Can I implement the Factory Method pattern in C++ without using new?

Since the AVR compiler is based on the gcc compiler, it is very likely to support the new keyword. What exactly is the error you're getting. I'm guessing it's a link/compiler error along the lines of an undefined function, namely, operator new. There is a difference between the new operator and operator new, the first is used to create objects and the latter is used to allocate memory for objects. The new operator calls operator new for the type of object being created, then initialises the object's v-table and calls the object's constructors. Reading this FAQ it says that operator new is not defined in the standard libraries. This is easy to fix, just define one:

void *operator new (size_t size)
{
  return some allocated memory big enough to hold size bytes
}

and you'll need to define a delete as well:

void operator delete (void *memory)
{
   free the memory
}

The only thing to add is the memory management, the allocation and freeing of blocks of memory. This can be done trivially, being careful not to clobber any existing allocated memory (the code, static / global data, the stack). You should have two symbols defined - one for the start of free memory and one for the end of the free memory. You can dynamically allocate and free any chunk of memory in this region. You will need to manage this memory yourself.

The big picture of the Factory Method is object creation, which means heap memory consumption. On an embedded system, you are constrained by RAM and need to make all your design decisions with your memory limits in mind. The ATmega328 only has 2 KB RAM. I would recommend against using dynamically allocated memory in such a tight space.

Without knowing your problem in more detail, I would recommend statically declaring a handful of instances of the class and re-use those instances in some fashion. This means you need to know when and why your objects are created and--JUST AS IMPORTANT--when and why they end; then you need to figure out how many you need to have active at one time and how many it is possible to have active at one time.

!!Dean