OOP for abap

good morning to all sap experts,

i have a question for any OOP programmers in abap. i have been working in abap for over 12 years, but have not yet transitioned to oop. but, since we have been upgrading to ecc 6.0, i am constantly working with oop classes, and objects. maybe someone can answer these questions, and also point me in the right direction online, to educate myself.

i have a class that is called CL_WTY_CLAIM and it is running a method called get_direct_references. within get_direct_references, this line of code is running.

co_ltext ?= co_ext_objects->get_ext_buffered_object(

co_ltext_class_id ).

firstly, what does "?=" mean?

secondly, if i go to get_ext_buffered_object, i find the following code.

ro_object = co_ext_objects->get_ext_buffered_object( io_id ).

how/where is the code running to fill the initial co_ltext field?

please and thank you.