Call by reference

Question

I am trying to learn 'call by reference'. I want to call a function fcn1 from the main loop and pass a reference of the local variable var to the function fcn1. Next step is to call a second function fcn2 from the function fcn1 and pass again the reference of the local loop variable var. I always get this error massage:

invalid conversion from 'int' to 'int*' [-fpermissive]

Here a small example code:

void fcn1(int *variable)
{
  fcn2(&variable);
}

void fcn2(int *variable)
{
  //do something with the variable
}

void setup()
{

}

void loop()
{
  int var = 1000;
  fcn1(&var);
}

I tried to solve that issue by using:

void fcn1(int *variable)
{
  fcn2(*variable);
}

But it's the same error.

By deleting the * respectively & it works fine, but is it still a reference?

Thanks a lot! If I interpret your answer and the link correctly, the following code works only with references, right? void fcn1(int &variable) { variable++; fcn2(variable); } void fcn2(int &variable) { Serial.println(variable); } void setup() { Serial.begin(9600); } void loop() { static int var = 1000; fcn1(var); delay(1000); } Many thanks! — piiq
– piiq, Commented Jan 31, 2018 at 20:47
Yes, however, it will not compile that way .. int & variable denotes a pointer, to increase it, you should use variable++. — Michel Keijzers
– Michel Keijzers, Commented Jan 31, 2018 at 20:50

tttapa · Accepted Answer · 2018-01-31 21:45:06Z

void fcn1(int *variable)
{
  fcn2(*variable);
}

This function takes a pointer to an integer as parameter, and then passes the value of the integer to the second function.

To do it by reference, use an ampersand (&, not to be confused with the address-of operator).

void fcn1(int &variable) {
  // Do something to variable
}

For example:

void increment(int &variable) { // Reference to an integer
  variable++;
}
void setup() {
  Serial.begin(115200);
  int a = 1;
  Serial.println(a); // 1
  increment(a);
  Serial.println(a); // 2
}
void loop() {}

You could achieve the same thing using pointers, but using references is much cleaner and easier to read.

void increment(int *variable) { // Pointer to an integer
  ++*variable; // dereference first, then increment value 
}
void setup() {
  Serial.begin(115200);
  int a = 1;
  Serial.println(a); // 1
  increment(&a); // address of a
  Serial.println(a); // 2
}
void loop() {}

Please note: this question is not specific to Arduino, it is just C++. For questions like this, it is best to google for 'C++ pass by reference' instead of 'Arduino pass by reference'.

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