What is it called where a child object gets killed if the parent object is killed?

The concept you're referring to is known as "cascading delete" in object-oriented programming. Here's a step-by-step explanation:

1. In object-oriented programming, objects are instances of classes. These objects can have relationships with each other, such as parent-child relationships.

2. A parent object can have one or more child objects. For example, in a database, a "Customer" object might have multiple "Order" objects associated with it.

3. A "cascading delete" refers to the situation where deleting a parent object also results in the deletion of all its child objects.

4. This is often used in databases to maintain data integrity. If a parent object no longer exists, it often doesn't make sense for its child objects to continue to exist, as they may now lack necessary context.

5. For example, if you delete a "Customer" object, you might also want to delete all the "Order" objects associated with that customer, since those orders no longer have a customer associated with them.

6. This is not always the desired behavior, however. Sometimes, you might want child objects to continue to exist even if their parent object is deleted. The appropriate behavior depends on the specific requirements of your application.