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  10 <title>THE BOOST MPL LIBRARY: Implementing Addition and Subtraction</title>
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  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 &lt;class T, class D&gt;
  23 quantity&lt;T,D&gt;
  24 operator+(quantity&lt;T,D&gt; x, quantity&lt;T,D&gt; y)
  25 {
  26   return quantity&lt;T,D&gt;(x.value() + y.value());
  27 }
  28
  29 template &lt;class T, class D&gt;
  30 quantity&lt;T,D&gt;
  31 operator-(quantity&lt;T,D&gt; x, quantity&lt;T,D&gt; y)
  32 {
  33   return quantity&lt;T,D&gt;(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&lt;float,<strong>length</strong>&gt; len1( 1.0f );
  40 quantity&lt;float,<strong>length</strong>&gt; 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&lt;float,<strong>mass</strong>&gt;( 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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