Arduino IDE/Visual Micro - Undefined reference to constructor
You're using a template class. A template class is a special corner case that breaks the "code in .cpp and definitions in .h files" rule.

Because it's a template it can't be pre-compiled - it needs to be specially compiled each time it is used, so that it can get the right values compiled in to the template place-holders, which are provided by the CU that has the template class included with it.

Because of that it is one of the rare cases where the code has to be in the header file.

For a full example you can take a look at my Average library which is a fully templated class.