建立box类，box的构造函数被传递了三个double值，每个double值表示盒子的一个边长，用box 类计算立方体的体积，并存储在一个double变量中，在box类中包含一个成员函数vol(),用来显示每个box对象的体积。-Establish box class, the box is the constructor is passed three double value, each double value represents a side length of the box, with box class to calculate the volume of the cube, and stored in a double variable in the box class contains a member The function vol () is used to display the volume of each box object.