项目一（直接抓狂，还是不对，求大神指点）

#include <iostream>
#include<string>
using namespace std;

class Time{
public:
Time(int=0,int=0,int=0);
void show_time( ); //根据is_24和from0，输出适合形式-20:23:5/8:23:5 pm/08:23:05 pm
void add_seconds(int); //增加n秒钟
void add_minutes(int); //增加n分钟
void add_hours(int); //增加n小时
static void change24();  //改变静态成员is_24，在12和24时制之间转换
static void changefrom0();   //改变静态成员from0，切换是否前导0
private:
static bool is_24; //为true时，24小时制，如20:23:5；为flase，12小时制，显示为8:23:5 pm
static bool from0; //为true时，前导0，8:23:5显示为08:23:05
int hour;
int minute;
int sec;
};
Time::Time(int h,int m,int s)
{
hour=h;
minute=m;
sec=s;
}
bool Time::is_24=true;
bool Time::from0=false;

void Time::show_time()
{
string c;
if(is_24)
{
if(from0)
{
if(hour<10)
cout<<"0"<<hour<<":";
else
cout<<hour<<":";
if(minute<10)
cout<<"0"<<minute<<":";
else
cout<<minute<<":";
if(sec<10)
cout<<"0"<<sec;
else
cout<<sec;
cout<<endl;
}
else
cout<<hour<<":"<<minute<<":"<<sec<<endl;
}

else{

if(hour>12)
{hour-=12;
c="pm";
}
else
c="am";
if(from0)
{
if(hour<10)
cout<<"0"<<hour<<":";
else
cout<<hour<<":";
if(minute<10)
cout<<"0"<<minute<<":";
else
cout<<minute<<":";
if(sec<10)
cout<<"0"<<sec;
else
cout<<sec;
cout<<" "<<c<<endl;
}
else
cout<<hour<<":"<<minute<<":"<<sec<<" "<<c<<endl;
}

}
void Time::change24()
{
is_24=!is_24;
}
void Time::changefrom0()
{
from0=!from0;
}
//增加n秒钟
void Time::add_seconds(int n)
{
sec=sec+n;
if(sec>59)
{

add_minutes(sec/60);
sec=sec%60;
}

if(minute>59)
{
add_hours(minute/60);
minute=minute%60;

}

if(hour>23)
{
hour=hour%24;
}
}

//增加n分钟

void Time::add_minutes(int n)
{
minute=minute+n;
if(minute>59)
{
add_hours(minute/60);
minute=minute%60;

}

if(hour>23)
{
hour=hour%24;
}

}
//增加n小时
void Time::add_hours(int n)
{
hour+=n;
if(hour>23)
{
hour=hour%24;
}

}
int main( ) //运行结果如图所示
{
Time t1(23,14,25),t2(8,45,6);
cout<<"24小时制，不导0："<<endl;
t1.show_time();
t2.show_time();
cout<<"10小时后，切换是否前导0："<<endl;
t1.add_hours(10);
t2.add_hours(10);
t1.changefrom0();
t2.changefrom0();
t1.show_time();
t2.show_time();
cout<<"换一种制式："<<endl;
t1.change24();
t2.change24();

t1.show_time();
t2.show_time();

}

直接抓狂了，经过个人分析，应该是函数change24与changefrom0没有实现其功能，可是定义函数，函数的赋值及调用都与其他同学的相同，问题究竟出在哪里啊？求指点！