59 lines
4.7 KiB
HTML
59 lines
4.7 KiB
HTML
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<title>THE BOOST MPL LIBRARY: Representing Quantities</title>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./dimensional-analysis.html" class="navigation-link">Dimensional Analysis</a> / <a href="./representing-quantities.html" class="navigation-link">Representing Quantities</a></td>
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<div class="section" id="representing-quantities">
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<h1><a class="toc-backref" href="./dimensional-analysis.html#id43" name="representing-quantities">Representing Quantities</a></h1>
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<p>The types listed above are still pure metadata; to typecheck real
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computations we'll need to somehow bind them to our runtime data.
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A simple numeric value wrapper, parameterized on the number type <tt class="literal"><span class="pre">T</span></tt>
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and on its dimensions, fits the bill:</p>
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<pre class="literal-block">
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template <class T, class Dimensions>
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struct quantity
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{
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explicit quantity(T x)
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: m_value(x)
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{}
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T value() const { return m_value; }
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private:
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T m_value;
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};
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</pre>
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<!-- @ quantity_declaration = len(stack) - 1 # Remember position for later -->
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<p>Now we have a way to represent numbers associated with dimensions.
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For instance, we can say:</p>
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<pre class="literal-block">
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quantity<float,length> l( 1.0f );
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quantity<float,mass> m( 2.0f );
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</pre>
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<p>Note that <tt class="literal"><span class="pre">Dimensions</span></tt> doesn't appear anywhere in the definition
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of <tt class="literal"><span class="pre">quantity</span></tt> outside the template parameter list; its <em>only</em>
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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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Because they do, we cannot make the mistake of assigning a length
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to a mass:</p>
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<pre class="literal-block">
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m = l; // compile-time type error
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</pre>
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<!-- @ example.wrap('void f() {', '}')
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compile('all', pop = 1, expect_error = True) -->
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