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+<title>Build Time Configuration</title>
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+<div class="section">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="boost_config.build_config"></a><a class="link" href="build_config.html" title="Build Time Configuration">Build Time Configuration</a>
+</h2></div></div></div>
+<p>
+ There are times when you want to control whether a build target gets built
+ or not, based on what features the compiler supports. For example, suppose
+ you have a test file "test_constexpr_128.cpp" which requires three
+ key features in order to build:
+ </p>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ The <code class="computeroutput"><span class="keyword">constexpr</span></code> keyword as detected
+ by BOOST_NO_CXX11_CONSTEXPR.
+ </li>
+<li class="listitem">
+ User defined literals, as detected by BOOST_NO_CXX11_USER_DEFINED_LITERALS.
+ </li>
+<li class="listitem">
+ The <code class="computeroutput"><span class="identifier">__int128</span></code> data type,
+ as detected by BOOST_HAS_INT128.
+ </li>
+</ul></div>
+<p>
+ Clearly we know that if these features are not supported by the compiler, then
+ there's simply no point in even trying to build the test program. The main
+ advantages being:
+ </p>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ Faster compile times - build configuration uses lightweight tests the results
+ of which are also cached.
+ </li>
+<li class="listitem">
+ Less noise in build output - there's no reason to be faced with pages of
+ template instantiation backtrace if we know the file can never compile
+ anyway.
+ </li>
+<li class="listitem">
+ Less noise in the online test results - the test will show up as blank,
+ rather than as a fail in the online test matrix.
+ </li>
+<li class="listitem">
+ A better experience for end users building all of Boost, if those libraries
+ which can not be built for the current target compiler are simply skipped,
+ rather than generating pages of error output.
+ </li>
+</ul></div>
+<p>
+ Returning to our example, the test case is probably executed in it's Jamfile
+ via the "run" rule:
+ </p>
+<pre class="programlisting"><span class="identifier">run</span> <span class="identifier">test_constexpr_128</span><span class="special">.</span><span class="identifier">cpp</span> <span class="special">;</span>
+</pre>
+<p>
+ We now need to make this target conditional on the necessary features. We can
+ do that by first importing the necessary rule at the start of the Jamfile:
+ </p>
+<pre class="programlisting"><span class="identifier">import</span> <span class="identifier">path</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">config</span><span class="special">-</span><span class="identifier">lib</span><span class="special">/</span><span class="identifier">checks</span><span class="special">/</span><span class="identifier">config</span> <span class="special">:</span> <span class="identifier">requires</span> <span class="special">;</span>
+</pre>
+<p>
+ Assuming that the test case is in the usual directory:
+ </p>
+<pre class="programlisting"><span class="identifier">libs</span><span class="special">/</span><span class="identifier">yourlib</span><span class="special">/</span><span class="identifier">test</span>
+</pre>
+<p>
+ then the import rule will actually be:
+ </p>
+<pre class="programlisting"><span class="identifier">import</span> <span class="special">../../</span><span class="identifier">config</span><span class="special">/</span><span class="identifier">checks</span><span class="special">/</span><span class="identifier">config</span> <span class="special">:</span> <span class="identifier">requires</span> <span class="special">;</span>
+</pre>
+<p>
+ Then add a "requires" rule invocation to the requirements section
+ of the target:
+ </p>
+<pre class="programlisting"><span class="identifier">run</span> <span class="identifier">test_constexpr_128</span><span class="special">.</span><span class="identifier">cpp</span>
+ <span class="special">:</span> <span class="special">:</span> <span class="special">:</span> <span class="special">#</span><span class="identifier">requirements</span><span class="special">:</span>
+ <span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cxx11_constexpr</span> <span class="identifier">cxx11_user_defined_literals</span> <span class="identifier">int128</span> <span class="special">]</span> <span class="special">;</span>
+</pre>
+<p>
+ Notice that multiple arguments can be added to the requires rule, and that
+ these are always the same as the Boost.Config macro name, but in lower case
+ and with the <span class="emphasis"><em>boost_no_</em></span> or <span class="emphasis"><em>boost_has_</em></span>
+ prefix removed.
+ </p>
+<p>
+ When building the above example, you will see at the start of the build process
+ the results of the configuration, for example GCC in C++11 mode gives:
+ </p>
+<pre class="programlisting"><span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">int128</span> <span class="special">:</span> <span class="identifier">yes</span>
+<span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">cxx11_constexpr</span> <span class="special">:</span> <span class="identifier">yes</span>
+<span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">cxx11_user_defined_literals</span> <span class="special">:</span> <span class="identifier">yes</span>
+</pre>
+<p>
+ That's all there is to this handy feature, should at any time you be unsure
+ of the feature-test names you can pass to the "requires" rule, then
+ search for the Boost.Config macro of interest in libs/config/checks/Jamfiles.v2,
+ and the name of the feature check will follow it.
+ </p>
+<p>
+ And finally, this feature is built around the Boost.Build built in rule <span class="emphasis"><em>check-target-builds</em></span>
+ which can be used to perform more generalized build-time feature testing. The
+ checks in this library are provided as a convenient shorthand without the need
+ for you to write the test cases yourself.
+ </p>
+</div>
+<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
+<td align="left"></td>
+<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Beman Dawes, Vesa Karvonen, John
+ Maddock<p>
+ Distributed under the Boost Software License, Version 1.0. (See accompanying
+ file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
+ </p>
+</div></td>
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