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27<a name="boost_config.build_config"></a><a class="link" href="build_config.html" title="Build Time Configuration">Build Time Configuration</a>
28</h2></div></div></div>
29<p>
30 There are times when you want to control whether a build target gets built
31 or not, based on what features the compiler supports. For example, suppose
32 you have a test file "test_constexpr_128.cpp" which requires three
33 key features in order to build:
34 </p>
35<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
36<li class="listitem">
37 The <code class="computeroutput"><span class="keyword">constexpr</span></code> keyword as detected
38 by BOOST_NO_CXX11_CONSTEXPR.
39 </li>
40<li class="listitem">
41 User defined literals, as detected by BOOST_NO_CXX11_USER_DEFINED_LITERALS.
42 </li>
43<li class="listitem">
44 The <code class="computeroutput"><span class="identifier">__int128</span></code> data type,
45 as detected by BOOST_HAS_INT128.
46 </li>
47</ul></div>
48<p>
49 Clearly we know that if these features are not supported by the compiler, then
50 there's simply no point in even trying to build the test program. The main
51 advantages being:
52 </p>
53<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
54<li class="listitem">
55 Faster compile times - build configuration uses lightweight tests the results
56 of which are also cached.
57 </li>
58<li class="listitem">
59 Less noise in build output - there's no reason to be faced with pages of
60 template instantiation backtrace if we know the file can never compile
61 anyway.
62 </li>
63<li class="listitem">
64 Less noise in the online test results - the test will show up as blank,
65 rather than as a fail in the online test matrix.
66 </li>
67<li class="listitem">
68 A better experience for end users building all of Boost, if those libraries
69 which can not be built for the current target compiler are simply skipped,
70 rather than generating pages of error output.
71 </li>
72</ul></div>
73<p>
74 Returning to our example, the test case is probably executed in it's Jamfile
75 via the "run" rule:
76 </p>
77<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>
78</pre>
79<p>
80 We now need to make this target conditional on the necessary features. We can
81 do that by first importing the necessary rule at the start of the Jamfile:
82 </p>
83<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>
84</pre>
85<p>
86 Assuming that the test case is in the usual directory:
87 </p>
88<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>
89</pre>
90<p>
91 then the import rule will actually be:
92 </p>
93<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>
94</pre>
95<p>
96 Then add a "requires" rule invocation to the requirements section
97 of the target:
98 </p>
99<pre class="programlisting"><span class="identifier">run</span> <span class="identifier">test_constexpr_128</span><span class="special">.</span><span class="identifier">cpp</span>
100 <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>
101 <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>
102</pre>
103<p>
104 Notice that multiple arguments can be added to the requires rule, and that
105 these are always the same as the Boost.Config macro name, but in lower case
106 and with the <span class="emphasis"><em>boost_no_</em></span> or <span class="emphasis"><em>boost_has_</em></span>
107 prefix removed.
108 </p>
109<p>
110 When building the above example, you will see at the start of the build process
111 the results of the configuration, for example GCC in C++11 mode gives:
112 </p>
113<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>
114<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>
115<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>
116</pre>
117<p>
118 That's all there is to this handy feature, should at any time you be unsure
119 of the feature-test names you can pass to the "requires" rule, then
120 search for the Boost.Config macro of interest in libs/config/checks/Jamfiles.v2,
121 and the name of the feature check will follow it.
122 </p>
123<p>
124 And finally, this feature is built around the Boost.Build built in rule <span class="emphasis"><em>check-target-builds</em></span>
125 which can be used to perform more generalized build-time feature testing. The
126 checks in this library are provided as a convenient shorthand without the need
127 for you to write the test cases yourself.
128 </p>
129</div>
130<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
131<td align="left"></td>
132<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2007 Beman Dawes, Vesa Karvonen, John
133 Maddock<p>
134 Distributed under the Boost Software License, Version 1.0. (See accompanying
135 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>)
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