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<title>The @code{unwind_protect} Statement - Untitled</title>
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<h3 class="section">10.8 The <code>unwind_protect</code> Statement</h3>
<p><a name="index-g_t_0040code_007bunwind_005fprotect_007d-statement-575"></a><a name="index-g_t_0040code_007bunwind_005fprotect_005fcleanup_007d-576"></a><a name="index-g_t_0040code_007bend_005funwind_005fprotect_007d-577"></a>
Octave supports a limited form of exception handling modelled after the
unwind-protect form of Lisp.
<p>The general form of an <code>unwind_protect</code> block looks like this:
<pre class="example"> unwind_protect
<var>body</var>
unwind_protect_cleanup
<var>cleanup</var>
end_unwind_protect
</pre>
<p class="noindent">where <var>body</var> and <var>cleanup</var> are both optional and may contain any
Octave expressions or commands. The statements in <var>cleanup</var> are
guaranteed to be executed regardless of how control exits <var>body</var>.
<p>This is useful to protect temporary changes to global variables from
possible errors. For example, the following code will always restore
the original value of the global variable <code>frobnosticate</code>
even if an error occurs in the first part of the <code>unwind_protect</code>
block.
<pre class="example"> save_frobnosticate = frobnosticate;
unwind_protect
frobnosticate = true;
...
unwind_protect_cleanup
frobnosticate = save_frobnosticate;
end_unwind_protect
</pre>
<p class="noindent">Without <code>unwind_protect</code>, the value of <var>frobnosticate</var>
would not be restored if an error occurs while evaluating the first part
of the <code>unwind_protect</code> block because evaluation would stop at the
point of the error and the statement to restore the value would not be
executed.
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