Re: instance_init() and the realize() functions

[Liviu Ionescu]

> On 18 Jul 2020, at 10:09, Pratik Parvati <pratikp@vayavyalabs.com> wrote:
> 
> The class_init() function is straight forward (it is similar to the 
> constructor in C++ OOP

The C++ constructor initialises class **instances**, i.e. C++ objects, not C++ 
classes.

In QEMU, the OOP functionality is implemented with nested structures used to 
store class and instance definitions, and callbacks as virtual methods.


The .class_init callbacks are called early, by a mechanism similar to C++ 
static constructors, and they initialise the structures used to store the class 
definitions. They are recursively chained, i.e. first the parent callback is 
called to initialise the parent structure members, then the current callback is 
called, to fill in its own members.

The .instance_init callback are automatically called when new **instances** of 
a class are created. Similarly, they are also recursively chained.

.instance_init may also create children objects, recursively.


.realize is a bit trickier. If .instance_init is the very first thing 
automatically called when creating an object, .realize is the very last thing, 
it is called manually when the whole hierarchy of objects is created and it 
signals that everything is ready, a kind of 'rien ne va plus'.

.realize usually has to manually call the parent .realize.

---

For me it was difficult to draw a line of what initialisations should be done 
in .instance_init and what in .realize, but given that .realise is called when 
the whole hierarchy is ready, it might do links between objects, which are not 
available when .instance_init is called.

A simple rule would be for .instance_init to ensure that all members have 
default values and properties associated with them, and defer all functional 
initialisations to .realize.


Hope this helps,

Liviu