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<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
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<H1><A NAME="SECTION0014100000000000000000">Basics</A></H1>
<P>
A memory pool is a region of contiguous memory locations
that is subdivided into non-overlapping areas which
are then allocated to the user.
For example, Figure <A HREF="page417.html#figpool1" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page417.html#figpool1"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> shows the <em>memory map</em><A NAME=30315> </A>
of a 128KB pool of memory.
The gray bars denote the blocks of memory that are currently in use;
the blank areas are unreserved and available for use.
<P>
<P><A NAME="30320"> </A><A NAME="figpool1"> </A> <IMG WIDTH=575 HEIGHT=557 ALIGN=BOTTOM ALT="figure30316" SRC="img1739.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img1739.gif" ><BR>
<STRONG>Figure:</STRONG> Memory Map of a Storage Pool<BR>
<P>
<P>
A memory pool supports the following two operations:
<DL ><DT><STRONG>Acquire</STRONG>
<DD>
The <em>acquire</em> operation locates in the pool
a region of contiguous, unused memory locations of a specified size
and returns a pointer to that region.
The region is marked <em>reserved</em>,
which indicates that the memory locations contained therein are in use.
<P>
Should there be insufficient memory to satisfy the request,
the acquire operation fails--the typical implementation throws an exception.
<DT><STRONG>Release</STRONG>
<DD>
The <em>release</em> operation returns a region of reserved
memory locations to the pool.
After a region is returned to the pool,
it is no longer reserved and can be allocated
in a subsequent <em>acquire</em> operation.
<P>
It is an error to attempt to release a region which is not reserved.
The behavior of the pool under these circumstances
is typically undefined.
<P>
</DL>
<P>
Program <A HREF="page417.html#progpool1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page417.html#progpool1h"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> gives the definition
of the <tt>StoragePool</tt> abstract class.
This class serves as the base class from which concrete memory pool
implementations are derived.
In addition to the virtual destructor,
two pure virtual member functions,
<tt>Acquire</tt> and <tt>Release</tt>, are defined.
<P>
<P><A NAME="30482"> </A><A NAME="progpool1h"> </A> <IMG WIDTH=575 HEIGHT=143 ALIGN=BOTTOM ALT="program30333" SRC="img1740.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img1740.gif" ><BR>
<STRONG>Program:</STRONG> <tt>StoragePool</tt> Class Definition<BR>
<P>
<P>
The <tt>Acquire</tt> function takes
a single argument of type <tt>sizet</tt>
and its return value is a pointer to <tt>void</tt>.
The purpose of this function is to acquire a region of memory locations,
the size of which is specified by the argument (in bytes).
A pointer to the region is returned.
We shall assume that if for any reason the request cannot be satisfied,
an exception of type <tt>badalloc</tt> is thrown.
<P>
The <tt>Release</tt> function takes as its lone argument
a pointer to <tt>void</tt>.
This pointer is assumed to be a pointer previously returned by
the <tt>Acquire</tt> function.
I.e., it is a pointer to a region of reserved memory locations.
The purpose of this function is to return the storage to the memory pool.
<P>
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