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- <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. You can also use any C++ standard feature-macro name with the
- leading underscores removed (see more below).
- </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>
- If you wish to make a build conditional on a C++ standard feature macro then
- you can specify these too, just remove the leading underscores from the name.
- For example:
- </p>
- <pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_constexpr</span> <span class="special">]</span>
- </pre>
- <p>
- To require C++11 style const-expressions. If you want to specify a macro from
- a particular standard, then you append an underscore followed by the (2 digit)
- year of the standard, for example:
- </p>
- <pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_constexpr_17</span> <span class="special">]</span>
- </pre>
- <p>
- For C++17 constepxr. If you don't specify a standard then you get the first
- version that introduced the macro. In addition there are only standard-specific
- rules for each version bump of the macro, so:
- </p>
- <pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_if_constexpr_17</span> <span class="special">]</span>
- </pre>
- <p>
- Is fine since the macro was introduced in C++17 and is the same as the un-versioned
- name, but:
- </p>
- <pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_if_constexpr_20</span> <span class="special">]</span>
- </pre>
- <p>
- Will result in a build error since there is no C++20 version bump for <code class="computeroutput"><span class="identifier">__cpp_if_constexpr</span></code>.
- </p>
- <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>
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