表达式求值，根据用户输入的算术表达式，输出相应结果，其中包括两个文件，头文件定义了各个类函数。只是支持加减乘除非运算。-Evaluates the expression, arithmetic expression based on user input, the output of the corresponding results, including the two files, header files define the various functions. The only support addition and subtraction multiplication unless computing.