数据结构作业，要求用内嵌类实现二叉树的迭代器，看网上的说法内嵌类可以访问外围类的private成员，不知道为什么报错？

#pragma once
#include<iostream>
#include<stack>
#include<queue>
using namespace std;
template<class T>
class BinaryTree;
template<class T>
class InorderIterator;
template<class T>
class TreeNode {
	friend class BinaryTree<T>;
public:
	TreeNode(T item) {
		leftChild = rightChild = NULL;
		data = item;
	}
private:
	T data;
	TreeNode<T>* leftChild;
	TreeNode<T>* rightChild;
};
template<class T>
class BinaryTree {
public:
	BinaryTree();
	void deleteBinaryTree();
	bool isEmpty();
	/*creates a binary tree whose left subtree is bt1,
	whose right subtree is bt2, and whose root node contains item*/
	BinaryTree(BinaryTree<T>* bt1, T item, BinaryTree<T>* bt2);
	BinaryTree<T> leftSubtree();
	BinaryTree<T> rightSubtree();
	void preorder();
	void preorder(TreeNode<T>* currentNode);
	void inorder();
	void inorder(TreeNode<T>* currentNode);
	void postorder();
	void postorder(TreeNode<T>* currentNode);
	void levelOrder();
	void inorderIter() {
		InorderIterator<T> it;
		while (it.next()) { cout << it.next() << endl; }
	}
	void visit(TreeNode<T>* currentNode) { cout << currentNode->data << endl; }
	T rootData();
private:
	TreeNode<T>* root;
	template<class T>
	class InorderIterator {
	public:
		InorderIterator() { this->currentNode = root; } //此处出错
		T* next() {
			while (currentNode) {
				s.push(currentNode);
				currentNode = currentNode->leftChild;
			}
			if (s.empty()) return NULL;
			currentNode = s.top();
			s.pop();
			T& temp = currentNode->data;
			currentNode = currentNode->rightChild;
			return &temp;
		}
	private:
		stack<TreeNode<T>*> s;
		TreeNode<T>* currentNode;
	};
	template<class T>
	class PreorderIterator {
	public:
		PreorderIterator() { this->currentNode = root; }
		T* next() {
			if (currentNode == NULL && s.empty()) return NULL;
			s.push(currentNode);
			if (currentNode->leftChild == NULL) {
				TreeNode<T>* temp = currentNode;
				currentNode = s.top()->rightChild;
				s.pop();
				return temp;
			}
			else {
				TreeNode<T>* temp = currentNode;
				currentNode = currentNode->leftChild;
				return temp;
			}
		}
	private:
		stack<TreeNode<T>*> s;
		TreeNode<T>* currentNode;
	};
};

错误 C2327 “BinaryTree<int>::root”: 不是类型名称、静态或枚举数
错误 C2065 “root”: 未声明的标识符

谢谢大佬！

求教一个java缓存机制问题

墨骨

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