struct Size: Equatable,Comparable,CustomStringConvertible { // 相等的协议
    var width: Double
    var height: Double
    
    var description: String {
        return "witth:" + "\(self.width)" + " " + "height" + "\(self.height)"
    }
}

// 相等协议的实现
func ==(left: Size,right: Size) -> Bool {
    return left.height == right.height && left.width == right.height
}

// 比较协议的实现
func <(left: Size,right: Size) ->Bool {
    if left.width * left.height != right.height * right.width {
        return left.width * left.height < right.height * right.width
    }
    
    return left.width * left.height > right.height * right.width
}

扩展协议

protocol student:CustomStringConvertible {
    var name: String{get}
    var height: Double {get}
    
}

// 扩展协议，在协议扩展中可以添加具体的逻辑
extension student {
    var description: String {
        return "name is" + " \(name)" + " " + "height is" + " \(height)"
    }
    
    func showInfo() {
        print("good student")
    }
}

// 扩展协议 CustomStringConvertible
extension CustomStringConvertible {
    var descriptionWithDate:String {
        return NSDate().description + " " + description
    }
}

class Person: student {
    var name: String = ""
    var height: Double = 0.0
    
    init(name: String,height: Double) {
        self.name = name
        self.height = height
    }
}

var person = Person(name: "jobs",height: 1.80)
print(person)
person.showInfo()
print(person.descriptionWithDate)

name is jobs height is 1.8

good student

2017-02-24 08:57:15 +0000 name is jobs height is 1.8

对已有的协议修改

protocol Student: CustomStringConvertible {
    var name: String{get}
    var height: Double {get}
    
}

// 扩展协议，在协议扩展中可以添加具体的逻辑
extension Student {
    var description: String {
        return "name is" + " \(name)" + " " + "height is" + " \(height)"
    }
    
    func showInfo() {
        print("good student")
    }
}

protocol HighSchoolStudent {
    var age: Int {get}
}

// 对已有的协议做修改 当类准守HighSchoolStudent的协议时 打印 good HighSchoolStudent
extension Student where Self:HighSchoolStudent {
    func showInfo() {
        print("name is" + " \(name)" + " " + "height is" + " \(height)" + " " + "age is " + "\(age)")
    }
}

// 扩展协议 CustomStringConvertible
extension CustomStringConvertible {
    var descriptionWithDate:String {
        return NSDate().description + " " + description
    }
}

class Person: Student {
    var name: String = ""
    var height: Double = 0.0
    
    init(name: String,height: Double) {
        self.name = name
        self.height = height
    }
}

class People: Student,HighSchoolStudent {
    internal var height: Double = 0.0

    internal var name: String = ""

    internal var age: Int = 0

    init(height:Double,name: String,age: Int) {
        self.name = name
        self.age = age
        self.height = height
    }
}

var person = Person(name: "jobs",height: 1.80)
print(person)
person.showInfo()
print(person.descriptionWithDate)

var people = People(height: 1.7,name: "zhangsan",age: 17)
people.showInfo()

name is jobs height is 1.8

good student

2017-02-24 09:44:12 +0000 name is jobs height is 1.8

name is zhangsan height is 1.7 age is 17