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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.tutorial.hierarchy"></a>Project Hierarchies</h2></div></div></div>
<p>So far we've only considered examples with one project
(i.e. with one user-written Boost.Jam file,
<code class="filename">Jamroot</code>). A typical large codebase would be
composed of many projects organized into a tree. The top of the
tree is called the <em class="firstterm">project root</em>. Every
subproject is defined by a file called
<code class="filename">Jamfile</code> in a descendant directory of the
project root. The parent project of a subproject is defined by
the nearest <code class="filename">Jamfile</code> or
<code class="filename">Jamroot</code> file in an ancestor directory. For
example, in the following directory layout:
</p>
<pre class="screen">
top/
|
+-- Jamroot
|
+-- app/
| |
| +-- Jamfile
| `-- app.cpp
|
`-- util/
|
+-- foo/
. |
. +-- Jamfile
. `-- bar.cpp
</pre>
<p>
the project root is <code class="filename">top/</code>. Because there is
no <code class="filename">Jamfile</code> in
<code class="filename">top/util/</code>, the projects in
<code class="filename">top/app/</code> and
<code class="filename">top/util/foo/</code> are immediate children of the
root project.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
When we refer to a “Jamfile,” set in normal
type, we mean a file called either
<code class="filename">Jamfile</code> or
<code class="filename">Jamroot</code>. When we need to be more
specific, the filename will be set as
“<code class="filename">Jamfile</code>” or
“<code class="filename">Jamroot</code>.”
</p>
</div>
<p>
Projects inherit all attributes (such as requirements)
from their parents. Inherited requirements are combined with
any requirements specified by the sub-project.
For example, if <code class="filename">top/Jamroot</code> has
</p>
<pre class="programlisting">
<include>/home/ghost/local
</pre>
<p>
in its requirements, then all of its sub-projects will have it
in their requirements, too. Of course, any project can add
include paths to those specified by its parents. <sup>[<a name="id1695733" href="#ftn.id1695733">5</a>]</sup>
More details can be found in
<a href="../advanced/jamfiles.html#bbv2.advanced.projects" title="Projects">the section called “Projects”</a>.
</p>
<p>
Invoking <span><strong class="command">bjam</strong></span> without explicitly specifying
any targets on the command-line builds the project rooted in the
current directory. Building a project does not automatically
cause its sub-projects to be built unless the parent project's
Jamfile explicitly requests it. In our example,
<code class="filename">top/Jamroot</code> might contain:
</p>
<pre class="programlisting">
build-project app ;
</pre>
<p>
which would cause the project in <code class="filename">top/app/</code>
to be built whenever the project in <code class="filename">top/</code> is
built. However, targets in <code class="filename">top/util/foo/</code>
will be built only if they are needed by targets in
<code class="filename">top/</code> or <code class="filename">top/app/</code>.
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a name="ftn.id1695733" href="#id1695733">5</a>] </sup>Many
features will be overridden,
rather than added-to, in sub-projects. See <a href="../reference/definitions.html#bbv2.reference.features.attributes" title="Feature Attributes">the section called “Feature Attributes”</a> for more
information</p></div>
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