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<div class="section" id="implementing-addition-and">
<h1><a class="toc-backref" href="./dimensional-analysis.html#id44" name="implementing-addition-and">Implementing Addition and Subtraction</a></h1>
<p>We can now easily write the rules for addition and subtraction,
since the dimensions of the arguments must always match.</p>
<pre class="literal-block">
template &lt;class T, class D&gt;
quantity&lt;T,D&gt;
operator+(quantity&lt;T,D&gt; x, quantity&lt;T,D&gt; y)
{
  return quantity&lt;T,D&gt;(x.value() + y.value());
}

template &lt;class T, class D&gt;
quantity&lt;T,D&gt;
operator-(quantity&lt;T,D&gt; x, quantity&lt;T,D&gt; y)
{
  return quantity&lt;T,D&gt;(x.value() - y.value());
}
</pre>
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<p>These operators enable us to write code like:</p>
<pre class="literal-block">
quantity&lt;float,<strong>length</strong>&gt; len1( 1.0f );
quantity&lt;float,<strong>length</strong>&gt; len2( 2.0f );

len1 = len1 + len2;   // OK
</pre>
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<p>but prevent us from trying to add incompatible dimensions:</p>
<pre class="literal-block">
len1 = len2 + quantity&lt;float,<strong>mass</strong>&gt;( 3.7f ); // <strong>error</strong>
</pre>
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