I got the following error:

warning: variable 'b' set but not used.

I think I do a really small thing wrong but I can't figure out what it is.

#define array_size(array) ((int)(sizeof(array) / sizeof((array)[0])))

typedef struct Boid {
  unsigned long start_animate_time;
} Boid;

Boid boids[4];


Boid create_boid() {
  unsigned long start_animate_time = 0L; 
  Boid b = {start_animate_time};
  return b;
}


void setup() {
  Serial.begin(9600);

  boids[0] = create_boid();
  boids[1] = create_boid();
  boids[2] = create_boid();
  boids[3] = create_boid();

  for (int i = 0; i < array_size(boids); i++) {
    Boid b = boids[i]; // <<<<<<<<<<<<< this line!
    b.start_animate_time = 456;
  } 
  

  for (int i = 0; i < array_size(boids); i++) {
    Boid b = boids[i];
    Serial.println(b.start_animate_time); // BUG always 0
  }
}


void loop() {

}

flock_01_ino_debug.ino:28:10: warning: variable 'b' set but not used [-Wunused-but-set-variable] Boid b = boids[i]; ^