c++ - Defining a template in .h file, vs declaring in .h file and defining in .cpp file

I have code that looks something like this (compiling using the Arduino IDE).

main sketch file:

#include "myHeader.h"


//setup stuff here, including setting up Serial & SerialBT

loop(){
  CheckForMessages(&Serial);   //Check USB first
  CheckForMessages(&SerialBT); //Then check Bluetooth
}

myHeader.h:

#include "Arduino.h"
//other includes

template  void CheckForMessages(T *port);

//declarations of other stuff

myHeader.cpp:

#include "myHeader.h"

template  void CheckForMessages(T *port){
  if(port->available() !=0){
    //rest of function...
}

//definitions of other stuff

When I compile, I get the "undefined reference" errors for both calls to CheckForMessages in the main sketch file. This goes away if I move the definition of the template into the .h file.

I have no issues splitting the declaration and definition of functions in this way as far as I can tell, can anyone explain to me why it isn't working for my templates?