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<title>Header-only libraries</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.faq.header-only-libraries"></a>Header-only libraries</h2></div></div></div>
<p>In modern C++, libraries often consist of just header files, without
any source files to compile. To use such libraries, you need to add proper
includes and, maybe, defines, to your project. But with large number of
external libraries it becomes problematic to remember which libraries are
header only, and which are "real" ones. However, with Boost.Build a
header-only library can be declared as Boost.Build target and all
dependents can use such library without remebering if it's header-only or not.
</p>
<p>Header-only libraries are declared using the <code class="computeroutput">alias</code> rule,
that specifies only usage requirements, for example:
</p>
<pre class="programlisting">
alias mylib
: # no sources
: # no build requirements
: # no default build
: <include>whatever
;
</pre>
<p>
The includes specified in usage requirements of <code class="computeroutput">mylib</code> are
automatically added to build properties of all dependents. The dependents
need not care if <code class="computeroutput">mylib</code> is header-only or not, and it's possible
to later make <code class="computeroutput">mylib</code> into a regular compiled library.
</p>
<p>
If you already have proper usage requirements declared for project where
header-only library is defined, you don't need to duplicate them for
the <code class="computeroutput">alias</code> target:
</p>
<pre class="programlisting">
project my : usage-requirements <include>whatever ;
alias mylib ;
</pre>
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