My code worked perfectly, but now when trying to make it into a template, I'm getting a lot of seemingly unrelated errors. What is going on here?

Question

I found a question similar to mine, but I was not able to figure out a solution based on their solution, so I decided to post mine here.

I wrote code for a linked list of integers, and I am now trying to convert it into a template so it can be a linked list of whatever. When I did so, I got a ton of errors that I can't figure out why they happened due to my changes (they seem related to scope, but I didn't change any scoping) or how to fix them.

The errors I am getting are:

• "Definition or redeclaration of 'LinkedList' cannot name the global scope"

• "Use of undeclared identifier 'root'"

• "Unknown type name 'Node'"

• "'LinkedList' is not a class, namespace, or enumeration"

Below is my code (I'll mark where I am getting errors):

  template <typename DataType>
  class LinkedList {
  public:


    struct Node {
      //Node(Node *aNext=nullptr) : next(aNext) {}
        DataType value;   //this is the value you want to save
        Node *next;  //this points to the next node in the list (or nullptr)
    };
    //friend class Iterator; //do Ineed this even though it's a nested class

    Node *root;

    //---------------------------------------------------------------

    //add a NESTED Iterator class...
    class Iterator {
    public:
        Iterator();//default constructor
        //Iterator() : current(nullptr) {}
        Iterator(Node* aNode);
        //Iterator(Node* aNode): current(aNode){};//constructor
        Iterator& operator=(const LinkedList::Iterator& aCopy) noexcept; //added this DELETE DUMMY?
        ~Iterator(); //dtor
        Iterator operator++();
        Iterator operator++(DataType);
        bool operator==(const Iterator &anIterator);
        bool operator!=(const Iterator &anIterator);
        DataType operator*();
        operator Node*();
        Node *current; //do I need to put LinkedList since it's a nested class?

      //add all the necessary operators

    protected:
    };

    //--------------------------------------------------------------

    LinkedList(); //default constructor...
    LinkedList(const LinkedList& aCopy); //copy ctor
    ~LinkedList(); //dtor
    LinkedList& operator=(const LinkedList& aCopy); //assignment operator

    void append(DataType value);
    void prepend(DataType value);
    void remove(DataType value);
    int size(); //needs to return unsigned int????
    Iterator begin();
    Iterator end();
    Iterator find(DataType aValue); //find



  protected:
  };

//class Iterator;
LinkedList<DataType>::LinkedList() {  //ERROR: "Definition or redeclaration of 'LinkedList' cannot name the global scope"
      root=nullptr;  //ERROR: Use of undeclared identifier 'root'
  }

LinkedList<DataType>::LinkedList(const LinkedList& aCopy){ //copy ctor //ERROR: Definition or redeclaration of 'LinkedList' cannot name the global scope
    Node *temp=aCopy.root;  //ERROR: Unknown type name 'Node'
    Node *newNode = new Node; //ERROR: Unknown type name 'Node'
    root=newNode;  //ERROR: Use of undeclared identifier 'root'

    while (temp != nullptr){
        newNode-> value=temp->value;
        temp=temp->next;
        if (temp !=nullptr){
            newNode->next=new Node;
            newNode=newNode->next;
        }
        else{ newNode->next=nullptr;}
    }
}

LinkedList& LinkedList::operator=(const LinkedList &aCopy){ //assignment operator
    while(root!=nullptr){
        Node* oneBefore= root;
        root =root->next;
        delete oneBefore;
    }
    Node *newNode= new Node;
    Node *temp=aCopy.root;
    root=newNode;

    while(temp!=nullptr){
        newNode->value=temp->value;
        temp=temp->next;
        if(temp!=nullptr){
            newNode->next=new Node;
            newNode=newNode->next;
        }
        else{newNode->next=nullptr;}
    }
    return *this;
}

LinkedList::~LinkedList(){ //dtor
    Node* oneBefore = nullptr;
    while(root!=nullptr){
        oneBefore=root;
        root=root->next;
        delete oneBefore;
    }
}
LinkedList::Iterator LinkedList::find(DataType aValue){
    Node* temp=root;
    Iterator myIterator = begin();
    for(myIterator = this->begin(); myIterator != this->end(); myIterator++){
        if(temp->value==aValue){
            return temp;
        }
        temp=temp->next;
    }
    return nullptr;
}

void LinkedList::append(DataType value){
    Node* newNode=new Node;
    newNode->value=value;
    if(root!=nullptr){
        Node* temp = root;
        while (temp->next !=nullptr){
            temp=temp->next;
        }
        newNode->next=nullptr;
        temp->next=newNode;
    }
    if(root==nullptr){
        newNode->next=nullptr;
        root=newNode;
    }

}

void LinkedList::prepend(DataType value){ //ERROR: 'LinkedList' is not a class, namespace, or enumeration
    Node* newNode=new Node;
    newNode->value=value;
    if (root!=nullptr){
        newNode->next=root;
        root=newNode;
    }
    if(root==nullptr){
        root=newNode;
        newNode->next=nullptr;
    }
}

void LinkedList::remove(DataType value){ //ERROR: 'LinkedList' is not a class, namespace, or enumeration
    if(root!=nullptr){
        Node *before=nullptr;
        Node *temp=root;
        if(temp->value==value){
            root=temp->next;
        }
        else{
            while(temp->value!=value &&temp->next != nullptr){
                before=temp;
                temp=temp->next;
            }
            if(temp->value==value){
                before->next=temp->next;
            }
        }
        delete temp;
    }
}

int LinkedList::size(){ //ERROR: 'LinkedList' is not a class, namespace, or enumeration
    Node* aNode = root;
    int numElements=0;
    while(aNode!=nullptr){
        aNode=aNode->next;
        numElements=numElements+1;
    }
    return numElements;
}

LinkedList::Iterator LinkedList::begin(){ //ERROR:'LinkedList' is not a class, namespace, or enumeration
    return LinkedList::Iterator(root);
}


LinkedList::Iterator LinkedList::end(){ //ERROR:'LinkedList' is not a class, namespace, or enumeration
    Node *aNode=root;
    while(aNode!=nullptr){
        aNode=aNode->next;
    }
    return LinkedList::Iterator(aNode);
}

LinkedList::Iterator::Iterator() : current(nullptr) {} //ERROR: 'LinkedList' is not a class, namespace, or enumeration

LinkedList::Iterator::Iterator(Node* aNode): current(aNode){ //ERROR: 'LinkedList' is not a class, namespace, or enumeration
};

LinkedList::Iterator LinkedList::Iterator::operator++(){//I have no idea what the difference is supposed to be between this one and the one below
    current=current->next;
    return *this;
}
LinkedList::Iterator LinkedList::Iterator::operator++(DataType){//I have no idea what the difference is supposed to be between this one and the one below
    current=current->next;
    return *this;
}

LinkedList::Iterator& LinkedList::Iterator::operator=(const LinkedList::Iterator& aCopy) noexcept{ //assignment operator
    current=aCopy.current;
    return *this;
}

bool LinkedList::Iterator::operator !=(const LinkedList::Iterator& aCopy){
    return current != aCopy.current;
}

bool LinkedList::Iterator::operator==(const LinkedList::Iterator& aCopy){
    return current==aCopy.current;
}

DataType LinkedList::Iterator::operator*(){
    return current->value;
}

LinkedList::Iterator::~Iterator(){}

Answer 1

You need to declare each of your implementations to be templates as well. For instance:

LinkedList<DataType>::LinkedList()

should be:

template<typename DataType>
LinkedList<DataType>::LinkedList()

Of course, every implementation of each function will need to become templates as well.