ceph/src/boost/libs/mpl/doc/tutorial/representing-quantities.html

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  10 <title>THE BOOST MPL LIBRARY: Representing Quantities</title>
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  17 <div class="section" id="representing-quantities">
  18 <h1><a class="toc-backref" href="./dimensional-analysis.html#id43" name="representing-quantities">Representing Quantities</a></h1>
  19 <p>The types listed above are still pure metadata; to typecheck real
  20 computations we'll need to somehow bind them to our runtime data.
  21 A simple numeric value wrapper, parameterized on the number type <tt class="literal"><span class="pre">T</span></tt>
  22 and on its dimensions, fits the bill:</p>
  23 <pre class="literal-block">
  24 template &lt;class T, class Dimensions&gt;
  25 struct quantity
  26 {
  27     explicit quantity(T x)
  28        : m_value(x)
  29     {}
  30
  31     T value() const { return m_value; }
  32  private:
  33     T m_value;
  34 };
  35 </pre>
  36 <!-- @ quantity_declaration = len(stack) - 1 # Remember position for later -->
  37 <p>Now we have a way to represent numbers associated with dimensions.
  38 For instance, we can say:</p>
  39 <pre class="literal-block">
  40 quantity&lt;float,length&gt; l( 1.0f );
  41 quantity&lt;float,mass&gt; m( 2.0f );
  42 </pre>
  43 <p>Note that <tt class="literal"><span class="pre">Dimensions</span></tt> doesn't appear anywhere in the definition
  44 of <tt class="literal"><span class="pre">quantity</span></tt> outside the template parameter list; its <em>only</em>
  45 role is to ensure that <tt class="literal"><span class="pre">l</span></tt> and <tt class="literal"><span class="pre">m</span></tt> have different types.
  46 Because they do, we cannot make the mistake of assigning a length
  47 to a mass:</p>
  48 <pre class="literal-block">
  49 m = l;    // compile-time type error
  50 </pre>
  51 <!-- @ example.wrap('void f() {', '}')
  52 compile('all', pop = 1, expect_error = True) -->
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