60 lines
4.4 KiB
HTML
60 lines
4.4 KiB
HTML
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: Implementing Addition and Subtraction</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./representing-quantities.html" class="navigation-link">Prev</a> <a href="./implementing.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./representing-quantities.html" class="navigation-link">Back</a> <a href="./implementing.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./dimensional-analysis.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
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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="./implementing-addition-and.html" class="navigation-link">Implementing Addition and Subtraction</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="implementing-addition-and">
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<h1><a class="toc-backref" href="./dimensional-analysis.html#id44" name="implementing-addition-and">Implementing Addition and Subtraction</a></h1>
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<p>We can now easily write the rules for addition and subtraction,
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since the dimensions of the arguments must always match.</p>
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<pre class="literal-block">
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template <class T, class D>
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quantity<T,D>
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operator+(quantity<T,D> x, quantity<T,D> y)
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{
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return quantity<T,D>(x.value() + y.value());
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}
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template <class T, class D>
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quantity<T,D>
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operator-(quantity<T,D> x, quantity<T,D> y)
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{
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return quantity<T,D>(x.value() - y.value());
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}
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</pre>
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<!-- @ example.append('void test1() {') # function prologue -->
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<p>These operators enable us to write code like:</p>
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<pre class="literal-block">
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quantity<float,<strong>length</strong>> len1( 1.0f );
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quantity<float,<strong>length</strong>> len2( 2.0f );
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len1 = len1 + len2; // OK
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</pre>
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<!-- @litre_translator.line_offset -= 7 -->
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<p>but prevent us from trying to add incompatible dimensions:</p>
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<pre class="literal-block">
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len1 = len2 + quantity<float,<strong>mass</strong>>( 3.7f ); // <strong>error</strong>
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</pre>
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<!-- @ stack[-1].append('}') # last 2 examples together
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compile('all', pop = 1, expect_error = True)
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stack[-1].append('}') # just the prior example
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compile('all', pop = None) -->
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