Linked List Class

[Cat32]

Hey guys! I was supposed to make a linked list class that linked boxes. I submitted it and my professor said this - "When you insert into a linked list you should be using the overloaded < operator to find where the new node belongs.
When you traverse the list you should print using the classes toString method. This is why every class needs the have a toString method and the overloaded operators for comparing." I'm not really sure what I'm supposed to do. I know how to fix the toString issue, but I don't know what she means about the overloaded < operator. Here's all of my code. Any and all help you can offer is greatly appreciated!

  #include <iostream>
#include <sstream>
using namespace std;

class numberList
{
    private:
    struct listNode
    {
        double value;
        struct listNode *next;
    };
    
    listNode *head;
    
    public:
    numberList()
    {
        head = nullptr;
    }
    ~numberList(){};
    
    void appendNode(double num)
    {
        listNode *newNode;
        listNode *nodePtr;
        
        newNode= new listNode;
        newNode->value = num;
        newNode->next = nullptr;
        
        if (!head)
        head=newNode;
        else
        {
            nodePtr=head;
            
            while (nodePtr->next)
            nodePtr = nodePtr->next;
            
            nodePtr->next = newNode;
        }
    }
    
    void insertNode(double num)
    {
        listNode *newNode;
        listNode *nodePtr;
        listNode *previousNode = nullptr;
        
        newNode = new listNode;
        newNode->value = num;
        
        if (!head)
        {
            head = newNode;
            newNode->next = nullptr;
        }
        
        else
        {
            nodePtr=head;
            previousNode = nullptr;
            
            while(nodePtr != nullptr && nodePtr->value<num)
            {
                previousNode=nodePtr;
                nodePtr = nodePtr->next;
            }
            

            if(previousNode == nullptr)
            {
                head = newNode;
                newNode->next=nodePtr;
            }
            else
            {
                previousNode->next = newNode;
                newNode->next = nodePtr;
            }
        }
    }
    
    void deleteNode(double num)
    {
        listNode *nodePtr;
        listNode *previousNode;
        
        if (!head)
            return;
        
        if (head->value == num)
        {
            nodePtr = head;
            delete head;
            head = nodePtr;
        }
        
        else
        {
            nodePtr = head;
            while (nodePtr !=nullptr && nodePtr->value !=num)
            {
                previousNode=nodePtr;
                nodePtr = nodePtr->next;
            }
            
            if (nodePtr)
            {
                previousNode->next = nodePtr->next;
                delete nodePtr;
            }
        }
    }
    
    
    void displayList() const
    {
        listNode *nodePtr;
        nodePtr = head;
        
        while (nodePtr)
        {
            cout << nodePtr->value << endl;
            nodePtr=nodePtr->next;
        }
    }
};


class Box
{
    private:
    double width;
    double length;
    double height;
    
    public:
    Box()
    {
        width=1;
        length=1;
        height=1;
    }; //default constructor;
    
    Box(double hei, double wid, double len)
    {
        if (hei>0)
        height=hei;
        
        if(wid>0)
        width=wid;
        
        if(len>0)
        length=len;
        
    }; //constructor
    
    void setWidth(double wid)
    {
        if(wid>0)
        width=wid;
    }
    
    void setHeight(double hei)
    {
        if(hei>0)
        height=hei;
    }
    
    void setLength(double len)
    {
        if(len>0)
        length=len;
    }
    
    double getWidth() const
    {
        return width;
    }
    
    double getHeight() const
    {
        return height;
    }
    
    double getLenght() const
    {
        return length;
    }
    
    double volume() const
    {
        return length*width*height;
    }
    
    string toString() const
    {
        string String=static_cast<ostringstream*>(&(ostringstream() << length << " x " << width << " x " << height) ) ->str();
        return String;
    }
};

int main()
{
    double len;
    double wid;
    double hei;
    double volume1;
    
    double len2;
    double wid2;
    double hei2;
    double volume2;
    
    double len3;
    double wid3;
    double hei3;
    double volume3;
    
    Box box1;
    cout << "Please enter the length, width, and height for your first box: " << endl;
    cin >> len;
    cin >> wid;
    cin >> hei;
    box1.setWidth(wid);
    box1.setLength(len);
    box1.setHeight(hei);
    volume1=box1.volume();
    cout << "Your first box is " << box1.toString() << endl;
    cout << "The volume of your first box is " << box1.volume() << endl;
    
    Box box2;
    cout << "Please enter the length, width, and height for your second box: " << endl;
    cin >> len2;
    cin >> wid2;
    cin >> hei2;
    box2.setWidth(wid2);
    box2.setLength(len2);
    box2.setHeight(hei2);
    volume2=box2.volume();
    cout << "Your second box is " << box2.toString() << endl;
    cout << "The volume of your second box is " << box2.volume() << endl;
    
    Box box3;
    cout << "Please enter the length, width, and height for your third box: " << endl;
    cin >> len3;
    cin >> wid3;
    cin >> hei3;
    box3.setWidth(wid3);
    box3.setLength(len3);
    box3.setHeight(hei3);
    volume3=box3.volume();
    cout << "Your third box is " << box3.toString() << endl;
    cout << "The volume of your third box is " << box3.volume() << endl;
    
    numberList list;
    cout << "Adding the boxes to the list now." << endl;
    list.insertNode(volume1);
    list.insertNode(volume2);
    list.insertNode(volume3);
    cout << "Your boxes are: " << endl;
    list.displayList();
    cout << "Appending your second box." << endl;
    list.appendNode(volume2);
    cout << "The list is now: " << endl;
    list.displayList();
    cout << "Deleting your first box from the list now." << endl;
    list.deleteNode(volume1);
    cout << "The list is now: " << endl;
    list.displayList();
    
    return 0;
}

Thank you in advance!

[cire]

"When you insert into a linked list you should be using the overloaded < operator to find where the new node belongs.

Perhaps you're supposed to overload operator< for the listNode class rather than using it on the value member?