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