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70 lines
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_staticassert.how"></a>How it works</h2></div></div></div>
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<p>
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<code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works as
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follows. There is class <code class="computeroutput"><span class="identifier">STATIC_ASSERTION_FAILURE</span></code>
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which is defined as:
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span>
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<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>
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<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>
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<span class="special">}</span>
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</pre>
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<p>
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The key feature is that the error message triggered by the undefined expression
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<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
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to be consistent across a wide variety of compilers. The rest of the machinery
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of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> is just
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a way to feed the <code class="computeroutput"><span class="keyword">sizeof</span></code> expression
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into a <code class="computeroutput"><span class="keyword">typedef</span></code>. The use of a macro
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here is somewhat ugly; however boost members have spent considerable effort
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trying to invent a static assert that avoided macros, all to no avail. The
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general conclusion was that the good of a static assert working at namespace,
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function, and class scope outweighed the ugliness of a macro.
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<td align="right"><div class="copyright-footer">Copyright © 2000, 2005 Steve Cleary and John Maddock<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
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http://www.boost.org/LICENSE_1_0.txt </a>)
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