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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.tutorial.libs"></a>Dependent Targets</h2></div></div></div>
<p>
      Targets that are &#8220;needed&#8221; by other targets are called
      <em class="firstterm">dependencies</em> of those other targets.  The
      targets that need the other targets are called
      <em class="firstterm">dependent</em> targets.
    </p>
<p>To get a feeling of target dependencies, let's continue the
      above example and see how <code class="filename">top/app/Jamfile</code> can
      use libraries from <code class="filename">top/util/foo</code>.  If
      <code class="filename">top/util/foo/Jamfile</code> contains

</p>
<pre class="programlisting">
lib bar : bar.cpp ;
</pre>
<p>

      then to use this library in <code class="filename">top/app/Jamfile</code>, we can
      write:

</p>
<pre class="programlisting">
exe app : app.cpp ../util/foo//bar ;
</pre>
<p>

      While <code class="computeroutput">app.cpp</code> refers to a regular source file,
      <code class="computeroutput">../util/foo//bar</code> is a reference to another target:
      a library <code class="filename">bar</code> declared in the Jamfile at
      <code class="filename">../util/foo</code>. 
    </p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>Some other build system have special syntax for listing dependent
      libraries, for example <code class="varname">LIBS</code> variable. In Boost.Build,
      you just add the library to the list of sources.
      </p>
</div>
<p>Suppose we build <code class="filename">app</code> with:
    </p>
<pre class="screen">
bjam app optimization=full define=USE_ASM
    </pre>
<p>
    Which properties will be used to build <code class="computeroutput">foo</code>? The answer is
    that some features are
    <em class="firstterm">propagated</em>&#8212;Boost.Build attempts to use
    dependencies with the same value of propagated features. The
    <code class="varname">&lt;optimization&gt;</code> feature is propagated, so both
    <code class="filename">app</code> and <code class="filename">foo</code> will be compiled
    with full optimization. But <code class="varname">&lt;define&gt;</code> is not
    propagated: its value will be added as-is to the compiler flags for
    <code class="filename">a.cpp</code>, but won't affect <code class="filename">foo</code>.
    </p>
<p>Let's improve this project further.
      The library
      probably has some headers that must be used when compiling
      <code class="filename">app.cpp</code>. We could manually add the necessary
      <code class="computeroutput">#include</code> paths to <code class="filename">app</code>'s
      requirements as values of the
      <code class="varname">&lt;include&gt;</code> feature, but then this work will 
      be repeated for all programs
      that use <code class="filename">foo</code>. A better solution is to modify
      <code class="filename">util/foo/Jamfile</code> in this way:

</p>
<pre class="programlisting">
project 
    : usage-requirements &lt;include&gt;.
    ;

lib foo : foo.cpp ;
</pre>
<p>

      Usage requirements are applied not to the target being declared
      but to its
      dependents. In this case, <code class="literal">&lt;include&gt;.</code> will be applied to all
      targets that directly depend on <code class="filename">foo</code>.
    </p>
<p>Another improvement is using symbolic identifiers to refer to
      the library, as opposed to <code class="filename">Jamfile</code> location.
      In a large project, a library can be used by many targets, and if 
      they all use <code class="filename">Jamfile</code> location,
      a change in directory organization entails much work.
      The solution is to use project ids&#8212;symbolic names
      not tied to directory layout. First, we need to assign a project id by
      adding this code to
      <code class="filename">Jamroot</code>:</p>
<pre class="programlisting">
use-project /library-example/foo : util/foo ;
      </pre>
<p>Second, we modify <code class="filename">app/Jamfile</code> to use the
      project id:

</p>
<pre class="programlisting">
exe app : app.cpp /library-example/foo//bar ;
</pre>
<p>
The <code class="filename">/library-example/foo//bar</code> syntax is used 
      to refer to the target <code class="filename">bar</code> in
      the project with id <code class="filename">/library-example/foo</code>.
      We've achieved our goal&#8212;if the library is moved to a different
      directory, only <code class="filename">Jamroot</code> must be modified.
      Note that project ids are global&#8212;two Jamfiles are not
      allowed to assign the same project id to different directories.
           
    </p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>If you want all applications in some project to link
      to a certain library, you can avoid having to specify it directly the sources of every
      target by using the
      <code class="varname">&lt;source&gt;</code> property. For example, if <code class="filename">/boost/filesystem//fs</code>
      should be linked to all applications in your project, you can add
      <code class="computeroutput">&lt;source&gt;/boost/filesystem//fs</code> to the project's requirements, like this:</p>
<pre class="programlisting">
project 
   : requirements &lt;source&gt;/boost/filesystem//fs
   ;   
      </pre>
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