帮我看一下这个程序问题处在哪里啊？

#include<iostream>
using namespace std;
class Time
{
public:
    Time(int,int,int);
    friend void display(Time &);
   private:
   int hour;
   int minute;
   int sec;
};
Time::Time(int h,int m,int s)
{  hour=h; minute=m;  sec=s;  }
void display(Time &)
{  cout<<t.hour<<":"<<t.minute<<":"<<t.sec<<endl; }
int main()
{
Time t1(12,56,23);
display(t1);
return 0;
}