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<title>Using the GNU Compiler Collection (GCC): C++ Module Preprocessing</title>
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Next: <a href="C_002b_002b-Compiled-Module-Interface.html#C_002b_002b-Compiled-Module-Interface" accesskey="n" rel="next">C++ Compiled Module Interface</a>, Previous: <a href="C_002b_002b-Module-Mapper.html#C_002b_002b-Module-Mapper" accesskey="p" rel="previous">C++ Module Mapper</a>, Up: <a href="C_002b_002b-Modules.html#C_002b_002b-Modules" accesskey="u" rel="up">C++ Modules</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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<a name="Module-Preprocessing"></a>
<h4 class="subsection">3.23.2 Module Preprocessing</h4>
<a name="index-C_002b_002b-Module-Preprocessing"></a>
<p>Modules affect preprocessing because of header units and include
translation. Some uses of the preprocessor as a separate step either
do not produce a correct output, or require CMIs to be available.
</p>
<p>Header units import macros. These macros can affect later conditional
inclusion, which therefore can cascade to differing import sets. When
preprocessing, it is necessary to load the CMI. If a header unit is
unavailable, the preprocessor issues a warning and continue (when
not just preprocessing, an error is emitted). Detecting such imports
requires preprocessor tokenization of the input stream to phase 4
(macro expansion).
</p>
<p>Include translation converts <code>#include</code>, <code>#include_next</code> and
<code>#import</code> directives to internal <code>import</code> declarations.
Whether a particular directive is translated is controlled by the
module mapper. Header unit names are canonicalized during
preprocessing.
</p>
<p>Dependency information can be emitted for macro import, extending the
functionality of <samp>-MD</samp> and <samp>-MMD</samp> options. Detection of
import declarations also requires phase 4 preprocessing, and thus
requires full preprocessing (or compilation).
</p>
<p>The <samp>-M</samp>, <samp>-MM</samp> and <samp>-E -fdirectives-only</samp> options halt
preprocessing before phase 4.
</p>
<p>The <samp>-save-temps</samp> option uses <samp>-fdirectives-only</samp> for
preprocessing, and preserve the macro definitions in the preprocessed
output. Usually you also want to use this option when explicitly
preprocessing a header-unit, or consuming such preprocessed output:
</p>
<div class="smallexample">
<pre class="smallexample">g++ -fmodules-ts -E -fdirectives-only my-header.hh -o my-header.ii
g++ -x c++-header -fmodules-ts -fpreprocessed -fdirectives-only my-header.ii
</pre></div>
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