一个关于C++类的问题

class Box
{
private:
double height
double width;
double length;

public:
void GetValue(double x, double y, double z)
{
length = x;
width = y;
height = z;

}
double vol()
{
return length*width * height;

}
double surface()
{
return 2*(x*y + y*z + x*z)
}
};

class box{
private:
double long;
double width;
double hight;
public:
box(double,double,double);
double tiji();
double biaomianji();
};
box::box(double l,double w,double h){
long=l;
width=w;
hight=h;
}
double box::tiji(){
return (long*width*hight);
}
double box::biaomianji(){
return (2*(long*width+long*hight+width*hight));
}