No offence, but you should make it clear in the beginning that you are not too familiar with (modern) C++. This blog post might teach how to write a simple JSON

No offence, but you should make it clear in the beginning that you are not too familiar with (modern) C++. This blog post might teach how to write a simple JSON parser, but it does not teach how to write good modern C++. There a many red flags which others have already pointed out.