If you are creating your own std::vector in c++ code (platformio in my case), and need to set the value on one of your JsonObject or JsonArray for Arduino, there’s a specific function/method you can use to do this, and below are some examples and how to do it.

First I must say thank you to @timemage in the #arduino channel on Freenode, who helped me to figure this out. 

In my situation I was storing a dynamic number of values into an array during the normal loop() in Arduino code, and at some point in there, I needed to send that data, along with a matching JSON structure before inserting it into a Mongo database … which btw MongoDB is .. sooooo … awesome, esp coming from MySQL … yikes!

My array was using just std::vector<double> but for the life of me I couldn’t figure out how to set that value in a key of a JsonObject I created using ArduinoJson.  With a lot of my coding skills being PHP or JavaScript (web based) , it was quite difficult to understand that I couldn’t just set the variable equal to that array .. and I think that comes with more experience in lower level programming languages.

My code for the vector looked like this, which uses STL container std::vector:

Set ArduinoJson array from std::vector

So to set the value of JsonArray& that we created, you have to use the .copyFrom which you can find documentation (https://arduinojson.org/api/jsonarray/copyfrom/), setting the first value equal to the variable used, calling the .data() function/method, and the second one equal to the variable calling .size(), like so:

That’s all there is to it.  Yes it may sound simple, but I spent hours trying to figure this out due to the way that memory is allocated for arrays, and really just only having a bit of C++ knowledge, so hopefully if someone else has this issue, this post helps you out 🙂

Working Code

Here’s the code that actually works (i’ll keep the 20x revisions that didn’t work — to myself hah):

Voila! Profit!