Error C2679 binary '<<': no operator found which takes a right-hand operand of type 'T' (or there is no acceptable conversion)

问题

I'm having problems with my friend function within my template class. For some reason it doesn't like the fact that I'm trying to use a variable that is type T in an operator overloading friend function.

#include <iostream>
#include <fstream>
#include <string>

template <typename T>
class LL
{
    struct Node
    {
        T mData;
        Node *mNext;

        Node();
        Node(T data);
    };

private:
    Node *mHead, *mTail;
    int mCount;

public:
    LL();
    ~LL();
    bool insert(T data);
    bool isExist(T data);
    bool remove(T data);
    void showLinkedList();
    void clear();
    int getCount() const;
    bool isEmpty();

    friend std::ofstream& operator<<(std::ofstream& output, const LL& obj)
    {
        Node* tmp;

        if (obj.mHead != NULL)
        {
            tmp = obj.mHead;

            while (tmp != NULL)
            {
                output << tmp->mData << std::endl; // "tmp->mData is causing the error
                tmp = tmp->mNext;
            }
        }

        return output;
    }

};

This is a linked list class, and I need the friend function operator overload to basically allow me to output any particular list of objects onto a text file. I hope someone can help me out.

来源：https://stackoverflow.com/questions/55652315/error-c2679-binary-no-operator-found-which-takes-a-right-hand-operand-of-t