7.29交通工具类

有一个交通工具类vehicle，将它作为基类派生出汽车类motor_ vehicle，
再将汽车类motor_ vehicle 作为基类派生出小汽车类car和卡车类truck，
声明这些类并定义一个虚函数用来显示各类信息。
#include <iostream>
using namespace std;
class vehicle
{public:
virtual void message( )    
{  cout << "vehicle message " << endl;  }
private:
        int wheels;    
float weight;  
};
class motor_vehicle: public vehicle   
{public:
void message( ){  cout << " motor_ vehicle message " << endl;  }
private:
        int passengers;  
};
class car: public motor_vehicle  
{public:
void message( ){  cout << "car message " << endl;  }
private:
        float  engine;  
};
class truck: public motor_vehicle  
{public:
void message( ){  cout << " truck message " << endl;  }
private:
        int loadpay ;  
};
int main( )
{    vehicle v, *p = NULL;    
motor_vehicle m;        
car c;          
truck t;         
p = &v;          
p -> message( );       
p = &m;          
p -> message( );        
p = &c;          
p -> message( );   
p = &t;         
p -> message( );        
        return 0;
}