Swift泛型的一些理解

protocol MyProtocol {
    //关联类型
    associatedtype ItemType
    mutating func append(_ item:ItemType)
    mutating func getObj(_ index:Int) -> ItemType
    //下标
    subscript(i:Int) -> ItemType{ get }
    var count: Int { get }
}

struct MyDataSturce:MyProtocol {
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> (T) {
        return items[i];
    }

    var items = [T]();
    mutating func getObj(_ index: Int) -> T {
        return items[index];
    }

    mutating func append(_ item: T) {
        items.append(item)
    }
}

struct MyDataSturce1:MyProtocol {
    var items = [T]()
    
    subscript(i: Int) -> T {
        return items[i]
    }

    mutating func getObj(_ index: Int) -> T {
        return items[index]
    }

    mutating func append(_ item: T) {
        items.append(item)
    }

    var count: Int {
        return items.count
    }
}

//结构体stack使用泛型T作为占位符
struct Stack {
    var items = [T]()
    mutating func push(_ item:T){
        items.append(item)
    }
    func pop() -> T {
        return items.last!
    }
    //查找目标在数组中的index  泛型S满足Equatable协议
    static func findIndex(ofString valueToFind:S,in array:[S]) -> Int?{
        for (index,value) in array.enumerated() {
            if value == valueToFind {
                return index;
            }
        }
        return nil;
    }
    //匹配两个数组是否相等 两个数组满足MyProtocol协议 并且两个ItemType相等 并且ItemType满足Equatable协议
    static func allMatch(_ array1:C1,_ array2:C2) -> Bool where C1.ItemType == C2.ItemType,C1.ItemType:Equatable{
        if array1.count != array2.count {
            return false
        }
        for i in 0.. T? {
        return items.isEmpty ? nil : items[items.count - 1];
    }
}
//扩展Stack 使用泛型满足Equatable这个协议
extension Stack where T:Equatable{
    func isTop(_ item:T) -> Bool {
        guard let topItem = items.last else {
            return false
        }
        return item == topItem
    }
}
//扩展MyProtocol协议 使ItemType满足Equatable协议
extension MyProtocol where ItemType:Equatable{
    func startWith(_ item:ItemType) -> Bool {
        return count >= 1 && self[0] == item;
    }
}
//扩展MyProtocol协议 使ItemType满足为Int类型
extension MyProtocol where ItemType == Int{
    var sum: Int {
        var ss = 0
        for i in 0..