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() {

  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]; ^

  • I get Symbol 'I' could not be resolved. Change I to i.
    – Juraj
    Jun 19, 2018 at 9:31
  • @Jurai No idea how that became a capital since I copied pasted all code. I guess it is a safari auto correct that happened after I added // <<<<<<<<<<<<< this line! Since that is the only thing I added in the browser.
    – clankill3r
    Jun 19, 2018 at 13:18

1 Answer 1


C++ is not Java. You can't just assign structs like that - it copies them (if anything).

Boid b = boids[I]; // <<<<<<<<<<<<< this line!
b.start_animate_time = 456;

Instead just access the array slice directly:

boids[i].start_animate_time = 456;

And later on as well:


Basically, you create a "Boid" variable called "b", copy the content of boids[i] to it, set a value in that variable, and then throw it away. Hence the "not used". If you want to reference a specific array slice as a separate variable either get a pointer to it:

Boid *b = &boids[i];
b->start_animate_time = 456;

Or use a reference:

Boid &b = boids[i];
b.start_animate_time = 456;

Of the two I prefer using a pointer, but I come from a C background and understand pointers, but it took me many years of trial and error to get to that point...

  • Thanks, I always struggle with references and pointers since my lack of experience with them.
    – clankill3r
    Jun 19, 2018 at 13:24
  • 1
    Or you can use some C++11 features: for (auto & boid : boids) { boid.start_animate_time = 456; Serial.println(boid.start_animate_time); } (two loops are not needed here at all)
    – KIIV
    Jun 19, 2018 at 15:13
  • @KIIV Indeed - though be careful using auto - the meaning seems to change from C++ version to C++ version. Like they can't make their mind up what it's for.
    – Majenko
    Jun 19, 2018 at 15:56

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