<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>How it works</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../boost_staticassert.html" title="Chapter 20. Boost.StaticAssert"> <link rel="prev" href="../boost_staticassert.html" title="Chapter 20. Boost.StaticAssert"> <link rel="next" href="test.html" title="Test Programs"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../boost_staticassert.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_staticassert.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="test.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="boost_staticassert.how"></a> How it works</h2></div></div></div> <p> <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works as follows. There is class <code class="computeroutput"><span class="identifier">STATIC_ASSERTION_FAILURE</span></code> which is defined as: </p> <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special">;</span> <span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special"><</span><span class="keyword">true</span><span class="special">>{};</span> <span class="special">}</span> </pre> <p> The key feature is that the error message triggered by the undefined expression <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special"><</span><span class="number">0</span><span class="special">>)</span></code>, tends to be consistent across a wide variety of compilers. The rest of the machinery of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> is just a way to feed the <code class="computeroutput"><span class="keyword">sizeof</span></code> expression into a <code class="computeroutput"><span class="keyword">typedef</span></code>. The use of a macro here is somewhat ugly; however boost members have spent considerable effort trying to invent a static assert that avoided macros, all to no avail. The general conclusion was that the good of a static assert working at namespace, function, and class scope outweighed the ugliness of a macro. </p> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2000, 2005 Steve Cleary and John Maddock<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top"> http://www.boost.org/LICENSE_1_0.txt </a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../boost_staticassert.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_staticassert.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="test.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>