Fields serialized as "null" when including other library that messes with nullptr definition.

[dr-gino]

Targeting ESP8266 using PlatformIO.
Long story short: I was including a library (Arduino-Temperature-Control-Library) before including ArduinoJson.h and it caused ArduinoJson to always generate null values for some fields that were supposed to contain non-0 integer values. (At first I though there were memory issues, but there was plenty available).
It seems that Arduino-Temperature-Control-Library does the following in its header file:

#ifndef nullptr
#define nullptr NULL
#endif

Including ArduinoJson.h before the other library fixes this, and obviously just redefining nullptr like that is bad behavior. However this bug took me hours to resolve as no obvious errors/warnings were thrown.

Two questions:

1. How exactly does it cause the behavior I was seeing?
2. Can anything be done in ArduinoJson to warn the user/be more defensive? The Arduino Temperature Control Library is pretty popular for reading out DS18b20 temperature sensors. I might not be the first one encountering this issue.

[bblanchon]

Hi @dr-gino,

Indeed #ifndef nullptr is a mistake: the condition is always false because nullptr is not a macro.
I saw that milesburton/Arduino-Temperature-Control-Library#155 is trying to fix this bug.

I can add a warning and disable support for nullptr accordingly, but I don't manage to reproduce the issue.
Can you share a minimal example with wandbox?
Here is a starting point: https://wandbox.org/permlink/W6P7jhJSPQaHscOA

Best regards,
Benoit

[dr-gino]

I'm unable to reproduce it on wandbox, probably because the toolchains are very different.
Here's some code that reliably reproduces it on an ESP8266 with the ArduinoJson@6.16.1 library.
Both the Arduino IDE and PlatformIO can be used to reproduce the erroneous output.
Unfortunately I don't have the hardware to do on chip debugging so investigating any further is a real pain.

#include <Arduino.h>
#define nullptr NULL
#include <ArduinoJson.h>

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

    StaticJsonDocument<100> jsonDoc;

    jsonDoc.add(1);

    String jsonString;
    serializeJson(jsonDoc, jsonString);

    Serial.println(jsonString);
}

void loop() {
    // do nothing.  
}

This prints out: [null] but if the #define nullptr NULL is removed it prints [1] as expected.

[bblanchon]

This fix was published in ArduinoJson 6.17.0.