swift protocol mutating

protocol base{

var number:Int{get};

mutating func 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);