What is the difference between widget object and widget instance?

Question

I am new to wordpress and following a tutorial to create a new widget. I have noticed in some situations $instance variable is used and some situations $this is used. Like

$instance['title']

$this->get_field_id( 'title' )

I know first line returns the title value and second line returns the id of the field. But ins't $this means the instantiated object of the class? Why do I have to use $instance variable too? Can't we use $this to get title value of widget?

Answer 1

The WP_Widget class can be a bit confusing as an object instance of it (a child class must be used) doesn't represent a specific instance of a widget. It is more of an object to handle all instances of this widget class.

$instance is a variable repeatedly used within the WP_Widget class (and hence also in all child classes). It doesn't actually hold a reference to an instance of a php Object. Instead it is just an array with settings for an incarnation of a WordPress widget. This array() gets saved to the DB and retrieved again to be able to save the settings for the different widgets.

$this is a pseudo-variable which references the current instantiation of the widget class. So this is actually a reference to a php object instance, but it is not for a specific widget instance.