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- <h1><a class="toc-backref" href="./dimensional-analysis.html#id43" name="representing-quantities">Representing Quantities</a></h1>
- <p>The types listed above are still pure metadata; to typecheck real
- computations we'll need to somehow bind them to our runtime data.
- A simple numeric value wrapper, parameterized on the number type <tt class="literal"><span class="pre">T</span></tt>
- and on its dimensions, fits the bill:</p>
- <pre class="literal-block">
- template <class T, class Dimensions>
- struct quantity
- {
- explicit quantity(T x)
- : m_value(x)
- {}
- T value() const { return m_value; }
- private:
- T m_value;
- };
- </pre>
- <!-- @ quantity_declaration = len(stack) - 1 # Remember position for later -->
- <p>Now we have a way to represent numbers associated with dimensions.
- For instance, we can say:</p>
- <pre class="literal-block">
- quantity<float,length> l( 1.0f );
- quantity<float,mass> m( 2.0f );
- </pre>
- <p>Note that <tt class="literal"><span class="pre">Dimensions</span></tt> doesn't appear anywhere in the definition
- of <tt class="literal"><span class="pre">quantity</span></tt> outside the template parameter list; its <em>only</em>
- 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.
- Because they do, we cannot make the mistake of assigning a length
- to a mass:</p>
- <pre class="literal-block">
- m = l; // compile-time type error
- </pre>
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