I am writing a C++ code split in multiple files, and I want to include a function my_function in a .h file.

Here is the file structure:

• main.cpp:

  #include <iostream>
  #include "library.h"
  
  int main() {
  
      int i=0;
  
      my_function(i);
  
  
      std::cout << "Hello, World!n";
      return 0;
  }
  

• in library.h I include the predeclaration of my_function:

  #ifndef library_h
  #define library_h
  
  #include <stdio.h>
  
  void my_function(int);
  
  #endif 
  

• and my_function is coded into library.cpp:

  #include "library.h"
  
  
  void my_function(int i){
  
      int j;
  
      j=i+1;
  
  }
  

I compile with g++ library.cpp main.cpp -o main.o and the compilation returns no errors. So far, so good.

The issue:

However, if I want my_function to take a template argument and I modify the files as follows:

• main.cpp:

  #include <iostream>
  #include "library.h"
  
  int main() {
  
      int i=0;
  
      my_function<int>(i);
  
      std::cout << "Hello, World!n";
      return 0;
  }
  
  

• library.h:

  #ifndef library_h
  #define library_h
  
  #include <stdio.h>
  
  template<class T> void my_function(T);
  
  #endif 
  

• library.cpp:

  #include "library.h"
  
  template<class T> void my_function(T i){
  
      T j;
  
      j=i+1;
  
  }
  

I get a compilation error:

 % g++ library.cpp main.cpp -o main.o
ld: Undefined symbols:
  void my_function<int>(int), referenced from:
      _main in main-2d776a.o
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Why ? How may I fix this? I do see that a similar issue has been addressed here, but I am not sure that it answers the question in my specific case.

Thank you