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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.tutorial.conditions"></a>Conditions and alternatives</h2></div></div></div>
<p>Sometimes, particular relationships need to be maintained
among a target's build properties. For example, you might want to set
specific <code class="computeroutput">#define</code> when a library is built as shared,
or when a target's <code class="computeroutput">release</code> variant is built.
This can be achieved with <em class="firstterm">conditional requirements</em>.
</p>
<pre class="programlisting">
lib network : network.cpp
: <span class="bold"><strong><link>shared:<define>NEWORK_LIB_SHARED</strong></span>
<variant>release:<define>EXTRA_FAST
;
</pre>
<p>
In the example above, whenever <code class="filename">network</code> is
built with <code class="computeroutput"><link>shared</code>,
<code class="computeroutput"><define>NEWORK_LIB_SHARED</code> will be in its
properties, too. Also, whenever its release variant is built,
<code class="computeroutput"><define>EXTRA_FAST</code> will appear in its properties.
</p>
<p>
Sometimes the ways a target is built are so different that
describing them using conditional requirements would be
hard. For example, imagine that a library actually uses
different source files depending on the toolset used to build
it. We can express this situation using <em class="firstterm">target
alternatives</em>:
</p>
<pre class="programlisting">
lib demangler : dummy_demangler.cpp ; # alternative 1
lib demangler : demangler_gcc.cpp : <toolset>gcc ; # alternative 2
lib demangler : demangler_msvc.cpp : <toolset>msvc ; # alternative 3
</pre>
<p>
When building <code class="filename">demangler</code>, Boost.Build will compare
requirements for each alternative with build properties to find the best match.
For example, when building with with <code class="computeroutput"><toolset>gcc</code>
alternative 2, will be selected, and when building with
<code class="computeroutput"><toolset>msvc</code> alternative 3 will be selected. In all other
cases, the most generic alternative 1 will be built.
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