Apple mach-o linker (id) error with C++ - c++

Question

I created a new project in Xcode 5.0 to implement a Queue class as following:
I created a .h file called QueueArray.h and it contains the following:
#ifndef __Queue__QueueArray__
#define __Queue__QueueArray__
#include <iostream>
template <class T>
class QueueArray
{
public:
QueueArray(int cap);
~QueueArray();
void Enqueue(T& val);
T Dequeue(void);
T GetFirst(void);
T GetLast(void);
bool IsEmpty(void);
bool IsFull(void);
void Clear(void);
private:
T* data;
int capacity, size, first, last;
};
#endif
and a .cpp file called QueueArray.cpp that contains the following:
#include "QueueArray.h"
using namespace std;
template <class T>
QueueArray<T>::QueueArray(int cap)
{
capacity = cap;
data = new T[capacity];
size = 0;
first = last = -1;
}
template <class T>
QueueArray<T>::~QueueArray(void)
{
delete [] data;
}
template <class T>
void QueueArray<T>::Enqueue(T& el)
{
if(IsFull() == true)
{
printf("\n Can't enqueue into a full queue!");
return;
}
if(IsEmpty() == true)
first = last = 0;
else if(last == capacity-1)//if at the last entry
last = 0; //wrap around to the first entry
else
last++;
data[last] = el;
size++;
}
template <class T>
T QueueArray<T>::Dequeue()
{
if(IsEmpty() == true)
{
printf("\n Can't dequeue from an empty queue!");
return -1;
}
T el = data[first];
if(first == last) //if only one element in queue
last = first = -1; //we'll get an empty queue
else if(first == capacity-1) //if at the last entry
first = 0; //wrap around to the first entry
else //normal case
first++;
size--;
return el;
}
template <class T>
T QueueArray<T>::GetFirst()
{
return data[first];
}
template <class T>
T QueueArray<T>::GetLast()
{
return data[last];
}
template <class T>
bool QueueArray<T>::IsEmpty(void)
{
return size == 0;
}
template <class T>
bool QueueArray<T>::IsFull(void)
{
return size == capacity;
}
and a main.cpp file that contains the following:
#include <iostream>
#include "QueueArray.h"
using namespace std;
int main(int argc, const char * argv[])
{
QueueArray<int> q(100);
for (int i=0; i<100; i++)
{
q.Enqueue(i);
}
for (int i=0; i<100; i++) {
cout<<q.Dequeue()<<endl;
}
return 0;
}
When I try to run the project, a message appears saying "Build Failed" and here is a screenshot of the errors:
How to fix that?

Answer 1

The compiler needs to see the definition of the Template before using it.
http://www.parashift.com/c++-faq-lite/templates-defn-vs-decl.html
There are more than one solution:
a. Move the definitions from .cpp file to the .h file
b. Use the export keyword if your compiler supports
http://www.parashift.com/c++-faq-lite/separate-template-fn-defn-from-decl-export-keyword.html
c. Add an inline declaration:
http://www.parashift.com/c++-faq-lite/separate-template-fn-defn-from-decl.html

Answer 2

You need to include the template definitions in-line with their declarations. An easy solution to this is to rename QueueArray.cpp to something like QueueArray_impl.h and #include it from the bottom of QueueArray.h
#ifndef __Queue__QueueArray__
#define __Queue__QueueArray__
#include <iostream>
template <class T>
class QueueArray
{
public:
QueueArray(int cap);
~QueueArray();
void Enqueue(T& val);
T Dequeue(void);
T GetFirst(void);
T GetLast(void);
bool IsEmpty(void);
bool IsFull(void);
void Clear(void);
private:
T* data;
int capacity, size, first, last;
};
#include "QueueArray_impl.h"
#endif