说明并定义一个长方形类CRectangle。其数据成员定义为私有，包括左下角点坐标x，y；x方向长度a与y方向长度b。定义公有成员函数作为CRectangle的外部接口，包括输入或修改长方形的坐标；输出该长方形坐标、长宽，周长与面积。 -Describe and define a rectangle class CRectangle. The data member is defined as private, including the lower-left corner point coordinates x, y　x direction and y direction of the length of a length of b. Public member function as CRectangle defined external interfaces, including input or modify the rectangular coordinates　the output of the rectangular coordinates, length and width, circumference and area.