hpp/cpp

"template function" or "template class"  must be written in .h file or .hpp file

It's a more common way to write Template-Function's implement in .hpp file.

Or , if you write template-function in cpp, you will get the error " symbol not defined"

Reason:Complier won't complie template (Just leave a "placeholder"), it is complied when it has been used.

But it won't compile twice or more if meets  the same template-type.