identifier">Unit</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Y</span> <span class="special">=</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">quantity</span><span class="special">;</span>
</pre>
<p>
      This class is templated on both unit type (<code class="computeroutput"><span class="identifier">Unit</span></code>)
      and value type (<code class="computeroutput"