C++ I keep getting an error about my linked list not being a templete

Question

I am learning about creating a linked list in my class and I have run into a problem. My linked list is not a template and I can't figure out how to fix it. I looked at some of my notes for the class and the code looks to be the same as the slides but I was wondering if someone would be able to explain what I am doing wrong. Thank you so much.

LinkedList.h

#ifdef LINKED_LIST_
#define LINKED_LIST_

#include "ListInterface.h"
#include "Node.h"

template<class ItemType>
class LinkedList : public ListInterface<ItemType>
{
    private:
        Node<ItemType>* headPtr;
        int itemCount;

        Node<ItemType>* getNodeAt (int postion) const;
    
    public:
        LinkedList();
        LinkedList(const LinkedList<ItemType>& aList);
        virtual ~LinkedList();
        bool isEmpty() const;
        int getLength() const;
        bool remove(int postion);
        void clear();
        ItemType getEntry(int position) const;
        ItemType replace(int position, const ItemType& newEntry);

};

#include "LinkedList.cpp"
#endif

LinkedList.cpp

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

using namespace std;

template<class ItemType>
LinkedList<ItemType>::LinkedList() : headPtr(nullptr), itemCount(0){
    
}

template<class ItemType>
LinkedList<ItemType>::~LinkedList(){

}

The error only says: LinkedList is not a template.

[Why can templates only be implemented in the header file?](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) You can do what you're attempting where you put the implementation in another file and include it, but it should not be named `.cpp` or compiled directly. — Retired Ninja, Sep 28 '22 at 20:20
`#ifdef LINKED_LIST_` + `#define LINKED_LIST_` is not how you define a headerguard (and also doesn't make logical sense - why would you redefine the already defined preprocessor definition?) — UnholySheep, Sep 28 '22 at 20:24
Delete LinkedList.cpp and place all the code in LinkedList.h — john, Sep 28 '22 at 20:24

metal · Answer 1 · 2022-09-28T21:45:34.077

Your header guards do no match. You want #ifndef or even #if !defined(LINKED_LIST_).
It is inadvisable to #include the cpp file in the header file. If you must do so because your professor (unwisely) requires it, I suggest you rename it LinkedList.inl (short for "inline") or similar to distinguish it from a regular cpp file.
The structure of your cpp file -- it has #include <iostream> and a using directive -- suggests you're trying to ALSO treat it as a compiled source file even though you directly included it above in the header. This will result in something like this after the preprocessor finishes copy-pasting the files around, which you can see live on Godbolt (modified since I don't have your other classes):

#include "ListInterface.h"
#include "Node.h"

template<class ItemType>
class LinkedList : public ListInterface<ItemType>
{
    private:
        Node<ItemType>* headPtr;
        int itemCount;

        Node<ItemType>* getNodeAt (int postion) const;
    
    public:
        LinkedList();
        LinkedList(const LinkedList<ItemType>& aList);
        virtual ~LinkedList();
        bool isEmpty() const;
        int getLength() const;
        bool remove(int postion);
        void clear();
        ItemType getEntry(int position) const;
        ItemType replace(int position, const ItemType& newEntry);

};

#include <iostream>

using namespace std;

template<class ItemType>
LinkedList<ItemType>::LinkedList() : headPtr(nullptr), itemCount(0){
    
}

template<class ItemType>
LinkedList<ItemType>::~LinkedList(){

}
//

#include <iostream>

using namespace std;

template<class ItemType>
LinkedList<ItemType>::LinkedList() : headPtr(nullptr), itemCount(0){
    
}

template<class ItemType>
LinkedList<ItemType>::~LinkedList(){

}

You can see I get the expected redefinition errors like:

<source>:41:23: error: redefinition of 'LinkedList<ItemType>'

You might try adding a third source file that includes the header (and can see the definitions, preferably because they're defined inline in the header), and compile only that one source.
Failing that, show us a minimal sample that reproduces the problem.

C++ I keep getting an error about my linked list not being a templete

1 Answers1