same shape in a variety of ways, and other code outside the module that uses the shape shouldn't have to account for the implementation details about the list of transformations. Wrapper types like JoinedShape and FlippedShape don't matter to the module's users, and they shouldn't be visible. The module's public interface consists of operations like joining and flipping a shape, and those operations return another Shape value.

Returning an Opaque Type

You can think of an opaque type like being the reverse of a generic type. Generic types let the code that calls a function pick the type for that function's parameters and return value in a way that's abstracted away from the function implementation. For example, the function in the following code returns a type that depends on its caller:

func max<T>(_ x: T, _ y: T) -> T where T: Comparable { ... }

The code that calls max(_:_:) chooses the values for x and y, and the type of those values determines the concrete type of T. The calling code can use any type that conforms to the Comparable protocol. The code inside the function is written in a general way so it can handle whatever type the caller provides. The implementation of max(_:_:) uses only functionality that all Comparable types share.

Those roles are reversed for a function with an opaque return type. An opaque type lets the function implementation pick the type for the value it returns in a way that's abstracted away from the code that calls the function. For example, the function in the following example returns a trapezoid without exposing the underlying type of that shape.

struct Square: Shape {
    var size: Int
    func draw() -> String {
        let line = String(repeating: "*", count: size)
        let result = Array<String>(repeating: line, count: size)
        return result.joined(separator: "\n")
    }
}

func makeTrapezoid() -> some Shape {
    let top = Triangle(size: 2)
    let middle = Square(size: 2)
    let bottom = FlippedShape(shape: top)
    let trapezoid = JoinedShape(
        top: top,
        bottom: JoinedShape(top: middle, bottom: bottom)
    )
    return trapezoid
}
let trapezoid = makeTrapezoid()
print(trapezoid.draw())
// *
// **
// **
// **
// **
// *

The makeTrapezoid() function in this example declares its return type as some Shape; as a result, the function returns a value of some given type that conforms to the Shape protocol, without specifying any particular concrete type. Writing makeTrapezoid() this way lets it express the fundamental aspect of its public interface --- the value it returns is a shape --- without making the specific types that the shape is made from a part of its public interface. This implementation uses two triangles and a square, but the function could be rewritten to draw a trapezoid in a variety of other ways without changing its return type.

This example highlights the way that an opaque return type is like the reverse of a generic type. The code inside makeTrapezoid() can return any type it needs to, as long as that type conforms to the Shape protocol, like the calling code does for a generic function. The code that calls the function needs to be written in a general way, like the implementation of a generic function, so that it can work with any Shape value that's returned by makeTrapezoid().

You can also combine opaque return types with generics. The functions in the following code both return a value of some type that conforms to the Shape protocol.

func flip<T: Shape>(_ shape: T) -> some Shape {
    return FlippedShape(shape: shape)
}
func join<T: Shape, U: Shape>(_ top: T, _ bottom: U) -> some Shape {
    JoinedShape(top: top, bottom: bottom)
}

let opaqueJoinedTriangles = join(smallTriangle, flip(smallTriangle))
print(opaqueJoinedTriangles.draw())
// *
// **
// ***
// ***
// **
// *

The value of opaqueJoinedTriangles in this example is the same as joinedTriangles in the generics example in the doc:OpaqueTypes#The-Problem-that-Opaque-Types-Solve section earlier in this chapter. However, unlike the value in that example, flip(_:) and join(_:_:) wrap the underlying types that the generic shape operations return in an opaque return type, which prevents those types from being visible. Both functions are generic because the types they rely on are generic, and the type parameters to the function pass along the type information needed by FlippedShape and JoinedShape.

If a function with an opaque return type returns from multiple places, all of the possible return values must have the same type. For a generic function, that return type can use the function's generic type parameters, but it must still be a single type. For example, here's an invalid version of the shape-flipping function that includes a special case for squares:

func invalidFlip<T: Shape>(_ shape: T) -> some Shape {
    if shape is Square {
        return shape // Error: Return types don't match.
    }
    return FlippedShape(shape: shape) // Error: Return types don't match.
}

If you call this function with a Square, it returns a Square; otherwise, it returns a FlippedShape. This violates the requirement to return values of only one type and makes invalidFlip(_:) invalid code. One way to fix invalidFlip(_:) is to move the special case for squares into the implementation of FlippedShape, which lets this function always return a FlippedShape value:

struct FlippedShape<T: Shape>: Shape {
    var shape: T
    func draw() -> String {
        if shape is Square {
           return shape.draw()
        }
        let lines = shape.draw().split(separator: "\n")
        return lines.reversed().joined(separator: "\n")
    }
}

The requirement to always return a single type doesn't prevent you from using generics in an opaque return type. Here's an example of a function that incorporates its type parameter into the underlying type of the value it returns:

func `repeat`<T: Shape>(shape: T, count: Int) -> some Collection {
    return Array<T>(repeating: shape, count: count)
}

In this case, the underlying type of the return value varies depending on T: Whatever shape is passed it, repeat(shape:count:) creates and returns an array of that shape. Nevertheless, the return value always has the same underlying type of [T], so it follows the requirement that functions with opaque return types must return values of only a single type.

Boxed Protocol Types

A boxed protocol type is also sometimes called an existential type, which comes from the phrase "there exists a type T such that T conforms to the protocol". To make a boxed protocol type, write any before the name of a protocol. Here's an example:

struct VerticalShapes: Shape {
    var shapes: [any Shape]
    func draw() -> String {
        return shapes.map { $0.draw() }.joined(separator: "\n\n")
    }
}

let largeTriangle = Triangle(size: 5)
let largeSquare = Square(size: 5)
let vertical = VerticalShapes(shapes: [largeTriangle, largeSquare])
print(vertical.draw())

In the example above, VerticalShapes declares the type of shapes as [any Shape] --- an array of boxed Shape elements. Each element in the array can be a different type, and each of those types must conform to the Shape protocol. To support this runtime flexibility, Swift adds a level of indirection when necessary --- this indirection is called a box, and it has a performance cost.

Within the VerticalShapes type, the code can use methods, properties, and subscripts that are required by the Shape protocol. For example, the draw() method of VerticalShapes calls the draw() method on each element of the array. This method is available because Shape requires a draw() method. In contrast, trying to access the size property of the triangle, or any other properties or methods that aren't required by Shape, produces an error.

Contrast the three types you could use for shapes:

• Using generics, by writing struct VerticalShapes<S: Shape> and var shapes: [S], makes an array whose elements are some specific shape type, and where the identity of that specific type is visible to any code that interacts with the array.

• Using an opaque type, by writing var shapes: [some Shape], makes an array whose elements are some specific shape type, and where that specific type's identity is hidden.

• Using a boxed protocol type, by writing var shapes: [any Shape], makes an array that can store elements of different types, and where those types' identities are hidden.

In this case, a boxed protocol type is the only approach that lets callers of VerticalShapes mix different kinds of shapes together.

You can use an as cast when you know the underlying type of a boxed value. For example:

if let downcastTriangle = vertical.shapes[0] as? Triangle {
    print(downcastTriangle.size)
}
// Prints "5".

For more information, see doc:TypeCasting#Downcasting.

Differences Between Opaque Types and Boxed Protocol Types

Returning an opaque type looks very similar to using a boxed protocol type as the return type of a function, but these two kinds of return type differ in whether they preserve type identity. An opaque type refers to one specific type, although the caller of the function isn't able to see which type; a boxed protocol type can refer to any type that conforms to the protocol. Generally speaking, boxed protocol types give you more flexibility about the underlying types of the values they store, and opaque types let you make stronger guarantees about those underlying types.

For example, here's a version of flip(_:) that uses a boxed protocol type as its return type instead of an opaque return type:

func protoFlip<T: Shape>(_ shape: T) -> Shape {
    return FlippedShape(shape: shape)
}

This version of protoFlip(_:) has the same body as flip(_:), and it always returns a value of the same type. Unlike flip(_:), the value that protoFlip(_:) returns isn't required to always have the same type --- it just has to conform to the Shape protocol. Put another way, protoFlip(_:) makes a much looser API contract with its caller than flip(_:) makes. It reserves the flexibility to return values of multiple types:

func protoFlip<T: Shape>(_ shape: T) -> Shape {
    if shape is Square {
        return shape
    }

    return FlippedShape(shape: shape)
}

The revised version of the code returns an instance of Square or an instance of FlippedShape, depending on what shape is passed in. Two flipped shapes returned by this function might have completely different types. Other valid versions of this function could return values of different types when flipping multiple instances of the same shape. The less specific return type information from protoFlip(_:) means that many operations that depend on type information aren't available on the returned value. For example, it's not possible to write an == operator comparing results returned by this function.

let protoFlippedTriangle = protoFlip(smallTriangle)
let sameThing = protoFlip(smallTriangle)
protoFlippedTriangle == sameThing  // Error

The error on the last line of the example occurs for several reasons. The immediate issue is that the Shape doesn't include an == operator as part of its protocol requirements. If you try adding one, the next issue you'll encounter is that the == operator needs to know the types of its left-hand and right-hand arguments. This sort of operator usually takes arguments of type Self, matching whatever concrete type adopts the protocol, but adding a Self requirement to the protocol doesn't allow for the type erasure that happens when you use the protocol as a type.

Using a boxed protocol type as the return type for a function gives you the flexibility to return any type that conforms to the protocol. However, the cost of that flexibility is that some operations aren't possible on the returned values. The example shows how the == operator isn't available --- it depends on specific type information that isn't preserved by using a boxed protocol type.

Another problem with this approach is that the shape transformations don't nest. The result of flipping a triangle is a value of type Shape, and the protoFlip(_:) function takes an argument of some type that conforms to the Shape protocol. However, a value of a boxed protocol type doesn't conform to that protocol; the value returned by protoFlip(_:) doesn't conform to Shape. This means code like protoFlip(protoFlip(smallTriangle)) that applies multiple transformations is invalid because the flipped shape isn't a valid argument to protoFlip(_:).

In contrast, opaque types preserve the identity of the underlying type. Swift can infer associated types, which lets you use an opaque return value in places where a boxed protocol type can't be used as a return value. For example, here's a version of the Container protocol from doc:Generics:

protocol Container {
    associatedtype Item
    var count: Int { get }
    subscript(i: Int) -> Item { get }
}
extension Array: Container { }

You can't use Container as the return type of a function because that protocol has an associated type. You also can't use it as constraint in a generic return type because there isn't enough information outside the function body to infer what the generic type needs to be.

// Error: Protocol with associated types can't be used as a return type.
func makeProtocolContainer<T>(item: T) -> Container {
    return [item]
}

// Error: Not enough information to infer C.
func makeProtocolContainer<T, C: Container>(item: T) -> C {
    return [item]
}

Using the opaque type some Container as a return type expresses the desired API contract --- the function returns a container, but declines to specify the container's type:

func makeOpaqueContainer<T>(item: T) -> some Container {
    return [item]
}
let opaqueContainer = makeOpaqueContainer(item: 12)
let twelve = opaqueContainer[0]
print(type(of: twelve))
// Prints "Int".

The type of twelve is inferred to be Int, which illustrates the fact that type inference works with opaque types. In the implementation of makeOpaqueContainer(item:), the underlying type of the opaque container is [T]. In this case, T is Int, so the return value is an array of integers and the Item associated type is inferred to be Int. The subscript on Container returns Item, which means that the type of twelve is also inferred to be Int.

Opaque Parameter Types

In addition to writing some to return an opaque type, you can also write some in the type for a parameter to a function, subscript, or initializer. However, when you write some in a parameter type that's just a shorter syntax for generics, not an opaque type. For example, both of the functions below are equivalent:

func drawTwiceGeneric<SomeShape: Shape>(_ shape: SomeShape) -> String {
    let drawn = shape.draw()
    return drawn + "\n" + drawn
}

func drawTwiceSome(_ shape: some Shape) -> String {
    let drawn = shape.draw()
    return drawn + "\n" + drawn
}

The drawTwiceGeneric(_:) function declares a generic type parameter named SomeShape, with a constraint that requires SomeShape to conform to the Shape protocol. The drawTwiceSome(_:) function uses the type some Shape for its argument. This creates a new, unnamed generic type parameter for the function with a constraint that requires the type to conform to the Shape protocol. Because the generic type doesn't have a name, you can't refer to that type elsewhere in the function.

If you write some before more than one parameter's type, each of the generic types are independent. For example:

func combine(shape s1: some Shape, with s2: some Shape) -> String {
    return s1.draw() + "\n" + s2.draw()
}

combine(smallTriangle, trapezoid)

In the combine(shape:with:) function, the types of the first and second parameter must both conform to the Shape protocol, but there's no constraint that requires them to be the same type. When you call combine(shape:with), you can pass two different shapes --- in this case, one triangle and one trapezoid.

Unlike the syntax for named generic type parameters, described in doc:Generics chapter, this lightweight syntax can't include a generic where clause or any same-type (==) constraints. In addition, using the lightweight syntax for very complex constraints can be hard to read.