Garbage collection – PB Docs 125

What garbage collection does

The PowerBuilder garbage collection mechanism checks memory automatically
for unreferenced and orphaned objects and removes any it finds, thus
taking care of most memory leaks. You can use garbage collection
to destroy objects instead of explicitly destroying them using the DESTROY statement.
This lets you avoid runtime errors that occur when you destroy an object
that was being used by another process or had been passed by reference to
a posted event or function.

When garbage collection occurs

Garbage collection occurs:

• When
  a reference is removed from an object

  A reference to an object is any variable whose value is the
  object. When the variable goes out of scope, or when it is assigned
  a different value, PowerBuilder removes a reference to the object,
  counts the remaining references, and destroys the object if no references
  remain.

• When the garbage collection interval
  is exceeded

  When PowerBuilder completes the execution of a system-triggered
  event, it makes a garbage collection pass if the set interval between
  garbage collection passes has been exceeded. The default interval
  is 0.5 seconds. The garbage collection pass removes any objects
  and classes that cannot be referenced, including those containing
  circular references (otherwise unreferenced objects that reference
  each other).

Exceptions to garbage collection

There are a few objects that are prevented from being collected:

• Visual objects

  Any object that is visible on your screen is not collected because
  when the object is created and displayed on your screen, an internal
  reference is added to the object. When any visual object is closed it
  is explicitly destroyed.

• Timing objects

  Any Timing object that is currently running is not collected
  because the Start function for a Timing object
  adds an internal reference. The Stop function
  removes the reference.

• Shared objects

  Registered shared objects are not collected because the SharedObjectRegister function
  adds an internal reference. SharedObjectUnregister removes
  the internal reference.

Controlling when garbage collection occurs

Garbage collection occurs automatically in PowerBuilder, but
you can use the functions GarbageCollect, GarbageCollectGetTimeLimit,
and GarbageCollectSetTimeLimit to force immediate
garbage collection or to change the interval between reference count
checks. By setting the interval between garbage collection passes
to a very large number, you can effectively turn off garbage collection.