共用数据的保护的问题

#include <iostream>
using namespace std;
class  Test
{
private:
    int  x, y;
public:
    Test(int a, int b)
    {
        x=a;
        y=b;
    }
    void Printxy(void)
    {
        x=2; //可能是意外修改
        cout<<"x*y="<<x*y<<endl;
    }
} ;
int main()
{
    Test p1(3,5);
    p1.Printxy( );
}

常对象

#include <iostream>
using namespace std;
class Time
{
public:
    Time( ):hour(0),minute(0),sec(0) {};
    Time(int h,int m,int s):hour(h),minute(m),sec(s) {};
    void set_time(int h,int m,int s)
    {
        hour=h;
        minute=m;
        sec=s;
    };
    void show_time( );
private:
    int hour;
    int minute;
    int sec;
};


void Time::show_time( )
{
    cout<<hour<<":"<<minute<<":"<<sec<<endl;
}
int main( )
{
    const Time t1(10,10,10);
    t1.set_time(20,20,20);
    t1.show_time();
    return 0;
}

常成员函数

#include <iostream>
using namespace std;
class Time
{
public:
    Time( ):hour(0),minute(0),sec(0) {};
    Time(int h,int m,int s):hour(h),minute(m),sec(s) {};
    void set_time(int h,int m,int s)
    {
        hour=h;
        minute=m;
        sec=s;
    };
    void show_time( ) const;
private:
    int hour;
    int minute;
    int sec;
};


void Time::show_time( ) const
{
    cout<<hour<<":"<<minute<<":"<<sec<<endl;
}
int main( )
{
    Time t1(10,10,10);
    t1.set_time(20,20,20);
    t1.show_time();
    return 0;
}