"undefined reference to ... "?

[dn90]

This is a portion of my Ring implementation file (Ring.cpp):

Ring.cpp

#include "ring.h"
#include <stdexcept> 

template<class T>
Ring<T>::Ring(): n(DEFAULT_RING_SIZE), elementCount(0),referenceIndex(0) {
  
  // allocate memory for the dynamic arrays:
  elements = new T[n];
  isElementStored = new bool[n];
  
  // make sure ring is cleared
  clearRing();
}

template<class T>
Ring<T>::Ring(int ringSize): n(ringSize), elementCount(0),referenceIndex(0) {
  
  // sanity checking ring size
  if( ringSize < 1 ) {
    n = DEFAULT_RING_SIZE;
  }
  
  // allocate memory
  elements = new T[n];
  isElementStored = new bool[n];
  
  // make sure ring is cleared
  clearRing();
}
...

This is a portion of my Ring header (ring.h) file:

Ring.h

#ifndef _ring_h_
#define _ring_h_

template<class T>
class Ring {
private:
  static const int DEFAULT_RING_SIZE = 10;
  int n;                   // this is the ring's size as noted above
  int elementCount;        // number of elements actually stored
  T* elements;             // dynamic array that holds the actual elements
  bool* isElementStored;   // dynamic array that keeps tab whether element is stored
  int referenceIndex;      // index to the dynamic arrays

public:

  /**
   * Default constructor. 
   * @post a ring of size DEFAULT_RING_SIZE (10) created. 
   *  there are no elements and reference is at the beginning
   */
  Ring();

  /**
   * Parameterized constructor. 
   * @param ringSize size of the new ring
   * @post a ring of size ringSize is created.
   *  if ringSize is an invalid size, it is set to DEFAULT_RING_SIZE,
   *  there are no elements, and reference is at the beginning
   */
  Ring(int ringSize);

  /**
   * Stores a value at the current reference
   * @param value a value of type T
   * @post 
   *  if the value is non-null, it is stored at the current reference
   *  if the value is null, the element at reference is effectively removed
   *  if there was no previous value at the current reference, the 
   *   number of elements is increased by 1
   */
  void set(const T& value);

  /**
   * Retrieves the value stored at current reference
   * @post no internal state is modified 
   * @return value stored at current reference
   *   if no values were stored at current reference, returns NULL
   */
  T get() const;

...

... and a portion of my main.cpp:

main.cpp

#include "ring.h"
#include <iostream>
#include <stdexcept> 
#include <cassert>
#include <string>

using namespace std;

int main() {

  // create a ring of strings of size 7 to hold days of the week 
  Ring<string> days(7);

  assert(days.getRingSize()==7);
  assert(days.getElementCount()==0);

  // populate the ring
  days.set("Monday"); 

  days.moveNext();
  days.set("Tuesday"); 
  
  days.moveNext();
  days.set("Wednesday"); 
  
  days.moveNext();
  days.set("Thursday"); 
  
  days.moveNext();
  days.set("Friday"); 
  
  days.moveNext();
  days.set("Saturday"); 
  
  days.moveNext();
  days.set("Sunday");

  assert(days.getElementCount()==7);

  days.remove();

  assert(days.getElementCount()==6);
  
  try {
    cout << days.get() << endl;
  } catch(std::runtime_error e) {
    cerr << "An exception was intentionally generated, and handled" << endl;
  }

...

Sorry, I omitted most of the code for each file but anytime I run g++ Ring.cpp main.cpp -o ringexec.exe in my console, I'm getting errors similar to this: "undefined reference to 'Ring<std::string>::Ring(int)".

What I don't understand is why I'm getting this error? Is main.cpp not recognizing the header file? I know that C++ is case sensitive but everything matches, unless I'm missing something.

[jlb]

Normally when dealing with templates you need to have both the definition and the implementation in the same file, usually the header.