模板化运算符重载中的成员函数不起作用

Question

My printTree function won't work in the overloaded ostream operator. Error and code below.

Error code : C3861 ('printTree': identifier was not found)

Description: 'printTree': function was not declared in the template definition context and can be found only via argument-dependent lookup in the instantiation context

Class Declaration

#pragma once
#include "BSTNode.h"
#include <iostream>
template <typename T> class BST;
template <typename T>
std::ostream& operator<<(std::ostream& out, const BST<T>& rhs);

template <typename ItemType>
class BST
{
private: 
    BSTNode<ItemType>* root;
    BSTNode<ItemType>* insert(ItemType* &data, BSTNode<ItemType>* root);
    BSTNode<ItemType>* remove(const ItemType &x, BSTNode<ItemType>* root);
    BSTNode<ItemType>* findMin(BSTNode<ItemType>* root) const;
    BSTNode<ItemType>* findMax(BSTNode<ItemType>* root) const;
    bool search(const ItemType &data, BSTNode<ItemType>* root) const;
    void destroyTree(BSTNode<ItemType>* root); //for destructor
    BSTNode<ItemType>* clone(BSTNode<ItemType>* root) const;
    void printTree(std::ostream& out, BSTNode<ItemType>* root) const;

public:  
    BST();
    ~BST();
    BST(const BST &rhs); 
    BSTNode<ItemType>* getRoot() const {return root;}
    bool isEmpty(); 
    void insert(ItemType* &data);
    bool remove(const ItemType &x);
    ItemType findMin() const; 
    ItemType findMax() const;
    bool search(const ItemType &data) const;
    friend std::ostream& operator<< <>(std::ostream & out ,const BST<ItemType> &rhs);  
    void printTree (std::ostream &out = std::cout) const; 
};

Relevant Methods

template <typename ItemType>
void BST<ItemType>::printTree(std::ostream& out, BSTNode<ItemType>* root) const
{
   if (root != nullptr)
   {
      printTree(out, root->getLeft());
      out << *(root->getData()) << std::endl;
      printTree(out, root->getRight());
   }
}


template <typename ItemType>
std::ostream& operator << <>(std::ostream & out , const BST<ItemType> &rhs)
{
   printTree(out, rhs.getRoot());
   return out; 
}

Solved:

Needed to add calling object to printTree function in operator overload definition

template <typename ItemType>
std::ostream& operator << <>(std::ostream & out , const BST<ItemType> &rhs)
{
   rhs.printTree(out, rhs.getRoot());
   return out; 
}

Answer 1

std::ostream& operator<<(std::ostream& out, const BST<T>& rhs);
friend std::ostream& operator<< <>(std::ostream & out ,const BST<ItemType> &rhs);

Have <> as a difference so I'm not sure if this is your error. Same with the method.