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 execution-time 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:
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
Posting events and functions When you post an event or function and pass an object reference, PowerBuilder adds
an internal reference to the object to prevent it from being garbage
collected between the time of the post and the actual execution
of the event or function. This reference is removed when the event
or function is executed.
- 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
Exceptions to garbage collection
There are a few objects that are prevented from being garbage
- Visual objects Any object that is visible on your screen is not garbage 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 garbage
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 garbage 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 functions to force immediate garbage collection
or to change the interval between reference count checks. Three
functions have been added to let you control when garbage collection
occurs: GarbageCollect, GarbageCollectGetTimeLimit, and GarbageCollectSetTimeLimit.
For information about these functions, see
the PowerScript Reference
. For an example illustrating
their use, see the Code Examples sample application, described in Chapter 1, “Using
You can use tracing and profiling to examine the effect of
changing the garbage collection interval on performance.
For information about tracing and profiling,
see the PowerBuilder User’s Guide