利用虚函数实现的多态性来求四种几何图形的面积之和。这四种几何图形是：三角形、矩形、正方形和圆。几何图形的类型可以通过构造函数或通过成员函数来设置。-The use of virtual functions to find four kinds of polymorphism of the area of ​ ​ geometry. The four geometry are: triangular, rectangular, square and round. Geometry types can be passed through the constructor or member function to set.