我定义了一个名为CanStack
的协议,其中包含一个名为Item
的关联类型:
CanStack.swift
// protocol definition
protocol CanStack:
ExpressibleByArrayLiteral, CustomStringConvertible
{
associatedtype Item
var items:[Item] { get }
init()
mutating func push(_ items: [Item])
mutating func pop() -> Item?
}
// protocol extension (default behavior)
extension CanStack {
public var isEmpty: Bool { return items.isEmpty }
// init by array
public init(_ items:[Item]) {
self.init()
self.push(items)
}
// init by variadic parameter
public init(_ items:Item...){
self.init()
self.push(items)
}
// push items by variadic parameter
public mutating func push(_ items:Item...){
self.push(items)
}
}
// conform to ExpressibleByArrayLiteral
extension CanStack {
public init(arrayLiteral items:Item...){
self.init()
self.push(items)
}
}
// conform to CustomStringConvertible
extension CanStack {
public var description: String {
return "["
+ items.map{"\($0)"}.joined(separator:", ")
+ " ⇄ in/out"
}
}
并定义了一个符合该协议的StackStruct
结构体,这个通用的结构体有一个类型参数Item
(与上面的关联类型名称完全相同):
StackStruct.swift
public struct StackStruct<Item> {
public private(set) var items = [Item]()
public init() { }
mutating public func push(_ items:[Item]) {
self.items += items
}
@discardableResult
mutating public func pop() -> Item? {
return items.popLast()
}
}
// adopt CanStack protocol
extension StackStruct: CanStack { }
然后我定义了另一个符合该协议的类Stack
:
Stack.swift
public class Stack<Item> {
public private(set) var items = [Item]()
public required init() {}
public func push(_ newItems:[Item]) {
items += newItems
}
@discardableResult
public func pop() -> Item? {
return items.popLast()
}
}
// adopt CanStack protocol
extension Stack: CanStack { }
and I have 3 test cases:
TestCases.swift
func testStackStruct() {
// init
var s1: StackStruct = [1,2,3] // expressible by array literal
var s2 = StackStruct([4,5,6]) // init by array
var s3 = StackStruct(7, 8, 9) // init by variadic parameter
// push
s1.push([4,5]) // array
s2.push(10, 11) // variadic
s3.push(20) // variadic
// pop
for _ in 1...4 { s1.pop() }
s2.pop()
s3.pop()
// print these stacks
example("stack struct", items:[s1,s2,s3])
}
func testStackClass_Var() {
// init
var s4: Stack = [1,2,3] // ⚠️ warning: s4 was never mutated; consider changing to let ...
var s5 = Stack([4,5,6]) // init by array
var s6 = Stack(7, 8, 9) // init by variadic parameter
// push
s4.push([4,5]) // array
s5.push(10, 11) // variadic
s6.push(20) // variadic
// pop
for _ in 1...4 { s4.pop() }
// print these stacks
example("stack class", items: [s4,s5,s6])
}
func testStackClass_Let() {
// init
let s7: Stack = [1,2,3] // expressible by array literal
let s8 = Stack([4,5,6]) // init by array
let s9 = Stack(7, 8, 9) // init by variadic parameter
// push
s7.push([4,5]) // array
s8.push(10, 11) // ⛔ Error: Extra argument in call
s9.push(20) // ⛔ Error: Cannot convert value of type 'Int' to expected argument type '[Int]'
// pop
for _ in 1...4 { s7.pop() }
// print these stacks
example("stack class", items: [s7,s8,s9])
}
我可以顺利运行第一个测试用例testStackStruct()
:
testStackStruct()
的输出结果:
[1 ⇄ in/out
[4, 5, 6, 10 ⇄ in/out
[7, 8, 9 ⇄ in/out
并且只提示编译器警告,运行testStackClass_Var()
用例:
⚠️ warning: s4 was never mutated; consider changing to let ...
testStackClass_Var()
的输出结果[1 ⇄ in/out
[4, 5, 6, 10, 11 ⇄ in/out
[7, 8, 9, 20 ⇄ in/out
但是testStackClass_Let()
案例甚至无法成功编译,我得到了两个编译器错误:
s8.push(10, 11) // ⛔ Error: Extra argument in call
s9.push(20) // ⛔ Error: Cannot convert value of type 'Int' to expected argument type '[Int]'
testStackClass_Var()
和testStackClass_Let()
之间唯一的区别是我使用了var
或let
来声明这些堆栈实例。
我无法确定我在哪里或者做错了什么,有人可以帮忙吗? 谢谢。
p.s.
我的小助手函数:
example.swift
import Foundation // string.padding() needs this
// print example
// usage:
// example("test something", items: [a,b,c]) {
// // example code here ...
// }
public func example<T>(
_ title: String, // example title
length: Int = 30, // title length
items: [T]? = nil, // items to print
run: (()->Void)? = nil // example code (optional)
){
// print example title
print(
("----- [ " + title + " ] ")
.padding(toLength: length, withPad: "-", startingAt: 0)
)
// run example
if let run = run { run() }
// print items if provided
if let items = items {
items.forEach{ print($0) }
}
// new line
print()
}
example
的代码吗? - Codoexample()
已添加在上方。 - lochiwei