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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. |
| </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> |
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| <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>) |
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