为什么要写成(day D)，不能写成(day &D）呢？求解释

Question

#include"iostream.h"enum day{Sunday,Monday,Tuesday,Wednesday,Thursday,Friday,Saturday};void nextday(day &D);void display(day D);int main(){day today=Sunday;for(int i=0;i<7;i++){cout<<"Data in today="<<today<<endl;display(today);nextday(today);}return 0;}void nextday(day &D){switch(D){case Sunday:D=Monday;break;case Monday:D=Tuesday;break;case Tuesday:D=Wednesday;break;case Wednesday:D=Thursday;break;case Thursday:D=Friday;break;case Friday:D=Saturday;break;case Saturday:D=Sunday;break;}}void display(day D)// 请问这里为什么要写成(day D)为什么不能写成(day &D）{switch(D){case Sunday:cout<<"Sunday"<<endl;break;case Monday:cout<<"Monday"<<endl;break;case Tuesday:cout<<"Tuesday"<<endl;break;case Wednesday:cout<<"Wednesday"<<endl;break;case Thursday:cout<<"Thursday"<<endl;break;case Friday:cout<<"Friday"<<endl;break;case Saturday:cout<<"Saturday"<<endl;break;}}

Answer 1

也成 day&D 也是可以的,即运行结果和写成day D是一样的 但并不是一种好的做法,因为你这样你就可以在函数中改变D的值了,但这个函数是不允许改变D的值的(把今天是星期几输出,你改它值干的) 最好写成 const day & D ,这样就可以了.

Answer 2

能改啊，将函数说明也改成void display(day &D)就行了

Answer 3

可以写成day&D,但是这样就能通过函数改变D的值，如1楼的最好写成const day&D,这样的引用即不能通过函数改变D的值