Why does the value of a string in memory change in Arduino?

Question

Arduino

I have a list of structures which is stored in a class as a pointer to the first element of the list:

Structure

struct SensorValue {
  String name;
  double value;
  bool constantly;
  SensorValue *next;
};

It is written correctly by calling the sendValue method like this:

Entry to the list

void Cloud::sendValue(String name, double value, bool constantly) {
  SensorValue newSensorValue;
  newSensorValue.name = name;
  newSensorValue.value = value;
  newSensorValue.constantly = constantly;
  newSensorValue.next = NULL;

  addSensorValue(&newSensorValue);
}

void Cloud::addSensorValue(SensorValue* value) {
  if (sensorsList == NULL) {
    sensorsList = value;
    return;
  }

  SensorValue* iter = sensorsList;
  while (iter->next != NULL) 
    iter = iter->next;

  (iter->next) = value;
}

But when accessing the first element from another method, the value of the name field is taken as if from thin air:

List access

String requestBody = "{\"login\":\"" + login + "\",";
  requestBody += "\'password\':\"" + password + "\",";
  requestBody += "\"deviceName\":\"" + device + "\",";
  requestBody += "\"deviceType\":\"Arduino\",\"sensorsValue\":{";

 SensorValue* iter = sensorsList;
  Serial.println(sensorsList->name);
  Serial.println(sensorsList->value);
  Serial.println(sensorsList->next == NULL);

In this example, the value sensorsList->name outputs the value "\"deviceName\":\"" + device + "\","; which was concatenated above. And the following 2 values ​​are displayed correctly. It turns out the problem is with the line
Why is this happening and how to fix it?

Answer 1

SensorValue newSensorValue;
 addSensorValue(&newSensorValue);

Because the address on the structure allocated on a stack undertakes.
You can't do that.
Allocate memory via new or copy.