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Passing pointers between setup and loop

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User

( 5 months ago )

 

I'm new to the Arduino world. The last time i was programming microprocessors, was back with the Z80 in assembly language.

I am also teaching myself C++ and seems to be going quite well.

I have decided to write my own 'library' for driving 7 segment LED's. The reason for writing my own, rather then using an existing one is to aid with the learning process. I'm quite pleased to say it works ( it may not be the most efficient code)

The books I have on C++ are great, but i do find sometimes the explanation of concepts seem to get lost.

what I would like to know, how do you go about initialising an object class in setup() and using it in loop()? I have this sneaky suspicion that it involves creating the object on the heap, and passing a pointer to the object over somehow.

I could just initialise it once in loop(), and make my own continuous loop afterwards, but there are other things done between the and of a loop() and the beginning of the next loop() (things like timer interrupts).

I will post the code im using, but not the library unless asked to, as it is quite a few lines.

# include "Leddisplay.h"
void setup() {

    byte Digit[9] = { 22, 23 , 24 ,25, 26, 27, 28, 29, 2 };

    Leddisplay *pRightDigit = new Leddisplay(Digit);



}

void loop() {

    pRightDigit->setDigit(5);
    pRightDigit->displayDigit();

}

^^ Obviously this does not compile ^^

# include "Leddisplay.h"
void setup() {





}

void loop() {


    byte Digit[9] = { 22, 23 , 24 ,25, 26, 27, 28, 29, 2 };
    Leddisplay *pRightDigit = new Leddisplay(Digit);

    pRightDigit->setDigit(5);
    pRightDigit->displayDigit();

    for (;;);   // or do.. while within which has other code to repeat
                // but wont allow anything to be executed between loop() iterations

}

^^ This does, and runs fine, but may not achieve what I want ^^

I'm not sure if I have used the right wording, or even asked the right question. Thank you for any help in advance. Dave

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User

( 5 months ago )

 

What you have to understand is the concept of scope.

Everything (well, almost everything) in C and C++ is enclosed in { and }. Those define a scope, and everything defined within that scope is available to any other scopes defined within that scope.

You can also assume that the whole sketch is surrounded by { and }. This scope is called the global scope. Anything defined within it is available anywhere in your sketch.

So by defining the object in the global scope it is available everywhere:

# include "Leddisplay.h"

Leddisplay *pRightDigit;

void setup() {
    byte Digit[9] = { 22, 23 , 24 ,25, 26, 27, 28, 29, 2 };
    pRightDigit = new Leddisplay(Digit);
}

void loop() {
    pRightDigit->setDigit(5);
    pRightDigit->displayDigit();
}

However the use of new etc is discouraged in low-memory systems. Better is to statically define the object:

# include "Leddisplay.h"

const byte Digit[9] = { 22, 23 , 24 ,25, 26, 27, 28, 29, 2 };
Leddisplay pRightDigit(Digit);

void setup() {
}

void loop() {
    pRightDigit.setDigit(5);
    pRightDigit.displayDigit();
}

Note that, when statically defined like this, things in the constructor aren't guaranteed to be run at the right time. They should be moved into a .begin() function that you then call from setup():

# include "Leddisplay.h"

const byte Digit[9] = { 22, 23 , 24 ,25, 26, 27, 28, 29, 2 };
Leddisplay rightDigit(Digit);

void setup() {
    rightDigit.begin();
}

void loop() {
    rightDigit.setDigit(5);
    rightDigit.displayDigit();
}

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