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| <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. |
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| <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>) |
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