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<H2><A NAME="sec:E.3"><SPAN class="sec-nr">E.3</SPAN> <SPAN class="sec-title">Memory 
usage of objects</SPAN></A></H2>

<P><A NAME="idx:objectmemoryusage:726"></A><A NAME="idx:memoryusage:727"></A>Currently 
an object consists of an object-header and an array of instance 
variables. The object-header includes various flags, a reference count 
and a pointer to the class. The size of an object header is 12 bytes. 
Each instance variable consumes an additional 4 bytes. For example a 
point object has `x' and `y' instance variables and thus consumes <VAR>12 
+ 2 * 4 = 20</VAR> bytes.

<P>The method <A NAME="idx:classgetinstancesize:728"></A>`<B>class<CODE>&lt;-</CODE>instance_size</B>' 
returns the size of an instance of this class in bytes. Note that the 
costs of supporting objects is not considered in this value. For example 
a box object has instance size:

<PRE class="code">
1 ?- get(class(box), instance_size, S).
S = 72
</PRE>

<P>But a box has an <B><CODE>&lt;-</CODE>area</B> instance variable 
consuming an additional 28 bytes.
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swi-prolog-doc 5.6.59-2 / usr / share / doc / swi-prolog-doc / UserGuide / sec-E.3.html
