$set$函数不能定义在$class\;Boxs$外面
而且构造函数怎么写?
放代码:

#include<bits/stdc++.h>
using namespace std;
class Boxs
{
	private:
		long double a;
		long double b;
	public:
		long double Area()
		{
			return (a*b);
		}
		long double leng()
		{
			return (2*(a+b));
		}
  		void set(long double a,long double b)
		{
			this->a=a;
			this->b=b;
		}
		~Boxs();
};
Boxs::~Boxs()
{
	cout<<"deleted a member in system stack sucsessfully!";
}
int main(){
	ios::sync_with_stdio(0);
	Boxs s;
	long double a,b;
	cin>>a>>b;
	s.set(a,b);
	cout<<s.Area()<<' '<<s.leng()<<endl;
	return 0;
}