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<H2><A NAME="SECTION000410000000000000000"></A><A NAME="ss:contexts"></A>
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AST Pointer Contexts--Begin and End
</H2>

<P>
The use of astAnnulastAnnul (&#167;<A HREF="node41.html#ss:annullingpointers">4.9</A>) is not completely
foolproof, however. Consider the following:

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<IMG
 WIDTH="298" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img58.png"
 ALT="\begin{terminalv}
astShow( astZoomMap( 2, 5.0, '''' ) );
\end{terminalv}">
<BR>
</SMALL>
<P>
This creates a ZoomMapZoomMap and displays it on standard output
(&#167;<A HREF="node36.html#ss:displayingobjects">4.4</A>). Using function invocations as
arguments to other functions in this way is very convenient because it
avoids the need for intermediate pointer variables. However, the
pointer generated by astZoomMapastZoomMap is still active, and since we have not
stored its value, we cannot use astAnnul to annul it. The ZoomMap will
therefore stay around until the end of the program.

<P>
A simple way to avoid this problem is to enclose all use of AST
functions between invocations of astBeginastBegin and astEndastEnd, for example:

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 ALT="\begin{terminalv}
astBegin;
astShow( astZoomMap( 2, 5.0, '''' ) );
astEnd;
\end{terminalv}">
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</SMALL>
<P>
When the expansion of astEnd (which is a macro) executes, every ObjectObject
pointer created since the previous use of astBegin (also a macro) is
automatically annulled and any Objects left without pointers are
deleted. This provides a simple solution to managing Objects and their
pointers, and allows you to create Objects very freely without needing
to keep detailed track of each one.  Because this is so convenient, we
implicitly assume that astBegin and astEnd are used in most of the
examples given in this document.  Pointer management is not generally
shown explicitly unless it is particularly relevant to the point being
illustrated.

<P>
If necessary, astBegin and astEnd may be nested, like blocks delimited
by ``{...}'' in C, to define a series of AST pointer
contexts. Each use of astEnd will then annul only those Object
pointers created since the matching use of astBegin.

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