Calling functions from an array of pointers in a library causes 'undefined reference to' error

Question

I am trying to create a library, where I have an array of pointers to functions in it. This array is used by other functions in the library. This is the code of the header file:

class Lib {
 public:
 static void call();
 static void (*caller[])();
 private:
};

This is the code of the .cpp-file:

#include "Lib.h"
void (*caller[2])();
void Lib::call() {
 caller[0]();
}

The sketch executing this is

#include <Lib.h>
void setup() {
 Lib::call();
}
void loop() {}

When I try to compile this, I get the following error:

undefined reference to `Lib::caller'
collect2: error: ld returned 1 exit status
Mehrere Bibliotheken wurden für "Lib.h" gefunden

Another weird thing is that the error about the multiple found libraries goes away if I take away the line

Lib::call();

However, if I put the same code directly into the sketch like so:

void (*caller[2])();
void call() {
 caller[0]();
}
void setup() {
 call();
}
void loop() {}

I get no error. What's the problem here?

Answer 1

void (*caller[2])();

should be:

void (*Lib::caller[2])();

Since you are defining storage space for a static member variable you have to tell the compiler what class it's a static member of.

• LukasFun: Thanks, that solves the part about the undefined reference. How does the problem with the multiple libraries originate from this though?
• Majenko: Probably because you do have two libraries named Lib.h somewhere?
• Majenko: (incidentally, I don't speak german, so had no clue what you were on about at first about multiple libraries...)
• LukasFun: No, there is just one Lib.h file. The funny thing is, that, as I wrote, this error goes away when not calling the function in the sketch. And it also goes away with your solution. I was just wondering where it originated from.
• Majenko: Probably some bug in the IDE then...