How to fix error: "LNK 2019 reference to unresolved symbol" c++?

Question

OOP

Good day, I ran into this problem:

I use only three files so far:
1.RandomAccess.cpp - the main file with the main function
2.unit.h - the header file of the template class unit
3.unit.cpp - the executable file of the template class unit

RandomAccess structure .cpp

#include <iostream>
#include "unit.h"

using namespace std;

void main() {
  setlocale(0, "");

  unit <int> nBlock;


}

unit.h structure:

#pragma once

#include <iostream>
using namespace std;

template <typename type>
class unit{

  template <typename type>
  friend istream& operator >>(istream&, unit<type>&);
  template <typename type>
  friend ostream& operator <<(ostream&, unit<type>&);

public:

  unit();
  unit(int);
  int size();

  type& operator[] (int);

private:
  int s;
  type* ptr;

  void to_null();
};

Executable file unit.cpp:

#include "unit.h"
#include <iostream>
#include <iomanip>

using namespace std;

#include <cstdlib>

// constructors
template <typename type>
unit<type>::unit() {
  s = 16;
  ptr = new type[16];
  to_null();
}

template <typename type>
unit<type>::unit(int cap) {
  s = (cap > 0) ? cap : 16;
  ptr = new type[s];
  to_null();
}

// operators
template <typename type>
type& unit<type>::operator[](int ix) {
  if (ix < 0 || ix > s) {
    cerr << "\n Ошибка индексации: индекс(" << ix << ") не пренадлежит этому юниту..";
    exit(1);
  }
  else return ptr[ix];
}

template<typename type>
istream& operator>>(istream& input, unit<type>& obj) {
  for (int i = 0; i < obj.size(); i++) {
    input >> obj[i];
  }
  return input;
}

template <typename type>
ostream& operator <<(ostream& output, unit<type>& obj) {
  output << "[";
  int edge = obj.size();
  for (int ix = 0; ix < edge; ix++) {
    output << obj[ix];
    if (ix + 1 != edge) output << setw(5);
  }
  output << "]";
  return output;
}

// public functions
template <typename type>
int unit<type>::size() {
  return s;
}

// private functions
template <typename type>
void unit<type>::to_null() {
  for (int ix = 0; ix < s; ix++) {
    ptr[ix] = 0;
  }
}

Answer 1

The implementation of the template class must be in the *.h file.