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Use of "static" with "RTClib"

I've got a question concerning the Arduino library "RTClib" by Adafruit and the use of the word "static".

Here you can see an excerpt from the example provided for the pcf8523 real-time clock:

#include "RTClib.h"
RTC_PCF8523 rtc;
void setup () {
 while (!Serial) {
 delay(1); // for Leonardo/Micro/Zero
 }
 Serial.begin(57600);
 if (! rtc.begin()) {
 Serial.println("Couldn't find RTC");
 while (1);
 }
 if (! rtc.initialized()) {
 Serial.println("RTC is NOT running!");
 }
}
void loop () {
 DateTime now = rtc.now();
 Serial.print(now.year(), DEC);
 /*
 do some more stuff
 */
}

In the beginning of loop() they wrote:

DateTime now = rtc.now();

Thus, with every iteration of loop() "new" gets defined anew (on the heap?!), right? This appears inefficient to me.

I thought about rewriting it this way:

static DateTime now; // static declaration, executed only once
now = rtc.now(); // assignment to "now" every time loop() starts over

In this case "new" should be allocated on the stack like a global variable, right?

I want to use "static" to increase overall performance by reducing work for the processor and the heap (thereby avoiding heap fragmentation). Does this make sense or will it cause more problems than it solves? Keep in mind that I want to learn both proper programming for Arduino and proper coding in general.

Link to the library: https://github.com/adafruit/RTClib

Thank you! :-)

EDIT: For some reason I CAN NOT declare DateTime now; in global scope or else the program won't run. Don't know why though. This is why I want to use static in the first place.

Answer*

You don't need to use the `static` keyword.

The cleanest solution is to define `DateTime` outside of the `loop` function:

 DateTime now;

This will allocate the memory, and call the constructor of the class `DateTime` initializing it.

In case you would need a basic variable (like an `int`), you should initialize it, like:

 int now = 0;

When you use it in `setup` or `loop` you use the created instance (without the class name to declare it):

 void loop() {
 ...
 now = rtc.now();
 }

Also, find a better name for `now`, because the variable is the same as the `rtc`'s function `now`.
 

**Update**

after your comment, it cannot be used as a global variable:

I didn't try it myself, but I suspect, the constructor calls a function that uses variables or a function that is not initialized at that point. Actually, it's quite bad practice.

Anyway, one way to circumvent and stick as much to what I wrote above, is to use a pointer instead:

 RTC_PCF8523* rtc = NULL;

Than you create an empty pointer, without calling the constructor. You will do this at the end of `setup`:

 void setup() {
 ...
 rtc = new RTC_PCF8523();
 }

This will cause the variable to be put on the stack.

Than when using it, you call:

 now = rtc->now();

Note you have to use the -> notation, as rtc is a pointer.

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1

Forgot to mention: Defining it outside of loop() in the global scope DOES NOT WORK! Don't ask me why, I spent hours just to find this problem. This is the reason why I wanted to use static in the first place. Will edit my post. And concerning the name "now": It's just what they used in the example, I won't use it, but good point!

blackdaw
– blackdaw

2020年04月22日 19:54:18 +00:00
Commented Apr 22, 2020 at 19:54
I updated my answer, just try if it helps.

Michel Keijzers
– Michel Keijzers

2020年04月22日 20:10:42 +00:00
Commented Apr 22, 2020 at 20:10
1

Re "I suspect, the constructor calls a function that uses variables or a function that is not initialized at that point": I can't see anything wrong in this constructor.

Edgar Bonet
– Edgar Bonet

2020年04月22日 20:34:22 +00:00
Commented Apr 22, 2020 at 20:34
@EdgarBonet ... me neither, I'm wondering why creating it as global variable wouldn't work.

Michel Keijzers
– Michel Keijzers

2020年04月22日 20:36:46 +00:00
Commented Apr 22, 2020 at 20:36
See the comments above beneath @EdgarBonet 's solution. It compiles and uploads but doesn't execute.

blackdaw
– blackdaw

2020年04月23日 17:16:07 +00:00
Commented Apr 23, 2020 at 17:16

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