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<title>Precompiled Headers</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.reference.precompiled_headers"></a>Precompiled Headers</h2></div></div></div>
<p>Precompiled headers is a mechanism to speed up compilation
by creating a partially processed version of some header files,
and then using that version during compilations rather then
repeatedly parsing the original headers. Boost.Build supports
precompiled headers with gcc and msvc toolsets.</p>
<p>To use precompiled headers, follow these steps:</p>
<div class="orderedlist"><ol type="1">
<li><p>Create a header that includes big headers used by your project.
It's better to include only headers that are sufficiently stable —
like headers from the compiler, and external libraries. Please wrap
the header in <code class="computeroutput">#ifdef BOOST_BUILD_PCH_ENABLED</code>, so that
the potentially expensive inclusion of headers is not done
when PCH is not enabled. Include the new header at the top of your
source files.</p></li>
<li>
<p>Declare a new Boost.Build target for the precompiled header
and add that precompiled header to the sources of the target whose compilation
you want to speed up:
</p>
<pre class="programlisting">
cpp-pch pch : header.hpp ;
exe main : main.cpp pch ;</pre>
<p>
You can use the <code class="computeroutput">c-pch</code> if you want to use the precompiled
header in C programs.
</p>
</li>
</ol></div>
<p>The <code class="filename">pch</code> example in Boost.Build distribution
can be used as reference.</p>
<p>Please note the following:</p>
<div class="itemizedlist"><ul type="disc">
<li><p>The inclusion of the precompiled header must be the
first thing in a source file, before any code or preprocessor directives.
</p></li>
<li><p>The build properties used to compile the source files
and the precompiled header must be the same. Consider using
project requirements to assure this.
</p></li>
<li><p>Precompiled headers must be used purely as a way to
improve compilation time, not to save the number of <code class="computeroutput">#include</code>
statements. If a source file needs to include some header, explicitly include
it in the source file, even if the same header is included from
the precompiled header. This makes sure that your project will build
even if precompiled headers are not supported.</p></li>
</ul></div>
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