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<h3 class="section">11.3 Obsolete Features</h3>

<p>CPP has some features which are present mainly for compatibility with
older programs.  We discourage their use in new code.  In some cases,
we plan to remove the feature in a future version of GCC.

<h4 class="subsection">11.3.1 Assertions</h4>

<p><a name="index-assertions-111"></a>
<dfn>Assertions</dfn> are a deprecated alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on.  Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

   <p>Assertions were intended to provide a more systematic way to describe
the compiler's target system.  However, in practice they are just as
unpredictable as the system-specific predefined macros.  In addition, they
are not part of any standard, and only a few compilers support them. 
Therefore, the use of assertions is <strong>less</strong> portable than the use
of system-specific predefined macros.  We recommend you do not use them at
all.

   <p><a name="index-predicates-112"></a>An assertion looks like this:

<pre class="smallexample">     #<var>predicate</var> (<var>answer</var>)
</pre>
   <p class="noindent"><var>predicate</var> must be a single identifier.  <var>answer</var> can be any
sequence of tokens; all characters are significant except for leading
and trailing whitespace, and differences in internal whitespace
sequences are ignored.  (This is similar to the rules governing macro
redefinition.)  Thus, <code>(x + y)</code> is different from <code>(x+y)</code> but
equivalent to <code>(&nbsp;x&nbsp;+&nbsp;y&nbsp;)<!-- /@w --></code>.  Parentheses do not nest inside an
answer.

   <p><a name="index-testing-predicates-113"></a>To test an assertion, you write it in an &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;.  For example, this
conditional succeeds if either <code>vax</code> or <code>ns16000</code> has been
asserted as an answer for <code>machine</code>.

<pre class="smallexample">     #if #machine (vax) || #machine (ns16000)
</pre>
   <p class="noindent">You can test whether <em>any</em> answer is asserted for a predicate by
omitting the answer in the conditional:

<pre class="smallexample">     #if #machine
</pre>
   <p><a name="index-g_t_0023assert-114"></a>Assertions are made with the &lsquo;<samp><span class="samp">#assert</span></samp>&rsquo; directive.  Its sole
argument is the assertion to make, without the leading &lsquo;<samp><span class="samp">#</span></samp>&rsquo; that
identifies assertions in conditionals.

<pre class="smallexample">     #assert <var>predicate</var> (<var>answer</var>)
</pre>
   <p class="noindent">You may make several assertions with the same predicate and different
answers.  Subsequent assertions do not override previous ones for the
same predicate.  All the answers for any given predicate are
simultaneously true.

   <p><a name="index-assertions_002c-canceling-115"></a><a name="index-g_t_0023unassert-116"></a>Assertions can be canceled with the &lsquo;<samp><span class="samp">#unassert</span></samp>&rsquo; directive.  It
has the same syntax as &lsquo;<samp><span class="samp">#assert</span></samp>&rsquo;.  In that form it cancels only the
answer which was specified on the &lsquo;<samp><span class="samp">#unassert</span></samp>&rsquo; line; other answers
for that predicate remain true.  You can cancel an entire predicate by
leaving out the answer:

<pre class="smallexample">     #unassert <var>predicate</var>
</pre>
   <p class="noindent">In either form, if no such assertion has been made, &lsquo;<samp><span class="samp">#unassert</span></samp>&rsquo; has
no effect.

   <p>You can also make or cancel assertions using command line options. 
See <a href="Invocation.html#Invocation">Invocation</a>.

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