I created a class in a pure C++ project, but when I try to allocate memory for it in main, I catch an unresolved external symbol. Why?

Question

C++ / C#

header file

#pragma once
template <typename T>
class Tree
{
public:
  Tree();
  ~Tree();
};

cpp file

#include "pch.h"
#include "Tree.h"


template <typename T>
Tree<T>::Tree()
{
}


template <typename T>
Tree<T>::~Tree()
{
}

main

int main()
{
  auto tvar = new Tree<int>;
    std::cout << "Hello World!\n"; 
}

If I transfer the implementation of the constructor and destructor to the header, everything works.

Answer 1

cpp file

template <typename T>
Tree<T>::Tree()
{
}

How do you think the compiler will know when compiling this cpp file that you want to work with Tree<int>in some other file?
Only if you make an explicit instantiation of the types you need in the same translation unit. Otherwise the object file resulting from copying this cpp file will be (almost) empty.
...and this is a typical solution to this problem.