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1 | <html> |
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| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>References</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</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">></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">&</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"><</span><span class="identifier">T</span><span class="special">>::</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"><</span><span class="identifier">T</span><span class="special">>::</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"><</span><span class="identifier">T</span><span class="special">>::</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><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"><<</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special"><<</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><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"><</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">>::</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"><</span><span class="identifier">T</span><span class="special">>::</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> | |
| 103 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 104 | <td align="left"></td> | |
| 105 | <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas | |
| 106 | Heller, John Fletcher<p> | |
| 107 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 108 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
| 109 | </p> | |
| 110 | </div></td> | |
| 111 | </tr></table> | |
| 112 | <hr> | |
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| 115 | </div> | |
| 116 | </body> | |
| 117 | </html> |