不同于结构体和枚举类型，继承是类特有的。
语法：
class MyClass:Superclass{ //MyClass:子类 Superclass：父类
//类的定义
}
EG:
class Animal {
var eyes :Int
var month :Int
var nose :Int
var voice :String
init(){
eyes =2
month =1
nose =1
voice ="voice"
}
func description() ->String {
return"\(eyes) eyes ; up to\(month) month"
}
}
class Cat: Animal {
overrideinit() { //重写init()
super.init()
voice ="miao"
}
}
var cat = Cat()
print(cat.@H_822_301@voice) //"miao\n"
2.重写属性
class Animal {
var eyes :Int
var month :Int
var nose :Int
var voice :String
init(){
eyes =0
month =0
nose =0
voice ="voice"
}
func description() ->String {
return"\(eyes) eyes ; up to\(month) month"
}
}
class Cat: Animal {
overrideinit() {
super.init()
voice ="miao"
}
overridevar eyes: Int{
get{
returnsuper.eyes
}
set{
super.eyes =max(newValue,100)
}
}
}
var cat = Cat()
print(cat.voice)
cat.eyes =1000
print(cat.eyes)
若不想被继承，可在前面加final.
