swift protocol mutating

protocol base{

   var number:Int{get};

    mutatingfunc changeNum();//如果protocol不写mutating那么struct、enum实现此协议将会报错

}

struct Derive:base {

   var number = 1;

   mutating func changeNum(){

       number = 2;

    }

}

var d:Derive =Derive()

println(d.number);

d.changeNum();

println(d.number);