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| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 27 | <a name="boost_staticassert.how"></a> How it works</h2></div></div></div> |
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| 28 | <p> |
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| 29 | <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works as |
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| 30 | follows. There is class <code class="computeroutput"><span class="identifier">STATIC_ASSERTION_FAILURE</span></code> |
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| 31 | which is defined as: |
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| 32 | </p> |
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| 33 | <pre class="programlisting"> |
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| 34 | <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> |
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| 35 | |
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| 36 | <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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| 37 | |
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| 38 | <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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| 39 | |
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| 40 | <span class="special">}</span> |
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| 41 | </pre> |
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| 42 | <p> |
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| 43 | The key feature is that the error message triggered by the undefined expression |
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| 44 | <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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| 45 | to be consistent across a wide variety of compilers. The rest of the machinery |
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| 46 | of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> is just |
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| 47 | a way to feed the <code class="computeroutput"><span class="keyword">sizeof</span></code> expression |
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| 48 | into a <code class="computeroutput"><span class="keyword">typedef</span></code>. The use of a macro |
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| 49 | here is somewhat ugly; however boost members have spent considerable effort |
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| 50 | trying to invent a static assert that avoided macros, all to no avail. The |
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| 51 | general conclusion was that the good of a static assert working at namespace, |
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| 52 | function, and class scope outweighed the ugliness of a macro. |
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| 53 | </p> |
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| 54 | </div> |
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| 55 | <table width="100%"><tr> |
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| 56 | <td align="left"></td> |
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| 57 | <td align="right"><small>Copyright © 2000, 2005 Steve Cleary and John Maddock</small></td> |
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