JsonDocument dynamic allocation problem

[Leo-C]

Hi,
the title not refers to DynamicJsonDocument, but it refers precisely to dinamic allocation of a JsonDocument with new !

My environment is:
Platform: ESP32 TTGO
IDE: Arduino IDE 1.8.13

The simple code below compile well, but crash at delete:

#include <Arduino.h>
#include <ArduinoJson.h>

void setup()
{
	Serial.begin(115200);
}

void loop()
{
	Serial.println("Mem before: " + String(ESP.getFreeHeap()));
	
	JsonDocument* doc = new DynamicJsonDocument(1024);
	delete doc;
	
	Serial.println("Mem after: " + String(ESP.getFreeHeap()));
}

(this is a minimal example for a scenario where I allocate dinamically a JsonDocument into a function, return a pointer, and only after use, I call destructor in a different scope with delete in a different scope)

I appreciate any help to understand my error

Many thanks in advance

Leonardo

[Leo-C]

Sorry,
I've found that following code works:

#include <Arduino.h>
#include <ArduinoJson.h>

void setup()
{
	Serial.begin(115200);
}

void loop()
{
	Serial.println("Mem before: " + String(ESP.getFreeHeap()));
	
	DynamicJsonDocument* doc = new DynamicJsonDocument(1024);
	delete doc;
	
	Serial.println("Mem after: " + String(ESP.getFreeHeap()));
}

(only changing doc from type JsonDocument to DynamicJsonDocument)

Then, JsonDocument is not a superclass of DynamicJsonDocument? or destructor is not virtual?

Many thanks

Leonardo

[bblanchon]

Hi Leonardo,

Thank you very much for opening this issue.

DynamicJsonDocument is not intended to be deleted from a pointer to base.
In fact, it has no virtual function at all, and I don't want to pay the overhead just for this edge case.
As you'll see, I made the destructor of JsonDocument not virtual but protected, so the compiler will issue an error next time you try to delete a JsonDocument.

Now, I don't understand why anyone would want to do that since DynamicJsonDocument is already in the heap.
It seems that you want to return a JsonDocument from a function, right?
Why don't you write something like this:

DynamicJsonDocument makeJsonDocument() {
  DynamicJsonDocument doc(1024);
  // ...
  return doc;
}

DynamicJsonDocument is moveable, so no copy of the buffer is done, even if RVO doesn't kick in.

Best regards,
Benoit

[Leo-C]

Many Thanks!

I admit that I did not know Copy-Elision, then your suggestion is good and efficient.

I also have attempted to write a function like:

JsonDocument makeJsonDocument() {
  DynamicJsonDocument doc(1024);
  // ...
  return doc;
}

but was not accepted by compiler ...

Leonardo

[bblanchon]

Yes, that's expected.
Allowing this would result in a slicing of DynamicJsonDocument and therefore to a memory leak.

[smuellener]

One question, does that mean that DynamicJsonDocument has built-in smart shared pointer functionality? In other words:

std::shared_ptr<DynamicJsonDocument> create_a_doc_for_usage()
{
    auto doc = std::make_shared<DynamicJsonDocument>(JSON_OBJECT_SIZE(1));
    (*doc)["Content"] = "Test";
    return doc;
}

would not bring any benefit over

DynamicJsonDocument create_a_doc_for_usage()
{
    auto doc = DynamicJsonDocument(JSON_OBJECT_SIZE(1));
    doc["Content"] = "Test";
    return doc;
}

Thank you for the clarification!

[bblanchon]

@smuellener, in a sense, it's more like unique_ptr because it doesn't implement reference counting.

[bblanchon]

ArduinoJson 6.17.3 includes the protected destructor.