[pbrowne011]

"Classic C++" and "Modern C++" refer to the language before and after C++11, respectively.

Some of the key differences are use of standard library and its containers, smart pointers, and other language features that look less like C. In this specific library, this refers to some of the techniques like bit manipulation, manual memory management and string parsing, and using things like enums to improve speed and reduce complexity.

An example of a more robust (but still "classic") library would be something like https://github.com/Tencent/rapidjson.