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267 lines
7.1 KiB
Markdown
267 lines
7.1 KiB
Markdown
# Swift 协议探究
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- 协议可以用来定义属性、方法、下标的声明，协议可以被类、枚举、结构体遵守（多个协议用逗号隔开）
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- 协议中定义的方法不能有默认参数值
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- 协议中定义属性必须是 `var`
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- 实现协议时定义的属性权限，要不小于协议中定义的属性权限
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- 协议定义属性是 `get`、`set` 时，用 `var` 存储属性或者 `get`、`set` 计算属性实现
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- 为了保证通用，协议中必须用 `static` 定义类型方法、类型属性、类型下标
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- 只有将协议中的实例方法标记为 `mutating`
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- 才可以允许结构体、枚举对象在方法里修改自身内存。否则编译器会报错：`Cannot assign to property: 'self' is immutable`
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- 类遵循协议，实现方法的时候不用加 `mutating`，枚举、结构体的实现需要加 `mutating`
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```swift
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protocol Drawable {
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func draw()
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}
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class Size: Drawable {
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var width: Int = 0
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func draw() {
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width = 10
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}
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}
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var size = Size()
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print(size.width) // 0
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size.draw()
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print(size.width) // 10
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struct Point: Drawable {
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var x : Int = 0
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var y: Int = 0
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func draw() {
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x = 10 // Cannot assign to property: 'self' is immutable
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y = 10 // Cannot assign to property: 'self' is immutable
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}
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}
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```
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要想修改需要加 `mumating`
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```swift
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struct Point: Drawable {
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var x : Int = 0
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var y: Int = 0
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mutating func draw() {
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x = 10
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y = 10
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}
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}
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var point = Point()
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print(point.x, point.y)
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point.draw()
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print(point.x, point.y)
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```
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- 协议中还可以定义初始化器 `init`，非 `final` 类实现协议时， `init` 方法必须加 `required`
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```swift
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protocol Drawable {
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init(x: Int, y: Int)
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}
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class Point: Drawable {
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var x: Int = 0
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var y: Int = 0
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required init(x: Int, y: Int) {
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self.x = x
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self.y = y
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}
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}
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final class Size: Drawable {
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var x: Int = 0
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var y: Int = 0
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init(x: Int, y: Int) {
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self.x = x
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self.y = y
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}
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}
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var point = Point(x: 10, y: 20)
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print(point.x , point.y) // 10 20
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var size = Size(x: 30, y: 40)
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print(size.x , size.y) // 30 40
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```
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- 如果协议声明了初始化器，某个类遵循协议并实现了初始化器。且该初始化器也恰好是父类指定初始化器，那么这个初始化必须同事加 `required` 和 `override`
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```swift
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protocol Drawable {
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init(x: Int, y: Int)
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}
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class Shape {
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init(x: Int, y: Int) {}
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}
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class Circle: Shape, Drawable {
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var x: Int = 0
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var y: Int = 0
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required override init(x: Int, y: Int) {
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super.init(x: x, y: y)
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self.x = x
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self.y = y
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}
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}
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var circle = Circle(x: 10, y: 20)
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print(circle.x , circle.y) // 10 20
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```
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- 协议也可以继承
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- 协议也可以组合
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```swift
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protocol Drawable {}
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protocol Colorable {}
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func test1(obj: Shape) {} // 参数接收 Shape 类或者 Shape 类的子类
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func test2(obj: Drawable) {} // 参数接收遵循 Drawable 的实例
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func test3(obj: Drawable & Colorable) {} // 参数接收同时遵循 Colorable 和 Drawable 2个协议的实例
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func test4(obj: Shape & Drawable & Colorable) {} // 参数接收同时遵循 Colorable 和 Drawable 2个协议，且是 Shape 的子类的实例
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```
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- 遵循 `CustomStringConvertible` 可以自定义打印的字符串内容
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```swift
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class Person: CustomStringConvertible {
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var name: String
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var age: Int
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init(name: String, age: Int) {
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self.name = name
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self.age = age
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}
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var description: String {
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"My name is \(name), age is \(age)"
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}
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}
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var p = Person(name: "杭城小刘", age: 28)
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print(p) // My name is 杭城小刘, age is 28
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```
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## Any、AnyObject
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Swift 提供了2种特殊的类型：Any、AnyObject
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- Any 可以代表任意类型（枚举、结构体、类、函数类型）
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- AnyObject：代表任意类类型。比如可以在协议后面加上 AnyObject 则代表只有类能遵循这个协议。编译器会做检查 `Non-class type 'point' cannot conform to class protocol 'Eatable'`
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```swift
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protocol Eatable: AnyObject {}
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class Person: Eatable { }
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struct point: Eatable {} // Non-class type 'point' cannot conform to class protocol 'Eatable'
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```
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## 关联类型
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关联类型的作用：给协议中用到的类型，定义一个占位名称
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协议中可以拥有多个关联类型
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```swift
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protocol Stackable {
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associatedtype Element
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mutating func push(_ element: Element)
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mutating func pop() -> Element
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func top() -> Element
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func size() -> Int
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}
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class Stack<Element>: Stackable {
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var elements = Array<Element>()
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func push(_ element: Element) {
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elements.append(element)
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}
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func pop() -> Element {
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elements.removeLast()
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}
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func top() -> Element {
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elements.last!
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}
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func size() -> Int {
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elements.count
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}
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}
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```
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## Swift 泛型本质
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```swift
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func swapValue<T>(_ value1: inout T, _ value2: inout T) {
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(value1, value2) = (value2, value1)
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}
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var i1 = 11
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var i2 = 22
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swapValue(&i1, &i2)
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var s1 = "Hello"
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var s2 = "world"
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swapValue(&s1, &s2)
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```
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在 `swapValue(&i1, &i2)` 处下断点可以看到下面的汇编代码：
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftGenericExploreDemo1.png" style="zoom:25%">
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可以看到：
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- 在第一处调用 `swapValue ` 方法的时候，将8字节的 metadata 信息保存到 `rdx` 寄存器了。也就是在调用 `swapValue` 方法的时候，分别将 `i1`（0x000000000000000b，也就是11）的地址值赋值给 rdi 寄存器，将 `i2`（0x0000000000000016，也就是22）的地址值赋值给 rsi 寄存器
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- 将 `Int` 的 `metadata` 赋值给 `rdx` 寄存器
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- 然后调用 `swapValue` 方法
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- 后续的 `String` 的 `SwapValue` 过程类似
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所以编译器最后在执行的时候，会将泛型真正的类型对应的 metadata 信息当作函数参数，传递进去，再去执行函数
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## 泛型类型约束
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泛型必须遵循协议，可以在方法后加 `<>`，在 `<>` 内写泛型 `T: 需要继承的类 & 协议` （也可以是其他名字，大多数语言都写 T），
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```swift
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protocol Runable {}
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class Person {}
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func swapValue<T: Person & Runable>(_ a: inout T, _ b: inout T) {
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(a, b) = (b, a)
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}
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```
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另一种场景是在方法参数是某个泛型且遵循协议后，对其泛型有更多限制，则用 `where` 去处理。如下例子
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```swift
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protocol Stackable {
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associatedtype Element: Equatable
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}
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class Stack<E: Equatable>: Stackable {
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typealias Element = E
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}
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func equal<S1: Stackable, S2: Stackable>(_ s1: S1, _ s2: S2)-> Bool
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where S1.Element == S2.Element, S1.Element: Hashable {
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return true
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}
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let s1 = Stack<Int>()
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let s2 = Stack<Int>()
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let s3 = Stack<String>()
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var result:Bool = equal(s1, s2)
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print(result) // true
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result = equal(s1, s3) // Global function 'equal' requires the types 'Stack<Int>.Element' (aka 'Int') and 'Stack<String>.Element' (aka 'String') be equivalent
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print(result)
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```
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