doc/html/boost_staticassert/how.html

   1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
   2 <html>
   3 <head>
   4 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   5 <title>How it works</title>
   6 <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css">
   7 <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
   8 <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
   9 <link rel="up" href="../boost_staticassert.html" title="Chapter&#160;37.&#160;Boost.StaticAssert">
  10 <link rel="prev" href="../boost_staticassert.html" title="Chapter&#160;37.&#160;Boost.StaticAssert">
  11 <link rel="next" href="test.html" title="Test Programs">
  12 </head>
  13 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  14 <table cellpadding="2" width="100%"><tr>
  15 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td>
  16 <td align="center"><a href="../../../index.html">Home</a></td>
  17 <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  19 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  20 <td align="center"><a href="../../../more/index.htm">More</a></td>
  21 </tr></table>
  22 <hr>
  23 <div class="spirit-nav">
  24 <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>
  25 </div>
  26 <div class="section">
  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="boost_staticassert.how"></a>How it works</h2></div></div></div>
  29 <p>
  30       <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works as
  31       follows. There is class <code class="computeroutput"><span class="identifier">STATIC_ASSERTION_FAILURE</span></code>
  32       which is defined as:
  33     </p>
  34 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span>
  35
  36 <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special">;</span>
  37
  38 <span class="keyword">template</span> <span class="special">&lt;&gt;</span> <span class="keyword">struct</span> <span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special">&lt;</span><span class="keyword">true</span><span class="special">&gt;{};</span>
  39
  40 <span class="special">}</span>
  41 </pre>
  42 <p>
  43       The key feature is that the error message triggered by the undefined expression
  44       <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;)</span></code>, tends
  45       to be consistent across a wide variety of compilers. The rest of the machinery
  46       of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> is just
  47       a way to feed the <code class="computeroutput"><span class="keyword">sizeof</span></code> expression
  48       into a <code class="computeroutput"><span class="keyword">typedef</span></code>. The use of a macro
  49       here is somewhat ugly; however boost members have spent considerable effort
  50       trying to invent a static assert that avoided macros, all to no avail. The
  51       general conclusion was that the good of a static assert working at namespace,
  52       function, and class scope outweighed the ugliness of a macro.
  53     </p>
  54 </div>
  55 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
  56 <td align="left"></td>
  57 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2005 Steve Cleary and John Maddock<p>
  58         Distributed under the Boost Software License, Version 1.0. (See accompanying
  59         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
  60         http://www.boost.org/LICENSE_1_0.txt </a>)
  61       </p>
  62 </div></td>
  63 </tr></table>
  64 <hr>
  65 <div class="spirit-nav">
  66 <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>
  67 </div>
  68 </body>
  69 </html>