C++ defines a counter class to complete the countdown function.

1. First, initialize it in the constructor and stipulate that if it is less than zero, it will be reset to zero.

Counter(int h,int m,int s)
	{
		if(h<0)
		{
			hour=0;
		}
		else if(m<0)
		{
			minute=0;
		}
		else if(s<0)
		{
			second=0;
		}
		else
		{
			hour=h;
			minute=m;
			second=s;
		}
	}

2. Implement the countdown function in the function countDown().

void countDown()
	{
		for( i=hour;i>=0;i--)
		{
			for( j=minute;j>=0;j--)
			{
				for( k=second;k>=0;k--)
				{     
					long temp=time(NULL);
		            while(time(NULL)==temp);
		            if(--second<=0)
		            {
		                second=59;
		                if(--minute<=0)
		                {
		                    minute=59;
		                    if(--hour<=0)
		                    {
		                   		hour=23;
 						    }
		                }
		            }
					Sleep(1000);
					system("cls");
					cout << "The remaining time is   ";
					if(i<10)
						cout << "0";
					cout << i << " : ";
				    if(j<10)						
						cout << "0";
					cout << j << " : ";						
					if(k<10)						
						cout << "0";
					cout << k << endl;						
				}
			}
		}
		system("color  f4");			
		cout<<"Time is up !!!" <<endl;
		system("pause");						
	}

3. Test program functionality.

int main()
{
	//测试计数器类 
	system("color  f1");
	int hour_;
	int minute_;
	int second_;
	cout << "Please enter the time ! " << endl;
	cout << "hour : "  <<endl;
	cin >> hour_;
	cout << "minute : "<<endl;
	cin >> minute_;
	cout << "second : "<<endl;
	cin >> second_;
	cout << endl;
	Counter text1(hour_,minute_,second_);
	text1.countDown();
    return 0；
}

The complete code is as follows:

#include<iostream>
#include<cstdlib>
#include<windows.h>
#include<cstring>
#include<ctime>
#include<math.h>
using namespace std;
//计数器类
class Counter
{
public:
	Counter(int h,int m,int s)
	{
		if(h<0)
		{
			hour=0;
		}
		else if(m<0)
		{
			minute=0;
		}
		else if(s<0)
		{
			second=0;
		}
		else
		{
			hour=h;
			minute=m;
			second=s;
		}
	}
	void countDown()
	{
		for( i=hour;i>=0;i--)
		{
			for( j=minute;j>=0;j--)
			{
				for( k=second;k>=0;k--)
				{     
					long temp=time(NULL);
		            while(time(NULL)==temp);
		            if(--second<=0)
		            {
		                second=59;
		                if(--minute<=0)
		                {
		                    minute=59;
		                    if(--hour<=0)
		                    {
		                   		hour=23;
 						    }
		                }
		            }
					Sleep(1000);
					system("cls");
					cout << "The remaining time is   ";
					if(i<10)
						cout << "0";
					cout << i << " : ";
				    if(j<10)						
						cout << "0";
					cout << j << " : ";						
					if(k<10)						
						cout << "0";
					cout << k << endl;						
				}
			}
		}
		system("color  f4");			
		cout<<"Time is up !!!" <<endl;
		system("pause");						
	}
	
private:
	int hour;
	int minute;
	int second;
	int i,j,k;
		
};
int main()
{
	//测试计数器类 
	system("color  f1");
	int hour_;
	int minute_;
	int second_;
	cout << "Please enter the time ! " << endl;
	cout << "hour : "  <<endl;
	cin >> hour_;
	cout << "minute : "<<endl;
	cin >> minute_;
	cout << "second : "<<endl;
	cin >> second_;
	cout << endl;
	Counter text1(hour_,minute_,second_);
	text1.countDown();
    return 0；
}