__id__

        # BasicObject.__id__

(from ruby core)
---
    obj.__id__       -> integer
    obj.object_id    -> integer

---

Returns an integer identifier for `obj`.

The same number will be returned on all calls to `object_id` for a given
object, and no two active objects will share an id.

Note: that some objects of builtin classes are reused for optimization.
This is the case for immediate values and frozen string literals.

BasicObject implements +__id__+, Kernel implements `object_id`.

Immediate values are not passed by reference but are passed by value:
`nil`, `true`, `false`, Fixnums, Symbols, and some Floats.

    Object.new.object_id  == Object.new.object_id  # => false
    (21 * 2).object_id    == (21 * 2).object_id    # => true
    "hello".object_id     == "hello".object_id     # => false
    "hi".freeze.object_id == "hi".freeze.object_id # => true