Implement a template of a Queue, using 2 Classes [duplicate] - c++

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Why can templates only be implemented in the header file?
(17 answers)
Closed last year.
I have a problem. I have implemented a Queue, while using a Class "Queue" and a Class "Element".
the problem i have now is, that I can't work out how to create the template for class Element.
If I don't use the template and just use int instead of T. Everything works fine. I already looked for many examples on the Internet. But nobody uses two classes, which is probably more efficient. I think my problem is that i don't know how to use the pointer in templates.
PS: The template in Queue.h and.cpp works i think, but if i start trying to create a template for Element it doesn't work.
My Queue.h file
#ifndef ELEMENT_H
#define ELEMENT_H
#include "Element.h"
template <class T>
class Queue{
public:
explicit Queue(int max_queue);
~Queue() = default;
void enqueue(T inhalt);
Element* dequeue();
Element* show();
bool isEmpty();
private:
Element<T>* front{};
Element<T>* back{};
int max;
int anzahl = 0;
};
#endif
My Queue.cpp file
#include "Queue.h"
#include <iostream>
#include <string>
template <class T>
Queue<T>::Queue(int max_queue){
max = max_queue;
}
template <class T>
void Queue<T>::enqueue(T inhalt){
Element* e = new Element(inhalt);
if(max > anzahl){
if(isEmpty()){
front = back = e;
}else{
back->setNext(e);
back = e;
}
} anzahl++;
}
template <class T>
Element* Queue<T>::dequeue(){
Element* e = front;
front = front->getNext();
e->setNext(nullptr);
return e;
}
template <class T>
bool Queue<T>::isEmpty()
{
return anzahl == 0;
}
template <class T>
Element* Queue<T>::show()
{
return front;
}
My Element.h file
#ifndef QUEUE_H
#define QUEUE_H
class Element{
public:
explicit Element(int);
~Element() = default;
int getInhalt()const;
void setInhalt(int);
Element*getNext()const;
void setNext(Element*);
protected:
int inhalt;
Element* next;
};
#endif
My Element.cpp file
#include <string>
#include "Element.h"
Element::Element( int inhalt_element )
{
inhalt = inhalt_element;
next = nullptr;
}
int Element::getInhalt() const {
return inhalt;
}
void Element::setInhalt(int inhalt) {
Element::inhalt = inhalt;
}
Element* Element::getNext() const {
return next;
}
void Element::setNext(Element *next) {
Element::next = next;
}
The warning I'm getting is main.cpp:(.text+0x1a): undefined reference to `Queue::Queue(int)'
And If I try to use a template for the element class. There are hundreds of lines in the warnings, so I know I'm thinking completely wrong.
I am still pretty novice if it is about programming so any help or any idea would be really appreciated.
Thank you

Answer 1

The implementation of template classes must be done in the .h
The following code compiles.
#include <iostream>
#include <string>
template <class T>
class Element{
public:
Element( T inhalt_element ){
inhalt = inhalt_element;
next = nullptr;
}
T getInhalt() const {
return inhalt;
}
void setInhalt(T inhalt) {
this->inhalt = inhalt;
}
Element<T>* getNext() const {
return next;
}
void setNext(Element<T> *next) {
this->next = next;
}
protected:
T inhalt;
Element<T>* next;
};
template <class T>
class Queue{
public:
Queue(int max_queue){
max = max_queue;
}
void enqueue(T inhalt){
Element<T>* e = new Element<T>(inhalt);
if(max > anzahl){
if(isEmpty()){
front = back = e;
}else{
back->setNext(e);
back = e;
}
} anzahl++;
}
Element<T>* dequeue(){
Element<T>* e = front;
front = front->getNext();
e->setNext(nullptr);
return e;
}
bool isEmpty(){
return anzahl == 0;
}
Element<T>* show(){
return front;
}
private:
Element<T>* front{};
Element<T>* back{};
int max;
int anzahl = 0;
};
int main() {
std::cout << "Hello World!";
Queue<int> queue(10);
return 0;
}