c++长方体与三角形类

#include<iostream>
using namespace std;
class encapsulation
{
public:
	void get_doul();    
	void get_volume();  
	void get_area();    
	void volume_play(); 
	void area_play(); 
private:
	double height;            
    double width;             
    double length;            
    double volume;             
    double area;              

}; 
void encapsulation::get_volume()  
{  
     volume=height*width*length;  
}  
void encapsulation::get_area()
{
	area=2*(height*width+height*length+width*length);
}
void encapsulation::get_doul()
{
	cout<<"define length width height of a cuboid"<<endl;
	cout<<"length:"<<endl;
	cin>>length;
	cout<<"width:"<<endl;
	cin>>width;
	cout<<"height:"<<endl;
	cin>>height;
	get_volume();
	get_area();
}
void encapsulation::area_play()
{
	cout<<"area:"<<area<<endl;
}
void encapsulation::volume_play()
{
	cout<<"volume:"<<volume<<endl;
}

//以上是长方体类以下是三角形类

class Triangle
{
public:
	void get_Triangle_doul();    
	void get_Triangle_perimeter();  
	void get_Triangle_area();    
	void Triangle_perimeter_play(); 
	void Triangle_area_play(); 
    bool is_Triangle();
private:
    double x;            
    double y;             
    double z;            
    double Triangle_perimeter;             
    double Triangle_area;              

};
void Triangle::get_Triangle_doul()
{
    cout<<"define x y z of a Triangle"<<endl;
	cout<<"x:"<<endl;
	cin>>x;
	cout<<"y:"<<endl;
	cin>>y;
	cout<<"z:"<<endl;
	cin>>z;

	
}
bool Triangle::is_Triangle()  
{    if(x+y>z&&x+z>y&&y+z>x)  
       return true;  
     else   
       return false;  
}  
void Triangle::get_Triangle_perimeter()  
{  
    Triangle_perimeter=x+y+z;  
}  
void Triangle::get_Triangle_area()  
{  
    double r;  
    r=(x+y+z)/2;  
    Triangle_area=sqrt(r*(r-x)*(r-y)*(r-z));  
} 
void Triangle::Triangle_area_play()
{
	cout<<"area:"<<Triangle_area<<endl;
}
void Triangle::Triangle_perimeter_play()
{
	cout<<"perimeter:"<<Triangle_perimeter<<endl;
}


//下面主函数

 
int main()
{
  
	int a;
	cout<<"input your selection(1 or 2)"<<endl;
	cout<<"1.cuboid or 2.Triangle"<<endl;
	cin>>a;
	if(a==1)
	{
		encapsulation cub1;
	cub1.get_doul();
	cout<<"The cuboid:"<<endl;
	cub1.area_play();
	cub1.volume_play();
	}
	else if(a==2)
	{
		Triangle tri1;
	tri1.get_Triangle_doul();
	if(tri1.is_Triangle())    
    {     
    tri1.get_Triangle_perimeter();
	tri1.get_Triangle_area();
	cout<<"The Triangle:"<<endl;
	tri1.Triangle_area_play();
	tri1.Triangle_perimeter_play();
    }    
    else    
        cout<<"error"<<endl;    
	}
	return 0;
}

有一点不明白，我在头文件上没有用cmath为什么程序照常运行（程序中用到了sqrt）难道是vs2010的原因吗