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<h4 class="subsection" id="Missing-Components-1"><span>37.4.4 Missing Components<a class="copiable-link" href="#Missing-Components-1"> ¶</a></span></h4>
<p>If a package relies on a component, such as another Octave package, that may
not be present it may be useful to install a function which informs users what
to do when a particular component is missing. The function must be written by
the package maintainer and registered with Octave using
<code class="code">missing_component_hook</code>.
</p>
<a class="anchor" id="XREFmissing_005fcomponent_005fhook"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-missing_005fcomponent_005fhook"><span><code class="def-type"><var class="var">val</var> =</code> <strong class="def-name">missing_component_hook</strong> <code class="def-code-arguments">()</code><a class="copiable-link" href="#index-missing_005fcomponent_005fhook"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-missing_005fcomponent_005fhook-1"><span><code class="def-type"><var class="var">old_val</var> =</code> <strong class="def-name">missing_component_hook</strong> <code class="def-code-arguments">(<var class="var">new_val</var>)</code><a class="copiable-link" href="#index-missing_005fcomponent_005fhook-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-missing_005fcomponent_005fhook-2"><span><code class="def-type"><var class="var">old_val</var> =</code> <strong class="def-name">missing_component_hook</strong> <code class="def-code-arguments">(<var class="var">new_val</var>, "local")</code><a class="copiable-link" href="#index-missing_005fcomponent_005fhook-2"> ¶</a></span></dt>
<dd><p>Query or set the internal variable that specifies the function to call when
a component of Octave is missing.
</p>
<p>This can be useful for packagers that may split the Octave installation into
multiple sub-packages, for example, to provide a hint to users for how to
install the missing components.
</p>
<p>When called from inside a function with the <code class="code">"local"</code> option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.
</p>
<p>The hook function is expected to be of the form
</p>
<div class="example">
<pre class="example-preformatted"><var class="var">fcn</var> (<var class="var">component</var>)
</pre></div>
<p>Octave will call <var class="var">fcn</var> with the name of the function that requires the
component and a string describing the missing component. The hook function
should return an error message to be displayed.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Parser.html#XREFmissing_005ffunction_005fhook">missing_function_hook</a>.
</p></dd></dl>
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