Ощибка при компиляции! Undefined symbol: ArrayStack::ArrayStack()

ArrayStack.cpp

        #include <cassert> // For assert// Header file
        #include "ArrayStack.h"
        template<class ItemType>
        ArrayStack<ItemType>::ArrayStack() : top(-1){   }
        
        template<class ItemType>
        bool ArrayStack<ItemType>::isEmpty() const
        {
           return top < 0;
        } // end isEmpty
        
        template<class ItemType>
        bool ArrayStack<ItemType>::push(const ItemType& newEntry)
        {
           bool result = false;
           if (top < DEFAULT_CAPACITY - 1) // Does stack have room for newEntry?
           {
               top++;
               items[top] = newEntry;
               result = true;
           } // end if
        
           return result;
        } // end push
        
        template<class ItemType>
        bool ArrayStack<ItemType>::pop()
        {
           bool result = false;
           if (!isEmpty())
           {
               top--;
               result = true;
           } // end if
        
           return result;
        } // end pop
        
        template<class ItemType>
        ItemType ArrayStack<ItemType>::peek() const
        {
           assert(!isEmpty()); // Enforce precondition during debugging
        
           // Stack is not empty; return top
           return items[top];
        } // end peek
        // End of implementation file.

ArrayStack.h

    #ifndef ARRAY_STACK_
    #define ARRAY_STACK_
    #include "StackInterface.h"
    
    const int MAX_STACK = 20;
    
    template<class ItemType>
    class ArrayStack : public StackInterface<ItemType>
    {
    private:
        static const int DEFAULT_CAPACITY = 50;
        ItemType items[MAX_STACK];
        int top;
    public:
        ArrayStack();
        bool isEmpty() const;
        bool push(const ItemType& newEntry);
        bool pop();
        ItemType peek() const;
    };
    // end ArrayStack
    #endif

StackInterface.h

#ifndef STACKINTERFACE_H_
#define STACKINTERFACE_H_

template <class ItemType>
class StackInterface
{
public:
    virtual bool isEmpty() const = 0;
    virtual bool push(const ItemType& newEntry) = 0;
    virtual bool pop() = 0;
    virtual ItemType peek() const = 0;
    virtual ~StackInterface() { }
}; 
# endif