C++ link library undefined reference

Question

EDIT Thanks to user Richard Critten, this issue boiled down to Templates, and my understanding of how they work. I had to move function implementations into the class header, otherwise they would not be compiled for some arbitrary type. See Why can templates only be implemented in the header file?

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I'm writing a template class, Tensor, which can be ininitalised to hold elements of type T, whose constructors takes some arguments.

I've found that if I try to create an object of class Tensor in any other file, it fails with undefined reference. I made sure that I'm including the header correctly, and even created a function unrelated to the class called test_boi() which can be called successfully by other files.

I won't go into the details of my testing setup, but I have a MWE with 3 files: tensor.cpp, tensor.h, and main.cpp. In tensor.cpp I have a main() which works when uncommented. It is identical to the one in main.cpp, which fails with the undefined reference error.

For building (with CMake), I make a library using tensor.h and tensor.cpp and add it to my executable. I think this might be my problem, but I'm not sure what I'm doing wrong. Making libraries is something I've just started doing, and there's a deluge of info out there.

The fact that I am able to call test_boi() from Main.exe (when I don't create objects) tells me that tensor.cpp has been compiled, and is accessible by Main.exe. The fact that I can create these objects when I put main() in tensor.cpp tells me that there isn't something broken in my classes. "Undefined reference" stackoverflow posts are rampant, so I haven't been able to find anything that fits my particular issue.

Relevant portion of my CMake script

add_library (libTensor tensor.h tensor.cpp)

add_executable(Main.exe test_main.cpp)
target_link_libraries(Main.exe libTensor)

#add_executable (Tensor.exe tensor.cpp)
#target_link_libraries (Tensor.exe Tensor)

My Class definition (tensor.h)

#ifndef TENSOR_H
#define TENSOR_H
#include <vector>

template<typename T>
class MyClass
{
  int width, height;
 public:
  MyClass(int n, int m)
  {
    height = n;
    width = m;
  };
  MyClass(int n, int m, std::vector<T> elems);
};

int test_bois();
#endif

My function implementations (tensor.cpp) ```

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

template<typename T>
Tensor<T>::Tensor(int n, int m)
{
  //  shape = (height, width);
  height = n;
  width = m;
}

template<typename T>
Tensor<T>::Tensor(int n, int m, std::vector<T> elems)
{
  //  shape = (height, width);
  height = n + 100;
  width = m + 100;
}

int test_bois()
{
  return 42;
}

// int main()
// {
//   std::vector<int> elems(10);
//   int height = 2;
//   int width = 2;
//   Tensor<int> *my_obj_zero = new Tensor<int>(height, width);
//   Tensor<int> *my_obj_one= new Tensor<int>(height, width, elems);
//   std::cout << my_obj_zero->shape() << std::endl;
//   std::cout << my_obj_one->shape() << std::endl;
//   std::cout << "Hello World!!" << std::endl;
//   return 0;
// }

main.cpp

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

int main()
{
  std::cout << test_boi() << std::endl;

  // this block commented out to get successful run
  std::vector<int> elems(10);
  int height = 2;
  int width = 2;
  Tensor<int> *my_obj_zero = new Tensor<int>(height, width);
  Tensor<int> *my_obj_one= new Tensor<int>(height, width, elems);
  std::cout << my_obj_zero->shape() << std::endl;
  std::cout << my_obj_one->shape() << std::endl;
  std::cout << "Hello World!!" << std::endl;

  return 0;
}