“undefined reference to”c++ template

You need to use the 
export
 
keyword. However, I don't think G++ has proper support, so you need to include the template function's definition in the header so the translation unit can use it. This is because the 
<int>
 
'version' of the template hasn't been created, only the 
<typename T>
 
'version.'

An easy way is to 
#include
 
the .cpp file. However, this can cause problems, e.g. when other functions are in the .cpp file. It will also likely increase the compile time.

A clean way is to move your template functions into its own .cpp file, and include that in the header 
or
 
use the 
export
 
keyword and compile it separately.

c++中template不支援聲明和實現分別放在不同的檔案中。可以通過include cpp檔案解決這個問題。