Step #3 : Convert all Functions to class methods (except ISRs)
As shown in Listing 5 below, all old forward declarations become protected member functions.

Just move them into the private section of the header file. All so far published functions become public members. In the implementation just prefix all these functions with Timer.This step is quite simple.

So far we have a class with methods but no data. All class methods use global data.

Listing 5. Additional methods.

Step #4: Turn the ISR into a class member
Interrupt-Service-Functions differ from ordinary functions in that they are being called by the hardware which simply jumps to the address saved in the interrupt vector table.

Of course, the hardware does not supply a "this" pointer. So an Interrupt-Vector is a pointer to the ISR and a unique resource. This is why the class function has to be static ("static function in OO" means "a function that doesn't have a this pointer").

But how does the function know which object it should refer to? (Congratulations if you were just asking this yourself!) Well, it doesn't, and I'll start worrying about this when it comes to having more than one instance of the timer. Let's just get on for now.

1. Remove the forward declaration in timer.cpp
void TimerTx_ISR (void) __irq;
and declarestatic void Tx_ISR (void) __irq;
in the protected section of the class declaration in the header instead.

2. Change the function name in the .cpp file accordingly
void Timer::Tx_ISR (void) __irq

3. Then adjust the interrupt vector to point to the new function
SET_VIC_VECT_ADDR(TIMER_ILVL, Tx_ISR)

Step #5: Global variables become protected member variables
All right. Do what it says and see what happens. Cut and paste the global vars from timer.cpp to the protected section of class Timer. Try to compile: All works fine. It is just the ISR that complains:

"nonstatic reference must be relative to a specific object".

I admit, this "let's worry later" of Step 4 didn't last long here. This is because the ISR uses member-variables now (instead of global vars). Consequently it wants a this-pointer and cannot be static. So, here I want the ISR both to be non-static (so it knows the object's state) and static (so it can be an ISR) at the same time.

Listing 6. Static and non-static ISR

Really I need two functions:

1. The static real ISR. It has to somehow know the object instance and call.
2. the non-static member ISR-Handler function.

This means that I have to save the object instance pointer globally somewhere so that the ISR can refer to it. The best place to do this saving is the constructor. The additional changes required are listed in Listing 6 above and Listing 7 below.

In this step I also changed the variable prefixes g_ (for global var) to m_ (for member var).

Listing 7. Store object pointer globally and call "class ISR"