c++运算符()和'using'声明:左操作数必须是l值错误

The example below illustrates a more complex but not dissimilar problem I've been trying to solve elegantly. I have a set of templates which must be specialized and, in doing so, implement one or both of two interfaces: Readable and Writable, in each specialization. Specific implements both interfaces, and is then tested using main:The example below illustrates a more complex bu