Of the design of a parsing and encoding library? Not at all. Do you parse as a stream or all in one go? Are values represented as a special "json" type, or as built-in types? How should arrays and objects be represented? Are integers and reals different types? Are trailing zero decimals significant? Do you allow construction of arrays and objects in any order, or only sequentially?

(Granted, I've written my own C++ JSON library which I believe answers all these questions in an intuitive way, following both the design principles of the C++ standard library, and the lowest-common-denominator semantics of JSON, but it's sufficiently opinionated that I doubt I could convince any significant portion of C++ users that it's the "right" way to do things. Even if it "is", demonstrating such is nowhere near as easy as it is for unique_ptr, vector, string, thread, etc., each of which are more or less the "obvious" designs given certain constraints such as RAII to which the standard library adheres.)