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<H2><A NAME="sec:E.1"><SPAN class="sec-nr">E.1</SPAN> <SPAN class="sec-title">Lifetime 
of an object</SPAN></A></H2>

<P><A NAME="idx:garbagecollection:702"></A><A NAME="idx:objectremoving:703"></A>Object 
lifetime management is a difficult issue in PCE/Prolog as PCE cannot be 
aware of all references to PCE objects stored in Prolog. Another 
complicating factor is that non-incremental garbage collection as 
performed by most Lisp systems is undesirable because they harm the 
interactive response of the system. For these reasons PCE performs <EM>incremental</EM> 
garbage collection. It distinguishes a number of prototypical 
`life-cycles' for objects. Heuristics tell the system which of these is 
applicable and when the object may be deleted.

<P>PCE distinguishes between <EM>global-</EM>, <EM>top level-</EM>, <EM>support-</EM>, <EM>argument-</EM> 
and <EM>answer-</EM> objects. <EM>Global</EM> objects are created and 
exist for the entire PCE session: <A NAME="idx:prolog:704"></A><B>@prolog</B>, 
class objects, etc. <EM>Top-level</EM> objects are the principal objects 
of the application. They should exist even if no other PCE object refers 
to them. An example of a top level object is a frame or hash_table 
representing a database in the application. <EM>Support</EM> objects 
only complete the definition of other objects. If this `other' object is 
removed, the support object may be removed as well. An example is the 
area attribute of a graphical. <EM>Argument</EM> objects are objects 
created to serve as an argument to a message. For example a graphical 
may be moved to a position described by a point object. The point may be 
deleted when the message is completed. Finally, <EM>answer</EM> objects 
are the result of some computation. For example <A NAME="idx:areagetsize:705"></A>`<B>area<CODE>&lt;-</CODE>size</B>' 
returns a size object. This object may be deleted when the code that 
requested the value is done with it.

<P>PCE maintains the following information on objects to support garbage 
collection. This information may be requested using the PCE inspector 
(see <A class="sec" href="inspector.html">section 12.5</A>).

<DL>
<LI><I>Protect Flag</I><BR>
This flag may be set using <A NAME="idx:objectsendprotect:706"></A>`<B>object<CODE>-&gt;</CODE>protect</B>'. 
When set, the object can not be freed by any means. This flag is set for 
most global and reusable objects: <A NAME="idx:prolog:707"></A><B>@prolog</B>, <A NAME="idx:pce:708"></A><B>@pce</B>, <A NAME="idx:display:709"></A><B>@display</B>, 
names, classes, etc.
<LI><I>Lock Flag</I><BR>
This flag indicates that the object may not be removed by the garbage 
collector. Locked objects can only be freed by sending an explicit
<A NAME="idx:objectsendfree:710"></A>`<B>object<CODE>-&gt;</CODE>free</B>' 
message' or using the predicate <A NAME="idx:free1:711"></A><A class="pred" href="sec-D.1.html#free/1">free/1</A>. 
It is used to avoid that `top level' objects such as frames are deleted 
by the garbage collector. It is also used to indicate that Prolog wants 
to be responsible for destruction of the object rather than PCE's 
garbage collector. The lock flag is automatically set on any object that 
has a named reference. If Prolog wants to store integer object 
references in the Prolog database locking is often necessary to protect 
the object for the PCE garbage collector. See also <A class="sec" href="pceprolog.html">section 
6</A>.
<LI><I>Answer Flag</I><BR>
This flag indicates that the object has been created as an answer of 
some computation or as a result of the Prolog predicate <A NAME="idx:new2:712"></A><A class="pred" href="sec-D.1.html#new/2">new/2</A>. 
The
<EM>answer</EM> status is cleared if the object is used to fill a slot 
of another object<SUP class="fn">19<SPAN class="fn-text">PCE assumes the 
object has become a <EM>support</EM> object. This is generally not 
correct for code objects. Class <A class="" href="summary.html#class:code">code</A> 
therefore has <A NAME="idx:classgetunanswer:713"></A>`<B>class<CODE>&lt;-</CODE>un_answer</B>: <A NAME="idx:off:714"></A><B>@off</B>', 
which implies that objects that fill a slot of a code object will not 
loose their `answer' status.</SPAN></SUP> or <A NAME="idx:objectsenddone:715"></A>`<B>object<CODE>-&gt;</CODE>done</B>' 
is invoked on the object.
<LI><I>Reference Count</I><BR>
PCE maintains the number of other objects referring to this object. When 
the reference count drops to zero and none of the protect, lock or 
answer flags are set PCE assumes the object is garbage and removes the 
object from the object base.
</DL>

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