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| 26 | <div class="titlepage"><div><div><h3 class="title"> |
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| 27 | <a name="boost_staticassert.how"></a> How it works</h3></div></div></div> |
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| 28 | <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> |
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| 29 | which is defined as:</p> |
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| 30 | <pre class="programlisting"><code class="literal"><span class="keyword">namespace</span><span class="identifier"> boost</span><span class="special">{</span><span class="keyword"> |
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| 31 | |
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| 32 | 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"> |
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| 33 | |
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| 34 | 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"> |
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| 35 | |
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| 36 | }</span></code></pre> |
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| 37 | <p> |
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| 38 | The key feature is that the error message triggered by the undefined |
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| 39 | 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 |
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| 40 | across a wide variety of compilers. The rest of the machinery of |
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| 41 | <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>. |
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| 42 | The use of a macro here is somewhat ugly; however boost members have spent |
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| 43 | considerable effort trying to invent a static assert that avoided macros, |
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| 44 | all to no avail. The general conclusion was that the good of a static assert |
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| 45 | working at namespace, function, and class scope outweighed the ugliness of a macro.</p> |
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