| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Values</title> |
| <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> |
| <link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> |
| <link rel="up" href="../starter_kit.html" title="Starter Kit"> |
| <link rel="prev" href="../starter_kit.html" title="Starter Kit"> |
| <link rel="next" href="references.html" title="References"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../starter_kit.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../starter_kit.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.starter_kit.values"></a><a class="link" href="values.html" title="Values">Values</a> |
| </h3></div></div></div> |
| <p> |
| Values are functions! Examples: |
| </p> |
| <pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span> |
| <span class="identifier">val</span><span class="special">(</span><span class="string">"Hello, World"</span><span class="special">)</span> |
| </pre> |
| <p> |
| The first evaluates to a nullary function (a function taking no arguments) |
| that returns an <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="number">3</span></code>. The second evaluates to a nullary function |
| that returns a <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span><span class="special">(&)[</span><span class="number">13</span><span class="special">]</span></code>, <code class="computeroutput"><span class="string">"Hello, |
| World"</span></code>. |
| </p> |
| <h5> |
| <a name="phoenix.starter_kit.values.h0"></a> |
| <span class="phrase"><a name="phoenix.starter_kit.values.lazy_evaluation"></a></span><a class="link" href="values.html#phoenix.starter_kit.values.lazy_evaluation">Lazy |
| Evaluation</a> |
| </h5> |
| <p> |
| Confused? <code class="computeroutput"><span class="identifier">val</span></code> is a unary |
| function and <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> invokes |
| it, you say? Yes. However, read carefully: <span class="emphasis"><em>"evaluates to a |
| nullary function"</em></span>. <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> |
| evaluates to (returns) a nullary function. Aha! <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> |
| returns a function! So, since <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> |
| returns a function, you can invoke it. Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> |
| </pre> |
| <p> |
| (See <a href="../../../../example/values.cpp" target="_top">values.cpp</a>) |
| </p> |
| <div class="sidebar"> |
| <div class="titlepage"></div> |
| <p> |
| <span class="inlinemediaobject"><img src="../../images/tip.png"></span> |
| Learn more about values <a class="link" href="../modules/core/values.html" title="Values">here.</a> |
| </p> |
| </div> |
| <p> |
| The second function call (the one with no arguments) calls the nullary function |
| which then returns <code class="computeroutput"><span class="number">3</span></code>. The need |
| for a second function call is the reason why the function is said to be |
| <span class="bold"><strong><span class="emphasis"><em>Lazily Evaluated</em></span></strong></span>. The |
| first call doesn't do anything. You need a second call to finally evaluate |
| the thing. The first call lazily evaluates the function; i.e. doesn't do |
| anything and defers the evaluation for later. |
| </p> |
| <h5> |
| <a name="phoenix.starter_kit.values.h1"></a> |
| <span class="phrase"><a name="phoenix.starter_kit.values.callbacks"></a></span><a class="link" href="values.html#phoenix.starter_kit.values.callbacks">Callbacks</a> |
| </h5> |
| <p> |
| It may not be immediately apparent how lazy evaluation can be useful by just |
| looking at the example above. Putting the first and second function call |
| in a single line is really not very useful. However, thinking of <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> as a callback function (and in most cases |
| they are actually used that way), will make it clear. Example: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">f</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="keyword">int</span> |
| <span class="identifier">main</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">print</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">));</span> |
| <span class="identifier">print</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="string">"Hello World"</span><span class="special">));</span> |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| (See <a href="../../../../example/callback.cpp" target="_top">callback.cpp</a>) |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../starter_kit.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../starter_kit.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
