How to use Arduino's Serial.print with char* and const char* types

Question

I require to build a simple Arduino function that returns either "char*" or "const char*" type and then I need to print that value.

However, I'm facing a problem: when I try to print the function's return value, actually nothing gets printed.

char *getID()
{ 
  char ID[15]{"123456789ABCDE"};

  // The actual value for ID is returned from another 
  // function as a String type, so, for simplicity's sake
  // I'm just using a random string instead of posting here that function
  String str{"EDCBA987654321"};
  
  // Write the String returned value into the ID buffer
  str.toCharArray(ID,str.length());

  
   // The following piece of code actually prints the value: EDCBA987654321
  //Serial.println("print ID from inside the function: ");
  //Serial.println(ID);

  return ID;
}


void setup() {
  Serial.begin(9600);
  while(!Serial);
}

void loop() {
  /**
   * Nothing gets printed when using the return value from the function
   */
   Serial.println("print id as the value returned by the \"getID\" function:");
   Serial.println(getID());
   delay(2000);
}

This is the output on the serial monitor:

If I uncomment the lines inside the "getID" function, then the "ID" value gets printed:

I don't know what am I missing over here. Thanks in advance and happy holidays.

Answer 1

There are two solutions for this, it all related to the fundamental understanding of string literal and array in C++, not specific to Arduino.

This will work:

char *getID()
{ 
  char *ID{"123456789ABCDE"};

  return ID;
}

In C++, a string literal has global scope, a pointer to a string literal which has global scope is of course point to the correct string literal in the memory. This is equivalent to directly using a global declared const char *ID{"123456789ABCDE"};.

or alternative this will also work:

char *getID()
{ 
  static char ID[15]{"123456789ABCDE"};

  return ID;
}

The problem with your original code is that ID[15] is an array which has local scope within the function, it is not a string literal, but merely an array of ID[15]{"1", "2", "3"... "E"};. Another problem is that you are returning a pointer to an array which immediately out of the scope when return. Therefore you need the modifier static to keep the array in memory even after returning from the function.