ArduinoJSON: Using a reference to a reference when parsing array of arrays

Question

I started to play around with the ArduinoJson library to parse data I got from one of my APIs (the format of the JSON data I get from my API can be improved for sure, but the software I had to implement the API with is rather limited in formatting JSON strings). The data is formatted as an array of arrays like this:

[["P101","1.2345","bar(a)"],["P102","1234.567","mbar"],["P103","2345.678","mbar"],["P104","2.3456","bar(a)"]]

In order to get a first impression about how much memory is needed I used ArduinoJSON's Assistant to analyse my dataset, calculate memory requirements and get a very basic code snippet for parsing the data. Code snippet generated by the assistant is as follows:

const size_t bufferSize = 4*JSON_ARRAY_SIZE(3) + JSON_ARRAY_SIZE(4) + 90;
DynamicJsonBuffer jsonBuffer(bufferSize);

const char* json = "[[\"P101\",\"1.2345\",\"bar(a)\"],[\"P102\",\"1234.567\",\"mbar\"],[\"P103\",\"2345.678\",\"mbar\"],[\"P104\",\"2.3456\",\"bar(a)\"]]";

JsonArray& root = jsonBuffer.parseArray(json);

JsonArray& root_ = root;

JsonArray& root_0 = root_[0];
const char* root_00 = root_0[0]; // "P101"
const char* root_01 = root_0[1]; // "1.2345"
const char* root_02 = root_0[2]; // "bar(a)"

JsonArray& root_1 = root_[1];
const char* root_10 = root_1[0]; // "P102"
const char* root_11 = root_1[1]; // "1234.567"
const char* root_12 = root_1[2]; // "mbar"

JsonArray& root_2 = root_[2];
const char* root_20 = root_2[0]; // "P103"
const char* root_21 = root_2[1]; // "2345.678"
const char* root_22 = root_2[2]; // "mbar"

JsonArray& root_3 = root_[3];
const char* root_30 = root_3[0]; // "P104"
const char* root_31 = root_3[1]; // "2.3456"
const char* root_32 = root_3[2]; // "bar(a)"

I did not test this, yet but had a look at the produced snippet and stumbled upon these lines:

JsonArray& root = jsonBuffer.parseArray(json);

JsonArray& root_ = root;

What is the exact advantage of using address-references root, root_ and e.g. root_1 here? Those should, as far as I understood the code, reference to the same data at the end. Is it for better readability only?

Answer 1

The advantage of reference above pointers:

• References cannot be NULL, so it's much safer to use them
• Address of a reference cannot be acquired, this means a calling function cannot mess up anything outside the type (value) being passed.

For passing a reference to an address the same advantages applies (cannot be NULL, address cannot be changed).

Also, address arithmetic (address + xxx or address - xxx) cannot be used, which prevents tricky programming errors where values outside a variable's memory space can be changed.

Answer 2

It is not a reference to reference. It is a copy of the reference.

The generator must handle more difficult JSON inputs then yours and to make output readable it has a strategy, which in simple case can look as unnecessary. Yes, it could use root[n][m].