json_hpp

Q: I started to read up on json.hpp, this looks like an immediate dealbreaker:
    https://json.nlohmann.me/home/faq/#number-precision
   If it really were printing doubles with only 15 digits of precision, that would be inadequate: 17 is needed in general.
A: Fortunately I think that doc is lying:  I tried printing some stuff out and it looks like it does some sort of smart adaptive thing that makes sure double->json->double is non-lossy.

Q: So wait a minute, when I say, in c++:
    json j = {{"a","b"},{"c","d"}};
  How does it know whether I want this json string:
    '[["a","b"],["c","d"]]'
  or this other json string:
    '{"a":"b", "c":"d"}' ?

A: ah, that's one of the edge cases described in the doc
  https://github.com/nlohmann/json. All the edge cases listed are:

  // a way to express the empty array []
  json empty_array_explicit = json::array();

      // ways to express the empty object {}
  json empty_object_implicit = json({});
  json empty_object_explicit = json::object();

  // a way to express an _array_ of key/value pairs [["currency", "USD"],
  ["value", 42.99]] json array_not_object = json::array({ {"currency", "USD"},
  {"value", 42.99} });

Q: does json.hpp have copies of everything?  or refs?
A: appears to be copies, as can be seen here.
    json j1 = {{"a", "b"}};
    json j2 = j1;
    j1["c"] = "d";
    j2["e"] = "f";
    LOG(INFO) << "          j1.dump() = " << j1.dump();
    LOG(INFO) << "          j2.dump() = " << j2.dump();

Q: given that assigning a parent to a child is deep copy operation,
   is creating a json structure from a ragged array horribly inefficient?
   In particular, a long parent-child chain, say of length 100.