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<title> How it works</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_staticassert.how"></a> How it works</h3></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"><code class="literal"><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></code></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.</p>
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