Undefined reference to `TESTCLASS::TESTCLASS()' - c++

Question

Although I read a few google-results for that error I can't find my problem for this error, not even while trying to reduce everything to its very basic content.
That's my testclass.h:
class TESTCLASS {
public:
TESTCLASS();
};
int x; // I added this for testing if the file is included from my main code file
x=10; // It is and throws this error: testclass.h:8:1: error: 'x' does not name a type, which I don't understand neither, but it't not the main problem here
testclass.cpp:
#include "testclass.h"
TESTCLASS::TESTCLASS() {
// do some stuff
}
and here's my main code file:
#include "lib/testclass.h"
TESTCLASS test;
void setup() {
// put your setup code here, to run once:
}
void loop() {
// put your main code here, to run repeatedly:
}
This throws the error
/var/folders/b5/qc8dstcn02v_hyvgxsq4w9vr0000gq/T//ccQOziAu.ltrans0.ltrans.o: In function `_GLOBAL__sub_I_test':
/Volumes/Daten/stefanherzog/Documents/Nextcloud/Programmierung/Arduino/200515_growboxLibrary_test/200515_growboxLibrary_test.ino:3: undefined reference to `TESTCLASS::TESTCLASS()'
collect2: error: ld returned 1 exit status
exit status 1
So even this is very basic I can't see the problem! I'm using an avr-g++ compiler within my Arduino IDE (v1.8.12).
Can someone please explain me what I'm doing wrong?

Answer 1

it looks like you don't send testclass.cpp to your compiler. If so, your problem does'nt come from your code but your compiling command line.
Using gcc you should have something like :
g++ main.cpp lib/testclass.cpp -o testclass
I don't know the compilation process for arduino but i hope it will helps you finding the solution.

Answer 2

Easiest put testclass.cpp into the same folder as your .ino file. It should show up as a separate tab.
Put testclass.h there as well. and remove the lib subfolder.
And remove the int x=10; definition from the .h file. If both units are including testclass.h, that should end up in a duplicate error.
BTW: an assignment x=10; outside a function is nonsense anyway.

Answer 3

When using subdirectories with the Arduino IDE the subdirectory needs to be named as utility. That's actually it!
Having this structure for example (in ../Arduino/libraries/):
./testclass
./testclass/testclass.h
./testclass/testclass.cpp
./testclass/sub
./testclass/sub/sub.h
./testclass/sub/sub.cpp
testclass.h:
#ifndef __TESTCLASS_H__
#define __TESTCLASS_H__
#include "utility/sub.h"
class TESTCLASS {
public:
TESTCLASS();
};
#endif
testclass.cpp:
#include "testclass.h"
TESTCLASS::TESTCLASS() {
// do some stuff
}
sub.h:
class SUBCLASS {
public:
SUBCLASS();
};
sub.cpp:
#include "sub.h"
SUBCLASS::SUBCLASS() {
// do some stuff
}
You can simply include the "main" testclass.h in your project and instantiate the class and even the subclass:
#include <testclass.h>
TESTCLASS test;
SUBCLASS sub;
void setup() {
// put your setup code here, to run once:
}
void loop() {
// put your main code here, to run repeatedly:
}