warning: variable 'b' set but not used

Question

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]; ^

• Juraj: I get Symbol 'I' could not be resolved. Change I to i.
• clankill3r: @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.

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:

Serial.println(boids[i].start_animate_time);

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...

• clankill3r: Thanks, I always struggle with references and pointers since my lack of experience with them.
• KIIV: 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)
• Majenko: @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.