Question

I understand that certain data type object have certain buffer size. E.g. a char is 1byte.
So, when creating a self-defined class object,

1. How much memory is allocated to
   the object a?
2. Is the amount of memory allocated
   different if the object is created
   on stack, or heap?
3. Is the amount of memory allocated
   fixed, or can be changed?

Creating a user-defined class instance:

Animal a; //stack memory
a.makeSound();

Animal *a = new Animal(); //heap memory
a->makeSound();

Answer 1

In both cases at least sizeof(Animal) bytes will be allocated.

In case of stack allocation some extra memory might be used for alignment. In case of heap memory some extra memory will likely be used for storing heap service data. You can influence the exact amount of memory by changing the class – for example for heap allocation you can define a custom operator new for that class and make it allocate whatever you want amount of memory.