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<h4 class="subsection">4.2.9 <code>__has_include</code></h4>
<a name="index-_005f_005fhas_005finclude"></a>

<p>The special operator <code>__has_include (<var>operand</var>)</code> may be used in
&lsquo;<samp>#if</samp>&rsquo; and &lsquo;<samp>#elif</samp>&rsquo; expressions to test whether the header referenced
by its <var>operand</var> can be included using the &lsquo;<samp>#include</samp>&rsquo; directive.  Using
the operator in other contexts is not valid.  The <var>operand</var> takes
the same form as the file in the &lsquo;<samp>#include</samp>&rsquo; directive (see <a href="Include-Syntax.html#Include-Syntax">Include Syntax</a>) and evaluates to a nonzero value if the header can be included and
to zero otherwise.  Note that that the ability to include a header doesn&rsquo;t
imply that the header doesn&rsquo;t contain invalid constructs or &lsquo;<samp>#error</samp>&rsquo;
directives that would cause the preprocessor to fail.
</p>
<p>The <code>__has_include</code> operator by itself, without any <var>operand</var> or
parentheses, acts as a predefined macro so that support for it can be tested
in portable code.  Thus, the recommended use of the operator is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_include
#  if __has_include (&lt;stdatomic.h&gt;)
#    include &lt;stdatomic.h&gt;
#  endif
#endif
</pre></div>

<p>The first &lsquo;<samp>#if</samp>&rsquo; test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used.  Only when that
test succeeds is it valid to use <code>__has_include</code> as a preprocessor
operator.  As a result, combining the two tests into a single expression
as shown below would only be valid with a compiler that supports the operator
but not with others that don&rsquo;t.
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_include &amp;&amp; __has_include (&quot;header.h&quot;)   /* not portable */
&hellip;
#endif
</pre></div>


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