If you need the template code to be usable by other translation units (.cpp files), you need to put the implementation in the .h file or else those other units won't be able to instantiate the template (expand it according to the types they use).
If your template function is only instantiated in one .cpp file, you can define it there. This happens sometimes when a class has a private member function which is a template (and it is only called from the implementation file, not the class header file).
