1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> 5<!-- Software License, Version 1.0. (See accompanying --> 6<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 7<head> 8<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 9<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> 10<title>THE BOOST MPL LIBRARY: Implementing Addition and Subtraction</title> 11<link rel="stylesheet" href="../style.css" type="text/css" /> 12</head> 13<body class="docframe"> 14<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> 15<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> 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, 20since the dimensions of the arguments must always match.</p> 21<pre class="literal-block"> 22template <class T, class D> 23quantity<T,D> 24operator+(quantity<T,D> x, quantity<T,D> y) 25{ 26 return quantity<T,D>(x.value() + y.value()); 27} 28 29template <class T, class D> 30quantity<T,D> 31operator-(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"> 39quantity<float,<strong>length</strong>> len1( 1.0f ); 40quantity<float,<strong>length</strong>> len2( 2.0f ); 41 42len1 = 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"> 47len1 = len2 + quantity<float,<strong>mass</strong>>( 3.7f ); // <strong>error</strong> 48</pre> 49<!-- @ stack[-1].append('}') # last 2 examples together 50compile('all', pop = 1, expect_error = True) 51 52stack[-1].append('}') # just the prior example 53compile('all', pop = None) --> 54</div> 55 56<div class="footer-separator"></div> 57<table class="footer"><tr class="footer"><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> 58</tr></table></body> 59</html> 60