Arduino, code hangs in weird way when a function is invoked - c++

Question

I have the following code behaving in a weird way, when I make a method call
#include <TimerOne.h>
#include <cppQueue.h>
#include <Arduino.h>
typedef struct Diagnostic {
String entityName;
uint16_t pin;
} Diag;
// Global setup
cppQueue q(sizeof(Diag *), 10, FIFO, false);
int ctr = 0;
void setup() {
// initialize digital pin LED_BUILTIN as an output.
pinMode(LED_BUILTIN, OUTPUT);
Serial.begin(9600);
}
// Add to queue
// the loop function runs over and over again forever
void loop() {
digitalWrite(LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level)
delay(2000); // wait for a second
Serial.println("After high");
digitalWrite(LED_BUILTIN, LOW); // turn the LED off by making the voltage LOW
delay(1000); // wait for a second
// Critical Section
Diag d;
Serial.println("String assignment");
d.entityName = "Front Door";
d.pin = LED_BUILTIN;
d_ptr = &d;
Serial.println("Enqueuing one");
q.push(&d);
Serial.println("Enqueued one");
Serial.println(freeMemory());
//q.pop(&d);
//Serial.println("Dequeued one" + d.entityName);
DequeueIsr();
//Serial.println("Atomic block");
//Serial.println(q.getCount());
//Serial.println("Loop end");
}
void DequeueIsr() {
Diag di;
if (!q.isEmpty())
{
Serial.println("Queue not empty");
q.pop(&di);
Serial.println("Dequeued one" + di.entityName);
Serial.println(freeMemory());
}
}
Whenever I try to pop my object in the DequeueIsr function, the code hangs after 2 or 3 iterations, there is no output on the serial monitor after that. If I just copy the code in the Dequeue method and run it in the main loop it works, it doesn't get stuck. Also this happens when I work with strings, if I remove the entityName field from Diag, it works inside the method.
Am I missing something crucial ?
What I really wanted to do was, dequeue the queue using an interrupt method using the TimerOne library. I have removed all timer related code and even a simple method call isn't working.

Answer 1

It looks like cppQueue copies objects using memcpy so it only works with trivially copyable objects, String is not trivially copyable so your code has undefined behaviour