Arduino sizeof Servo array objects is.. wrong?

Question

After searching for a quite long while over the internet, I have no choice but to try asking someone if they can explain me this apparently strange situation.

I'm doing some tests using some servo motors, trying to move them almost together using millis() and Servo object.

It is, of course, working either for single Servos and multiple servos.

The case, more detailed, is about this:

1. Move one, two or more servo motors together.
2. Force them to take ALMOST the same time to accomplish any action.
3. Have a dynamic number of servos.

About the first two points I had no problems to solve them, I was easily able to make a function and then make a library to accomplish such a job.

However, since I'm NOT that used to C++ and I was wondering if it actually was possible to create an array of Servo objects, like you can actually do, on arduino, with strings:

String stringArray[] = {"string1", "string2", "and so on, I love it."};

So, after wondering a while, I've tried this:

Servo servObject;
Servo servObject_2;
Servo servObject_3;

Servo servos[] = {servObject, servObject_2, servObject_3};

And you know what's cool? my function is actually working correctly by forcing the amount of elements of the servos array.

What I mean by that is that if I force my function to know that it is going to have 3 elements into the array it will, of course, work; However, for some reason, if I use sizeof(servos); the value returned, surprisingly, is 2.... Moreover, if I only push two servo objects instead of three into the servo array, its sizeof returns 6.

By googling, I've found out a couple of discussions about the fact that, for some reasons, in some cases, sizeof returns an incorrect value (if I'm not wrong it was the correct value -1), but in my case it apparently is not following any logic at all.

The board I'm using is an Arduino nano w/ ATMEGA 328.

Any idea of why it is not returning the correct value? Am I actually going totally wrong because I shouldn't make an array of servo objects?

ps: I didn't post the whole code because it would've been useless, since the sizeof is the first function called in the loop()

Regards,

briosheje

Answer 1

What you describe seems to be a common issue that many C or C++ developers can get (or have got) with sizeof.

Take a look at the following code:

char a[3];

int size(char b[]) {
    return sizeof b;
}

void setup() {
    int size1 = sizeof a;
    int size2 = size(a);
}

In that code, size1 will be evaluated, by the compiler, to 3 which is the real size of a variable, whereas size2 will be 2.

Why so?

This is just because sizeof a is evaluated by the compiler, and the compiler knows that a is an array of 3 char; but when a is passed as an argument to function size(), it is converted to a pointer (to the first element of the array), hence its size is the size of a pointer, which is 2 bytes on AVR ATmega MCU.

This subtlety of sizeof may explain strange values as you describe.

How to fix the issue?

Most C functions that take an array as argument will require another argument that tells the size of the array; that's the way (the only one I think) to deal with this problem.

Here is a sample code:

Servo allServos[] = {...};
#define NUM_SERVOS (sizeof(allServos) / sizeof(Servo))
...
void handleServos(Servo *servos, int numServos) {
    for (int i = 0; i < numServos; i++) {
        servos[i].write(...);
        ...
    }
}

void loop() {
    handleServos(allServos, NUM_SERVOS);
}

It is common practice in C to declare an array and immediately after, define its size as above.