Undefined reference to Class even though I compiled all cpp files - c++

Question

first of all I am kind of new to coding, so it might be that I just missed something really obvious. Still I would appreciate any help.
I created some header and some code files in one directory, namely "main.cpp","elements.cpp","queue.cpp", "elements.hpp" and "queue.hpp". In "main.cpp" I included both .hpp files.
Then I compiled: g++ main.cpp elements.cpp queue.cppto get an executable file with all three .cpp files linked.
Still I get the following error message /usr/bin/ld: /tmp/ccUjQcqj.o: in function main': main.cpp:(.text+0x33): undefined reference to Queue::Queue()'.
Queue is a class declared in "queue.hpp".
Here is the code for "queue.hpp":
#include "elements.hpp"
class Queue{
Element Q[10];
int start;
int count;
void print(bool dir,int count);
public:
Queue(); //Standard Konstruktor Deklaration
void push(Element e);
void pop();
Element& top();
void print(bool dir=true);
int size();
};
Here is the code for "queue.cpp":
#include "elements.hpp"
#include <iostream>
class Queue{
Element Q[10];
int start;
int count;
void print(bool dir,int count){
if(count>0 && dir==0){
Q[count-1].print();
print(dir,count-1);
}
if(count>0 && dir==1){
print(dir,count-1);
Q[count-1].print();
}
}
public:
Queue():start(0),count(0){}
void push(Element e){
if (count==10){
for(int i=0;i<9;i++){
Q[i]=Q[i+1];
}
Q[9]=e;
}
else{Q[start+count]=e;
count++;
}}
void pop(){
for(int i=0;i<9;i++){
Q[i]=Q[i+1];
}
count--;
}
Element& top(){
return Q[0];
}
void print(bool dir=true){
print(dir,count);
}
int size(){
return count;
}
};
I do not understand why there is an undefined reference here, since I included "queue.hpp" in main and the constructor in "queue.cpp" is also not missing.
Thank you for your help!