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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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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 <class T, class Dimensions> | |
| 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<float,length> l( 1.0f ); | |
| 41 | quantity<float,mass> 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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