JsonArray.size contains always 0 after successful deserialization about arduinojson HOT 6 CLOSED

Or does it make sense to use the JsonDocument object as a central container for my stations list?

So I would pass a reference to the object to my load/deserialize function, like this

void loadStations(const char *filename, JsonDocument& doc);

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Hi @bblanchon!
Thanks for your comment - OMG. Got it.
This code works

  JsonArray arr = doc.as<JsonArray>();
  int count = arr.size();
  Serial.printf("arr.size()=%d \n", count);

Furthermore: Reading your FAQs (https://arduinojson.org/v7/faq/why-must-i-create-a-separate-config-object/) I understand it is not a good idea to use a `JsonDocument object as my permanent container for my stations array.

Again thanks for your support.

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Hello @bblanchon!

Still got one open question. Do I have to copy the strings from the JsonDocument item["name"] and item["url"] or can I use the existing ones by using their address (like shown below)?

I'm now using these two global variables

stationInfo_t * stationInfo;  // array of structures
int countElements = -1;     // array_len

and changed my function int loadStations(const char *filename) using new to allocate memory for my array of this struct

typedef struct stationInfo_s {
  const char* name;
  const char* url;
} stationInfo_t;

int loadStations(const char *filename) {
  Serial.printf("Reading stations from: %s\n", filename); 
  
  File file = SPIFFS.open(filename);

  if(!file) {
    Serial.printf("ERROR: Failed to open: %s\n", filename);
    return countElements;
  } 

  Serial.printf("Successfully opened: %s (%d bytes)\n", filename, file.size());
  
  JsonDocument doc;

  DeserializationError error = deserializeJson(doc, file);

  if (error) {
    Serial.print(F("deserializeJson() failed: "));
    Serial.println(error.f_str());
    return countElements;
  }

  JsonArray arr = doc.as<JsonArray>();
  countElements = arr.size();
  Serial.printf("Successfully deserialized %d elements\n", countElements);

  stationInfo = new stationInfo_t[countElements];

  int i = 0;
  for (JsonObject item : doc.as<JsonArray>()) {    
    stationInfo[i].name = item["name"];
    stationInfo[i].url = item["url"];
    Serial.printf("%d\t %s\t %s\n", i, stationInfo[i].name, stationInfo[i].url);
    i++;
  }

  file.close();
  return countElements;
}

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Answering my own question: Yes, I do have to duplicate the strings. I've used strndup to allocate memory and making a copy.

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Hi @chess-levin,

I didn't find the time to tell you about as<JsonArray>(), but fortunately, you saw my commit for the ArduinoJson Troubleshooter.

About the configuration structure, I would avoid strdup() because it's a recipe for memory leaks.
Instead, I would use fixed-sized char arrays (as shown in JsonConfigFile.ino) or Strings.

Best regards,
Benoit

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Thanks for your feedback. I've learned a lot yesterday ;-)
As I plan to read the presets just once (@ setup function) from the json-file and then hold them unchanged in memory, I think the risk of memory leaks is low.

Your library is a great piece of software. Thank you for all your work!!!

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