c++ - 模板类中的c++运算符重载Im正在尝试构建重载<<以打印树

Question

#pragma once

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


template<class T> 
class BinarySearchTree {
public:
	BinarySearchTree() {
		root = NULL;
	};

	class Node{
	public:
		Node* left;
		Node* right;
		Node* parent;
		T data;
	};
	Node* root;
	int Size(Node*);
	void insert(T);
	bool Exsist(T);
	void PrintTree(Node*);
	T Size();
	bool isEmpty() const {
		return root==NULL;
	}
	template < class T>
	friend ostream& operator<< (ostream& os, const Node& a);
	
};


template <class T>
void BinarySearchTree<T>::insert(T d)
{
	Node* t = new Node;
	Node* parent;
	t->data = d;
	t->right = NULL;
	t->left = NULL;
	parent = NULL;
	if(isEmpty())
		root = t;
	else
	{
		Node* curr;
		curr = root;
		while(curr)
		{
			parent = curr;
			if(t->data > curr->data)
				curr = curr->right;
			else
				curr = curr->left;
		}

		if(t->data < parent->data)
			parent->left = t;
		else
			parent->right = t;
	}
}


template <class T>
bool BinarySearchTree<T>::Exsist(T d)
{
	bool found = NULL;
	Node* curr = root;
	while(curr != NULL && !found)
	{
		if( d == curr->data )
		{
		cout << "The number is in the tree" << endl;
		found = true;
		}
		else if( d > curr->data)
			curr = curr->right;
		else if(d < curr->data)
			curr = curr->left;
	}
	if( !found )
		cout << "The number not found" << endl;
	return found;
}


template <class T>
int BinarySearchTree<T>::Size(Node* a)
{
		if (a == NULL) return 0;
	else
		return(1 + Size(a->left) + Size(a->right));

}

template < class T>
ostream& operator<< (ostream& os, const Node& a) //line 102
{
    PrintTree(a);
	os << endl;
	return os;
}

template <class T>
void BinarySearchTree<T>::PrintTree(Node* tree)
{
	if (tree!=NULL) {
		cout << tree->data << ", " ;
		PrintTree(tree->right);
		PrintTree(tree->left);
		
	}
}

-编辑-
我更改了它，现在仅在实现上有错误

错误1错误C4430:缺少类型说明符-假定为int。注意:C++不支持default-int 102
错误2错误C2143:语法错误:在'＆'102之前缺少'，'

Answer 1

声明上存在语法错误，实现由重载运算符代替

template < class T>
ostream& operator<< (ostream& os, const Node<T>& a); 

在声明和执行中。