There are two different flavours of initialisations, selectable in config.h :

i)  #undef INITIALIZATION_BY___INIT
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Initialisation is done at compile time. This is fast and costs no extra code
in the program.
Allowed expressions currently include integer literals, string literals,
integer operators, string addition, bracketing, array constructors, the empty
mapping and order_alist() .
When an object with initialised variables is cloned or inherited, all
initialised variables are copied from the blueprint.
A special application of this feature is to have an initialised non-empty
array or a mapping; it will be shared by all clones or inheriting objects
unless an assignment to the variable - as opposed to an assignment to an element
of the array/mapping - is done in all clones etc.
To prevent unauthorised chenges in initialised arrays/mappings, you can declare
the variables as private or use a nomask reset/create that checks for undesired
inheritance.

ii) #define INITIALIZATION_BY___INIT
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Creates a function names __INIT() from all variable initialisations and from
calls to __INIT() in all inherited objects, and runs this function at object
creation time.
Any efun can be used in the expressions for variable initialisations, even ones
with severe side effects, like destruct() or shutdown() .
The code created for __INIT() is a little worse than a medium-skilled
lpc-programmer would generate, because it is scattered all over the program.