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| <div class="section" id="representing-quantities"> |
| <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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