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<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
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<H2><A NAME="SECTION0014110000000000000000">C++ Magic</A></H2>
<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> defines the abstract interface, <tt>StoragePool</tt>.
I.e., given an instance <tt>p</tt>
of a concrete class derived from <tt>StoragePool</tt>,
say <tt>SomePool</tt>,
we can call the member functions like this:
<PRE>SomePool p;
void* ptr = p.Acquire (100);
// ...
p.Release (ptr);</PRE>
This code sequence first acquires and subsequently releases
a 100-byte area of memory.
<P>
What is the relationship between the <tt>StoragePool</tt> class
and the operators <tt>new</tt> and <tt>delete</tt>?
In a C++ program, the <tt>new</tt> operator is typically used
in a statement of the form
<PRE>T* tptr = new T;</PRE>
where <tt>T</tt> is the name of a type.
The <tt>new</tt> operator creates a new instance of type <tt>T</tt>
and returns a pointer to that instance.
This operation involves three distinct steps:
<OL><LI> Sufficient storage to hold an instance of type <tt>T</tt>
(i.e., <IMG WIDTH=69 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline68040" SRC="img1741.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img1741.gif" > bytes) is acquired; and then,<LI> the constructor for <tt>T</tt> is called to initialize the
the object instance; and then,<LI> a pointer to the object is returned.
</OL>
The C++ compiler accomplishes the first step by calling
the function <tt>operator new</tt>, the prototype of which is:
<PRE>void* operator new (size_t);</PRE>
I.e., the function <tt>operator new</tt> takes an argument that specifies
the amount of storage to acquire (in bytes),
and returns a pointer to that storage.
<P>
Similarly, the statement <tt>delete tptr</tt> releases the object
to which <tt>tptr</tt> points and returns it to the pool of available storage.
This operation involves two distinct steps:
<OL><LI> The destructor for object in question is called to finalize
the object instance; and then,<LI> the memory locations once occupied by the object are released.
</OL>
The C++ compiler accomplishes the second step by calling
the function <tt>operator delete</tt>,
the prototype of which is:
<PRE>void operator delete (void*);</PRE>
<P>
The programmer can take over the management of dynamic storage by
overloading (or redefining) the functions <tt>operator new</tt>
and <tt>operator delete</tt>.
For example,
given an instance <tt>p</tt>
of a concrete class derived from <tt>StoragePool</tt>,
say <tt>SomePool</tt>,
we can overload the functions like this:
<PRE>SomePool p;
void* operator new (size_t bytes)
{ return p.Acquire (bytes); }
void operator delete (void* vptr)
{ p.Release (vptr); }</PRE>
In this case, the storage pool is represented by
a statically allocated global variable <tt>p</tt>.
All dynamic storage is acquired from
and released back to the storage pool <tt>p</tt>.<A NAME="tex2html800" HREF="footnode.html#30487" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/footnode.html#30487"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/foot_motif.gif"></A>
<P>
<BR> <HR>
<UL>
<LI> <A NAME="tex2html7096" HREF="page419.html#SECTION0014111000000000000000" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page419.html#SECTION0014111000000000000000">Working with Multiple Storage Pools</A>
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