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<a name="Pushdef"></a>
<div class="header">
<p>
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<hr>
<a name="Temporarily-redefining-macros"></a>
<h3 class="section">5.6 Temporarily redefining macros</h3>

<a name="index-macros_002c-temporary-redefinition-of"></a>
<a name="index-temporary-redefinition-of-macros"></a>
<a name="index-redefinition-of-macros_002c-temporary"></a>
<a name="index-definition-stack"></a>
<a name="index-pushdef-stack"></a>
<a name="index-stack_002c-macro-definition"></a>
<p>It is possible to redefine a macro temporarily, reverting to the
previous definition at a later time.  This is done with the builtins
<code>pushdef</code> and <code>popdef</code>:
</p>
<dl>
<dt><a name="index-pushdef"></a>Builtin: <strong>pushdef</strong> <em>(<var>name</var>, <span class="roman">[</span><var>expansion</var><span class="roman">]</span>)</em></dt>
<dt><a name="index-popdef"></a>Builtin: <strong>popdef</strong> <em>(<var>name</var>&hellip;)</em></dt>
<dd><p>Analogous to <code>define</code> and <code>undefine</code>.
</p>
<p>These macros work in a stack-like fashion.  A macro is temporarily
redefined with <code>pushdef</code>, which replaces an existing definition of
<var>name</var>, while saving the previous definition, before the new one is
installed.  If there is no previous definition, <code>pushdef</code> behaves
exactly like <code>define</code>.
</p>
<p>If a macro has several definitions (of which only one is accessible),
the topmost definition can be removed with <code>popdef</code>.  If there is
no previous definition, <code>popdef</code> behaves like <code>undefine</code>.
</p>
<p>The expansion of both <code>pushdef</code> and <code>popdef</code> is void.
The macros <code>pushdef</code> and <code>popdef</code> are recognized only with
parameters.
</p></dd></dl>

<div class="example">
<pre class="example">define(`foo', `Expansion one.')
&rArr;
foo
&rArr;Expansion one.
pushdef(`foo', `Expansion two.')
&rArr;
foo
&rArr;Expansion two.
pushdef(`foo', `Expansion three.')
&rArr;
pushdef(`foo', `Expansion four.')
&rArr;
popdef(`foo')
&rArr;
foo
&rArr;Expansion three.
popdef(`foo', `foo')
&rArr;
foo
&rArr;Expansion one.
popdef(`foo')
&rArr;
foo
&rArr;foo
</pre></div>

<p>If a macro with several definitions is redefined with <code>define</code>, the
topmost definition is <em>replaced</em> with the new definition.  If it is
removed with <code>undefine</code>, <em>all</em> the definitions are removed,
and not only the topmost one.  However, POSIX allows other
implementations that treat <code>define</code> as replacing an entire stack
of definitions with a single new definition, so to be portable to other
implementations, it may be worth explicitly using <code>popdef</code> and
<code>pushdef</code> rather than relying on the GNU behavior of
<code>define</code>.
</p>
<div class="example">
<pre class="example">define(`foo', `Expansion one.')
&rArr;
foo
&rArr;Expansion one.
pushdef(`foo', `Expansion two.')
&rArr;
foo
&rArr;Expansion two.
define(`foo', `Second expansion two.')
&rArr;
foo
&rArr;Second expansion two.
undefine(`foo')
&rArr;
foo
&rArr;foo
</pre></div>

<a name="index-local-variables"></a>
<a name="index-variables_002c-local"></a>
<p>Local variables within macros are made with <code>pushdef</code> and
<code>popdef</code>.  At the start of the macro a new definition is pushed,
within the macro it is manipulated and at the end it is popped,
revealing the former definition.
</p>
<p>It is possible to temporarily redefine a builtin with <code>pushdef</code>
and <code>defn</code>.
</p>
<hr>
<div class="header">
<p>
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m4-doc 1.4.18-1 / usr / share / doc / m4-doc / Pushdef.html
