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| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> |
| |
| <span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{};</span> |
| |
| <span class="keyword">extern</span> <span class="keyword">const</span> <span class="identifier">none_t</span> <span class="identifier">none</span><span class="special">;</span> <span class="comment">// see below</span> |
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| <span class="special">}</span> <span class="comment">// namespace boost</span> |
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| Variable <code class="computeroutput"><span class="identifier">none</span></code> has external |
| linkage, however it is not required to link with any library to obtain |
| its definition. Only by including this header file, the definition becomes |
| available, by means of using template instantiation. |
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