Extend jsonString R6 class to support ordered json type

[cgiachalis]

This PR extends jsonString R6 constructor with an option ordered_json to preserve the insertion order of object keys using the type nlohmann::ordered_json from the vendor library. For convenience, a new R6 class ojsonString was added which represents an ordered_json type and inherits from jsonString.

For reference: Order of object keys and object order.

Example using ordered_json to preserve the insertion order:

j <- "{\"Jan\":1,\"Feb\":2,\"Mar\":3}"

# default case
jsonString$new(j, ordered_json = FALSE)

{
    "Feb": 2,
    "Jan": 1,
    "Mar": 3
}

# preserve order
jsonString$new(j, ordered_json = TRUE)

{
    "Jan": 1,
    "Feb": 2,
    "Mar": 3
}

# ojsonString always preserves order
ojsonString$new(j)

{
    "Jan": 1,
    "Feb": 2,
    "Mar": 3
}

Code Changes:

At C++ level, I created a new Rcpp class oJsonString to represent the ordered_json type. Practically, it is a replication of what has been done with JsonString Rcpp class with a minor code modification in addProperty method where I use the emplace_back and not emplace method because it couldn't compile.

At R level, the R6 constructor gets a new option ordered_json to select between preserving the order or not (default).

Internally, using the ordered_json argument we can switch to the appropriate Rcpp class JsonString or oJsonString at initialisation.

For user's convience, a new R6 class ojsonString was added as a wrapper that simply inherits from jsonString and defaults to ordered_json.

In addition, a new private state variable .is_ordered_json is introduced intended for methods that need to inherit the json type i.e., when using copy, unflatten , update etc.

Also, I added a private method getPtr() to abstract away a common pattern found across methods; and extend it a bit to guard against mixing json types . e.g., when adding a property from default json type to ordered json will raise an error. See below :

jstring <- jsonString$new("{\"a\":[1,2,3],\"b\":\"hello\"}") # json
ppty <- ojsonString$new("{\"f\":\"hello\"}")                 # ordered_json

jstring$addProperty("v1", ppty) # Error
jstring$merge(ppty)             # Error

Lastly, jsonString R6 interface has two new read only active fields: json_type and is_ordered_json.

Notes

• I am working on some unit tests for ojsonString that can be used for jsonString too

• Is it worth adding an abstract R6 base class so that both ojsonString and jsonString to inherit from?

closes #1