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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.modules.core.references"></a><a class="link" href="references.html" title="References">References</a> |
| </h4></div></div></div> |
| <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> |
| </pre> |
| <p> |
| Values are immutable constants. Attempting to modify a value will result |
| in a compile time error. When we want the function to modify the parameter, |
| we use a reference instead. For instance, imagine a lazy function <code class="computeroutput"><span class="identifier">add_assign</span></code>: |
| </p> |
| <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> |
| </pre> |
| <p> |
| Here, we want the first function argument, x, to be mutable. Obviously, |
| we cannot write: |
| </p> |
| <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> |
| </pre> |
| <p> |
| In C++, we can pass in a reference to a variable as the first argument |
| in our example above. Yet, by default, the library forces arguments passed |
| to partially applied functions to be immutable values (see <a class="link" href="values.html" title="Values">Values</a>). |
| To achieve our intent, we use: |
| </p> |
| <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> |
| </pre> |
| <p> |
| 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> |
| before but instead holds a reference to a variable. |
| </p> |
| <p> |
| 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> |
| objects directly. Instead we use: |
| </p> |
| <pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span> |
| </pre> |
| <p> |
| For example (where <code class="computeroutput"><span class="identifier">i</span></code> is |
| an <code class="computeroutput"><span class="keyword">int</span></code> variable): |
| </p> |
| <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> |
| </pre> |
| <h6> |
| <a name="phoenix.modules.core.references.h0"></a> |
| <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 |
| a Reference</a> |
| </h6> |
| <p> |
| References are actors. Hence, references can be evaluated. Such invocation |
| gives the reference's identity. Example: |
| </p> |
| <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> |
| <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> |
| <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> |
| </pre> |
| <p> |
| prints out "3 Hello World" |
| </p> |
| <h6> |
| <a name="phoenix.modules.core.references.h1"></a> |
| <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 |
| References</a> |
| </h6> |
| <p> |
| Another free function |
| </p> |
| <pre class="programlisting"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span> |
| </pre> |
| <p> |
| 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> |
| 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> |
| 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> |
| but when the data to be passed as argument to a function is heavy and expensive |
| 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> |
| offers a lighter alternative. |
| </p> |
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| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas |
| Heller, John Fletcher<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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