How to take care of error: use of deleted function c++?

Question

This is my code :

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>
#include "dummy.h"
using namespace std;

#ifndef SORT_H
#define SORT_H

template <class T>
class LinkedList {
    struct Node {
        T data;
        Node * next;
    };

    Node * head;

public:
    LinkedList() {
        head = NULL;
    }
    LinkedList(T value) {
        head -> data = value;
        head -> next = NULL;
    }

    ~LinkedList() {
        while(head != NULL) {
            Node * n = head->next;
            delete head;
            head = n;
        }
    }

    void add(T value) {
        Node * n = new Node;  // the error starts here probably
        n->data = value;
        n->next = head;
        head = n;
    }

    void print() {
        Node *curr = head;
        while (curr) {
            cout << curr->data << endl;
            curr = curr->next;
        }
    }
};

#endif

and when I try to compile it I get the error: use of deleted function 'LinkedList::Node::Node()'

I understand that it is probably because I can't know before-hand if I will get any arguments while creating the sorted list or not, how can I do that? To make it compile and run?

The Dummy class is like that:

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>

using namespace std;

#ifndef DUMB_H
#define DUMB_H

class Dummy {
    int num_of_teeth;
public:
    Dummy(int num) {
        num_of_teeth = num;
    }
    ~Dummy() {};
    void add(int num) {
        num_of_teeth += num;
    }
    void remove() {
        num_of_teeth --;
    }
    void print() {
        int num = num_of_teeth;
        while (num > 0) {
            cout << "D";
            if ((num-1) == (num_of_teeth/2)) {
                cout << "\n";
            }
            num --;
            if (num == 0) {
                cout << "\n";
            }
        }
    }
    friend ostream& operator << (ostream& output, Dummy& dumb)
    {
        output << dumb.num_of_teeth;
        return output;
    }
};

#endif

My main is:

#include <iostream>
#include "sortedList.h"
#include "dummy.h"

int main() {
    std::cout << "Hello, World!" << std::endl;

    Dummy teeth(24);
    teeth.add(7);
    Dummy slime(11);
    slime.add(1);
    Dummy josh(32);
    LinkedList<Dummy> teeth_list;
    teeth_list.add(teeth);  // The first time I try to put teeth into the list
    teeth_list.add(slime);
    teeth_list.add(josh);
    teeth_list.print();

    LinkedList <string> myList;
    myList.add("yossi");
    myList.add("Rikki");
    myList.add("Pavel <3");
    myList.print();

    return 0;
}

The build log:

====================[ Build | exe_name | Debug ]================================
"C:\Program Files\JetBrains\CLion 2021.1.1\bin\cmake\win\bin\cmake.exe" --build C:\Users\User\CLionProjects\ex2.2\cmake-build-debug --target exe_name -- -j 3
Scanning dependencies of target exe_name
[ 33%] Building CXX object CMakeFiles/exe_name.dir/main.cpp.obj
[ 66%] Building CXX object CMakeFiles/exe_name.dir/sortedList.cpp.obj
In file included from C:\Users\User\CLionProjects\ex2.2\main.cpp:2:
C:\Users\User\CLionProjects\ex2.2\sortedList.h: In instantiation of 'void LinkedList<T>::add(T) [with T = Dummy]':
C:\Users\User\CLionProjects\ex2.2\main.cpp:14:25:   required from here
C:\Users\User\CLionProjects\ex2.2\sortedList.h:38:20: error: use of deleted function 'LinkedList<Dummy>::Node::Node()'
         Node * n = new Node;
                    ^~~~~~~~
C:\Users\User\CLionProjects\ex2.2\sortedList.h:13:12: note: 'LinkedList<Dummy>::Node::Node()' is implicitly deleted because the default definition would be ill-formed:
     struct Node {
            ^~~~
C:\Users\User\CLionProjects\ex2.2\sortedList.h:13:12: error: no matching function for call to 'Dummy::Dummy()'
In file included from C:\Users\User\CLionProjects\ex2.2\sortedList.h:5,
                 from C:\Users\User\CLionProjects\ex2.2\main.cpp:2:
C:\Users\User\CLionProjects\ex2.2\dummy.h:14:5: note: candidate: 'Dummy::Dummy(int)'
     Dummy(int num) {
     ^~~~~
C:\Users\User\CLionProjects\ex2.2\dummy.h:14:5: note:   candidate expects 1 argument, 0 provided
C:\Users\User\CLionProjects\ex2.2\dummy.h:11:7: note: candidate: 'constexpr Dummy::Dummy(const Dummy&)'
 class Dummy {
       ^~~~~
C:\Users\User\CLionProjects\ex2.2\dummy.h:11:7: note:   candidate expects 1 argument, 0 provided
mingw32-make.exe[3]: *** [CMakeFiles\exe_name.dir\build.make:81: CMakeFiles/exe_name.dir/main.cpp.obj] Error 1
mingw32-make.exe[3]: *** Waiting for unfinished jobs....
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:94: CMakeFiles/exe_name.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:101: CMakeFiles/exe_name.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:136: exe_name] Error 2

Answer 1

Ok, so this is a fun one. Have a look here:

template <class T>
class LinkedList {
    struct Node {
        T data; // Here you create a T

When you create a T in node, how do you do it? Well you have to call its constructor. Now lets look at your Dummy. It doesn't have a default constructor, you must call it with Dummy(int). So therefore in Node your T which is now Dummy must be constructed with this constructor. But how!?

That is the conundrum presented by you to your compiler. The compiler has answered, in a strange but sensible way. I can't construct this Node because the default constructor is deleted. It is deleted because its T (Dummy) cannot be default constructed.

A potential fix for this is to add a new constructor to Node, one that takes and copies a T, aka:

struct Node {
    Node(const T &in) : data(in) {}
    T data;

You should experiment with this a little.

---

A little side note. You have an issue. Think about how you add subsequent nodes,, vs when you construct the first one. Your constructor LinkedList(T value) { has undefined behaviour because it dereferences a null pointer.