Error in multiple files programming with C-plusplus(but no problem in ONE file) [duplicate]

here met an error in writing c++ in multiple files.

I am searching for a long time on net. But no use. Please help or try to give some ideas how to achieve this. Thanks in advance!

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I designed two class:btNode and binaryTree in btNode.h and binaryTree.h respectively. Then the member functions are implemented in their own cpp files. and then I defined an object in main.cpp and call the member functions.

below is the result and error when I run my makefile

anc@anc:~/qs/my-Leetcode/code/binaryTreeClass$ make
g++    -c -o main.o main.cpp
g++    -c -o btNode.o btNode.cpp
g++    -c -o binaryTree.o binaryTree.cpp
g++ -o test main.o btNode.o binaryTree.o
main.o: In function `main':
main.cpp:(.text+0x26): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::binaryTree()'
main.cpp:(.text+0x69): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0xca): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x12b): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x18c): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x1e7): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.o:main.cpp:(.text+0x23f): more undefined references to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::insert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)' follow
main.o: In function `main':
main.cpp:(.text+0x2c8): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::_current_os'
main.o: In function `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::preorder(std::ostream&)':
main.cpp:(.text._ZN10binaryTreeINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE8preorderERSo[_ZN10binaryTreeINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE8preorderERSo]+0x29): undefined reference to `btNode<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::preorder(btNode<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >*, std::ostream&) const'
main.o: In function `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::clear()':
main.cpp:(.text._ZN10binaryTreeINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE5clearEv[_ZN10binaryTreeINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE5clearEv]+0x2a): undefined reference to `binaryTree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::clear(btNode<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >*)'
collect2: error: ld returned 1 exit status
makefile:5: recipe for target 'test' failed
make: *** [test] Error 1

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and here is my makefile:

objects = main.o btNode.o binaryTree.o
cc = g++

test : $(objects)
    $(cc) -o test $(objects)

main.o : binaryTree.cpp binaryTree.h btNode.cpp btNode.h
binaryTree.o : binaryTree.h btNode.cpp btNode.h
btNode.o : btNode.h binaryTree.h

.PHONY:clean
clean:
    -rm test $(objects)
    @echo "rm test files..."

---

here is one of the two header file

#ifndef _BTNODE_H
#define _BTNODE_H

#include<iostream>
using namespace std;

template <typename elemType>
class binaryTree;

template<typename valType>
class  btNode{
    friend class binaryTree<valType>;

    public:
        btNode(const valType &val) : _val(val) {
            _cnt = 1;
            _lchild = _rchild = 0;
        }
        const valType& value() const {return _val;}
        int occurs() const {return _cnt;}
        
        void insert_val(const valType&);
        
        void remove_val(const valType&, btNode*&);
        void inorder(btNode* , ostream&) const;
        void preorder(btNode* , ostream&) const;
        void postorder(btNode* , ostream&) const;

        static void lchild_leaf(btNode*, btNode*);
        
    private:
        valType _val;
   
        int _cnt;
        btNode *_lchild;
        btNode *_rchild;
        void display_val(btNode*, ostream&) const;
};

#endif

---

main.cpp

#include<iostream>
#include<string>
#include"binaryTree.h"

using namespace std;

int main() {

    binaryTree<string> bt;

    bt.insert("Piglet");
    bt.insert("Eeyore");
    bt.insert("Roo");
    bt.insert("Tigger");
    bt.insert("Chris");
    bt.insert("Pooh");
    bt.insert("Kanga");

    cout << "Preorder traversal:\n";
    bt.preorder();

    return 0;
}

The question is when I write all the code in ONE file(just sum of all the code,no modify), there is no problem and obtain the desired results.

But I want to use the style of multi file programming, because it's good. pls help me find the problem, thanks very much.