[求助]如何用构造函数解决这两个题?

1:
class tree{
int age;
public:
tree() : age(0){}
void grow(int x){age = x;}
int get_age(){return age;}
};
2:
class square{
double l;
public:
square(double x) : l(x){}
double comput_area(){return l*l;}
};

能给我完整的程序吗???
搞到头痛了....

实在是..看不下去了..不看了...泡吧去..