为什么我在声明本身中收到“未在范围内声明”错误？

Question

I'm trying to overload the << operator for cout and my custom linked list class. However, I'm getting the error "ptr was not declared in this scope" on the line of the actual declaration itself in the very last method in LinkedList.hpp ( LinkedList<T>::Node* ptr = list.getHead(); ). Am I missing something?

Here's the code:

// LinkedList.hpp

#ifndef LINKED_LIST_HPP
#define LINKED_LIST_HPP

#include <stdexcept>
#include <iostream>

template <typename T>
class LinkedList {
    public:
        class Node {
            private:
                T _data;
                Node* _next;
            public:
                Node(T data);
                T getData();
                Node* getNext();
        };
        LinkedList();
        ~LinkedList();
        int size();
        LinkedList<T>::Node* getHead();
    private:
        LinkedList<T>::Node* _head;
        int _size;
};

template <typename T>
std::ostream& operator<<(std::ostream& strm, LinkedList<T>& list);

#include "LinkedList.cpp"
#endif



// LinkedList.cpp

template <typename T>
LinkedList<T>::Node::Node(T data) {
    _data = data;
    _next = nullptr;
}

template <typename T>
T LinkedList<T>::Node::getData() {
    return _data;
}

template <typename T>
typename LinkedList<T>::Node* LinkedList<T>::Node::getNext() {
    return _next;
}

template <typename T>
LinkedList<T>::LinkedList() {
    _head = nullptr;
    _tail = nullptr;
    _size = 0;
}

template <typename T>
LinkedList<T>::~LinkedList() {
    Node* ptr = _head;
    while (ptr != nullptr) {
        _head = _head->getNext();
        delete ptr;
        ptr = _head;
    }
}

template <typename T>
int LinkedList<T>::size() {
    return _size;
}

template <typename T>
typename LinkedList<T>::Node* LinkedList<T>::getHead() {
    return _head;
}

template <typename T>
std::ostream& operator<<(std::ostream& o, LinkedList<T>& list) {
    if (list.size() == 0) {
        o << "NULL";
    }
    else {
        LinkedList<T>::Node* ptr = list.getHead();
        while (ptr->getNext() != nullptr) {
            o << ptr->getData() << " -> ";
        }
        o << ptr->getData();
    }
    return o;
}

Answer 1

This seems like an issue that Node is a dependent type, and so you need to do this:

typename LinkedList<T>::Node* ptr = list.getHead();

See this answer for more details on when this is necessary and why: Where and why do I have to put the "template" and "typename" keywords?