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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 10 | <title>THE BOOST MPL LIBRARY: Implementing Addition and Subtraction</title> | |
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| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="implementing-addition-and"> | |
| 18 | <h1><a class="toc-backref" href="./dimensional-analysis.html#id44" name="implementing-addition-and">Implementing Addition and Subtraction</a></h1> | |
| 19 | <p>We can now easily write the rules for addition and subtraction, | |
| 20 | since the dimensions of the arguments must always match.</p> | |
| 21 | <pre class="literal-block"> | |
| 22 | template <class T, class D> | |
| 23 | quantity<T,D> | |
| 24 | operator+(quantity<T,D> x, quantity<T,D> y) | |
| 25 | { | |
| 26 | return quantity<T,D>(x.value() + y.value()); | |
| 27 | } | |
| 28 | ||
| 29 | template <class T, class D> | |
| 30 | quantity<T,D> | |
| 31 | operator-(quantity<T,D> x, quantity<T,D> y) | |
| 32 | { | |
| 33 | return quantity<T,D>(x.value() - y.value()); | |
| 34 | } | |
| 35 | </pre> | |
| 36 | <!-- @ example.append('void test1() {') # function prologue --> | |
| 37 | <p>These operators enable us to write code like:</p> | |
| 38 | <pre class="literal-block"> | |
| 39 | quantity<float,<strong>length</strong>> len1( 1.0f ); | |
| 40 | quantity<float,<strong>length</strong>> len2( 2.0f ); | |
| 41 | ||
| 42 | len1 = len1 + len2; // OK | |
| 43 | </pre> | |
| 44 | <!-- @litre_translator.line_offset -= 7 --> | |
| 45 | <p>but prevent us from trying to add incompatible dimensions:</p> | |
| 46 | <pre class="literal-block"> | |
| 47 | len1 = len2 + quantity<float,<strong>mass</strong>>( 3.7f ); // <strong>error</strong> | |
| 48 | </pre> | |
| 49 | <!-- @ stack[-1].append('}') # last 2 examples together | |
| 50 | compile('all', pop = 1, expect_error = True) | |
| 51 | ||
| 52 | stack[-1].append('}') # just the prior example | |
| 53 | compile('all', pop = None) --> | |
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