Question

Okay, I’m trying to implement a templated class of an array-based queue called Queue.
Here’s how I did it. First, is this the right way to implement a templated class?
The problem is, when I try to compile, I get the error messages

undefined reference to
‘Queue::Queue()’
undefined reference to
‘Queue::~Queue()’

Any ideas what’s wrong? Am I declaring it wrong or implementing it wrong?

queue.h

#ifndef __QUEUE_H__
#define __QUEUE_H__

template <class T>
class Queue
{
    int first;
    T* array;

  public:
    Queue();
    ~Queue;
    void enqueue(T next);
    T dequeue();
    bool is_empty();
    bool is_full();

};

#endif

queue.cpp

#include "queue.h"

template <class T> Queue<T>::Queue(){
    ...
}
template <class T> Queue<T>::~Queue(){
    ...
}
template <class T> void Queue<T>::enqueue(T next){
    ...
}
template <class T> T Queue<T>::dequeue(){
    ...
}
template <class T> bool Queue<T>::is_empty(){
    ...
}
template <class T> bool Queue<T>::is_full(){
    ...
}

main.cpp

#include "queue.h"
#include <iostream>

using namespace std;

int main(){
  Queue<int> test;
  return 0;
}

Answer 1

Several issues:

• The cause of your problem – C++ does not really support splitting templates into .h and .cpp files – you need to put everything in the header

• The name __QUEUE_H__ is reserved for the C++ implementation, as are all names that contain double-underscores or begin with an underscore and an uppercase letter. You are not allowed to create such names in your own code.

• You probably will find it more convenient to implement the queue in terms of a std::deque, rather than a C-style array

• I assume you are doing this as a learning exercise, but if not you should know that the C++ Standard Library already contains a std::queue template class.