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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="phoenix.modules.core.references"></a><a class="link" href="references.html" title="References">References</a>
  28 </h4></div></div></div>
  29 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  30 </pre>
  31 <p>
  32           Values are immutable constants. Attempting to modify a value will result
  33           in a compile time error. When we want the function to modify the parameter,
  34           we use a reference instead. For instance, imagine a lazy function <code class="computeroutput"><span class="identifier">add_assign</span></code>:
  35         </p>
  36 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">add_assign</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">x</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">;</span> <span class="special">}</span> <span class="comment">// pseudo code</span>
  37 </pre>
  38 <p>
  39           Here, we want the first function argument, x, to be mutable. Obviously,
  40           we cannot write:
  41         </p>
  42 <pre class="programlisting"><span class="identifier">add_assign</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="comment">// error first argument is immutable</span>
  43 </pre>
  44 <p>
  45           In C++, we can pass in a reference to a variable as the first argument
  46           in our example above. Yet, by default, the library forces arguments passed
  47           to partially applied functions to be immutable values (see <a class="link" href="values.html" title="Values">Values</a>).
  48           To achieve our intent, we use:
  49         </p>
  50 <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
  51 </pre>
  52 <p>
  53           This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  54           before but instead holds a reference to a variable.
  55         </p>
  56 <p>
  57           We normally don't instantiate <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  58           objects directly. Instead we use:
  59         </p>
  60 <pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span>
  61 </pre>
  62 <p>
  63           For example (where <code class="computeroutput"><span class="identifier">i</span></code> is
  64           an <code class="computeroutput"><span class="keyword">int</span></code> variable):
  65         </p>
  66 <pre class="programlisting"><span class="identifier">add_assign</span><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">),</span> <span class="number">2</span><span class="special">)</span>
  67 </pre>
  68 <h6>
  69 <a name="phoenix.modules.core.references.h0"></a>
  70           <span class="phrase"><a name="phoenix.modules.core.references.evaluating_a_reference"></a></span><a class="link" href="references.html#phoenix.modules.core.references.evaluating_a_reference">Evaluating
  71           a Reference</a>
  72         </h6>
  73 <p>
  74           References are actors. Hence, references can be evaluated. Such invocation
  75           gives the reference's identity. Example:
  76         </p>
  77 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
  78 <span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"Hello World"</span><span class="special">;</span>
  79 <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">&lt;&lt;</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">s</span><span class="special">)();</span>
  80 </pre>
  81 <p>
  82           prints out "3 Hello World"
  83         </p>
  84 <h6>
  85 <a name="phoenix.modules.core.references.h1"></a>
  86           <span class="phrase"><a name="phoenix.modules.core.references.constant_references"></a></span><a class="link" href="references.html#phoenix.modules.core.references.constant_references">Constant
  87           References</a>
  88         </h6>
  89 <p>
  90           Another free function
  91         </p>
  92 <pre class="programlisting"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span>
  93 </pre>
  94 <p>
  95           may also be used. <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code>
  96           creates an <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  97           object. This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
  98           but when the data to be passed as argument to a function is heavy and expensive
  99           to copy by value, the <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code>
 100           offers a lighter alternative.
 101         </p>
 102 </div>
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 105 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014 Joel de Guzman, Dan Marsden, Thomas Heller, John
 106       Fletcher<p>
 107         Distributed under the Boost Software License, Version 1.0. (See accompanying
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