How to write an interface and implementation of a template class in c++?

Question

C++ / C#

Comrades, good day!
Today I spent quite a lot of time, and did not understand how this can be done.
I want to figure out how to write separately (interface and implementation) template classes in c++.
For example, based on std queue and mutex, I want to make a blocking queue, but I don’t know what objects I will have to use in development.
I declare an interface:

#ifndef QUEUE_BLOCK_T_HPP
#define QUEUE_BLOCK_T_HPP

#include <queue>
#include <mutex>

template <class T>
class block_queue{

public:
  block_queue(int = 100); // default size
  ~block_queue();
  void push(const T&);
private:
  std::mutex lock;
  std::queue<T> q;
  int max_size;
};
#endif

And the implementation:

#include "queue_block_t.hpp"

template <class T>
block_queue<T>::block_queue(int max_size_) : max_size(max_size_){}

template <class T>
block_queue<T>::~block_queue(){}

template <class T>
void block_queue<T>::push(const T& elem){
  lock.lock();
  q.push(elem); 
  lock.unlock();
}

I use, for example, like this:

#include "queue_block_t.hpp"

int main(int argc, char **argv) {
  block_queue<int> q(500);
  q.push(10);
  return 0;
}

As a result I get:

main.cpp:7: undefined reference to `block_queue<int>::block_queue(int)'
main.cpp:8: undefined reference to `block_queue<int>::push(int const&)'
main.cpp:7: undefined reference to `block_queue<int>::~block_queue()'
main.cpp:7: undefined reference to `block_queue<int>::~block_queue()'

Why are these functions undeclared?
This is how it works:

#ifndef QUEUE_BLOCK_T_HPP
#define QUEUE_BLOCK_T_HPP

#include <iostream>
#include <queue>
#include <mutex>

template <class T>
class block_queue{

public:
  block_queue(int max_size__ = 100) : max_size(max_size__) { }
  ~block_queue(){ }
  void push(const T& elem){
     lock.lock();
     q.push(elem);
     lock.unlock();
  }
private:
  std::mutex lock;
  std::queue<T> q;
  int max_size;
};
#endif

Thanks to all!

Answer 1

you cannot separate the header and the implementation of the template class, this is impossible