reusing a json document - JsonDocument.clear() not clearing pool

[proddy]

I'm finding the JsonDocument.clear() function doesn't always reset and clear the pool. I've read https://arduinojson.org/v6/how-to/reuse-a-json-document/ and I suspect it's because my json object is dangling after each clear() but wanted to report it here anyway since it used to work < 6.19.0.

Environment

ESP8266, ESP32, PlatformIO, also standalone with G++ and gcc

Reproduction

Copy into wandbox:

#include <iostream>

#include "ArduinoJson.h"

using namespace std;

int main() { 
  DynamicJsonDocument doc(1024);
  JsonObject json = doc.to<JsonObject>(); // I need the JsonObject to pass to other functions later
    
  for (int i = 1; i < 5; i++) {
      doc.clear();
      printf("Adding object %d:\n", i);
      printf(" Before: ");
      serializeJson(json, cout);
      cout<<endl;
      
      json["name"] = "name";
      json["index"] = i;
      
      printf(" After: ");
      serializeJson(json, cout);
      cout<<endl;
  }
   
}

Expected output:

Adding object 1:
 Before: {}
 After: {"name":"name","index":1}
Adding object 2:
 Before: {}
 After: {"name":"name","index":2}
Adding object 3:
 Before: {}
 After: {"name":"name","index":3}
Adding object 4:
 Before: {}
 After: {"name":"name","index":4}

Actual output:

Adding object 1:
 Before: {}
 After: {"name":"name","index":1}
Adding object 2:
 Before: {"name":"name","index":1}
 After: {"name":"name","index":2}
Adding object 3:
 Before: {"name":"name","index":2}
 After: {"name":"name","index":3}
Adding object 4:
 Before: {"name":"name","index":3}
 After: {"name":"name","index":4}

(the JsonObject json still has data after the JsonDocument clear() )

[bblanchon]

Hi @proddy,

This is the expected behavior, and I think the documentation is pretty clear about it:

every reference acquired before clearing the JsonDocument is invalidated. In other words, you must not use a JsonArray, JsonObject, or JsonVariant created before calling JsonDocument::clear().

If you're surprised (or disappointed) by this behavior, think of std::vector::clear(): "All iterators, pointers and references related to this container are invalidated." This is a known issue in C and C++ collections; there is nothing we can do about it.

To fix you program, you just have to move JsonObject json = doc.to<JsonObject>() inside the loop.
Also, you wouldn't notice the issue if you passed the JsonDocument to serializeJson() instead of the dangling JsonObject.

Best regards,
Benoit

[bblanchon]

PS: your program gives the same output with 6.19.1, 6.18.5, 6.17.3, 6.16.1, 6.15.2, and 6.14.1:
https://wandbox.org/permlink/mpgRuSKdZTEV8aPH
https://wandbox.org/permlink/ojmV9U9BE7tO459z
https://wandbox.org/permlink/Zcw2gGnEdQfULZZt
https://wandbox.org/permlink/JuuJdN22qd0cSxZx
https://wandbox.org/permlink/o4ybNCyae8iA9k3E
https://wandbox.org/permlink/KMkYBMQ02UHxkitF

[proddy]

thanks, makes sense.