关于C++中报Error:new types may not be defined in a return type问题

#include <iostream>
using namespace std;
class Clock
{
public:
Clock(int NewH,int NewM,int NewS);
void ShowTime();
private:
int Hour,Minute,Second;
}
Clock::Clock(int NewH,int NewM,int NewS)
{
Hour = NewH;
Minute = NewM;
Second = NewS;
}

void Clock::ShowTime()
{
cout<<Hour<<":"<<Minute<<":"<<Second;
}
int main()
{
Clock c(0,0,0);
c.ShowTime();
system("pause");
return 0;
}