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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. This can be achieved with 
    <em class="firstterm">conditional
    requirement</em>. For example, you might want to set
    specific <code class="computeroutput">#defines</code> when a library is built as shared,
    or when a target's <code class="computeroutput">release</code> variant is built in
    release mode.
    </p>
<pre class="programlisting">
lib network : network.cpp 
    : <span class="bold"><strong>&lt;link&gt;shared:&lt;define&gt;NEWORK_LIB_SHARED</strong></span>
     &lt;variant&gt;release:&lt;define&gt;EXTRA_FAST
    ;
</pre>
<p>

      In the example above, whenever <code class="filename">network</code> is
      built with <code class="computeroutput">&lt;link&gt;shared</code>,
      <code class="computeroutput">&lt;define&gt;NEWORK_LIB_SHARED</code> will be in its
      properties, too.
    </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 : &lt;toolset&gt;gcc ;   # alternative 2
lib demangler : demangler_msvc.cpp : &lt;toolset&gt;msvc ; # alternative 3
</pre>
<p>
        In the example above, when built with <code class="literal">gcc</code>
        or <code class="literal">msvc</code>, <code class="filename">demangler</code>
        will use a source file specific to the toolset.  Otherwise, it
        will use a generic source file,
        <code class="filename">dummy_demangler.cpp</code>.
    </p>
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