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 26<div class="titlepage"><div><div><h3 class="title">
 27<a name="phoenix.starter_kit.values"></a><a class="link" href="values.html" title="Values">Values</a>
 28</h3></div></div></div>
 29<p>
 30        Values are functions! Examples:
 31      </p>
 32<pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span>
 33<span class="identifier">val</span><span class="special">(</span><span class="string">"Hello, World"</span><span class="special">)</span>
 34</pre>
 35<p>
 36        The first evaluates to a nullary function (a function taking no arguments)
 37        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
 38        that returns a <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span><span class="special">(&amp;)[</span><span class="number">13</span><span class="special">]</span></code>, <code class="computeroutput"><span class="string">"Hello,
 39        World"</span></code>.
 40      </p>
 41<a name="phoenix.starter_kit.values.lazy_evaluation"></a><h5>
 42<a name="id801318"></a>
 43        <a class="link" href="values.html#phoenix.starter_kit.values.lazy_evaluation">Lazy Evaluation</a>
 44      </h5>
 45<p>
 46        Confused? <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> is a unary
 47        function, you say? Yes it is. However, read carefully: <span class="emphasis"><em>"evaluates
 48        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>
 49        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>
 50        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>
 51        returns a function, you can invoke it. Example:
 52      </p>
 53<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 54</pre>
 55<p>
 56        (See <a href="../../../../example/values.cpp" target="_top">values.cpp</a>)
 57      </p>
 58<div class="sidebar">
 59<div class="titlepage"></div>
 60<p>
 61        <span class="inlinemediaobject"><img src="../../images/tip.png" alt="tip"></span> Learn more about values <a class="link" href="../modules/core/values.html" title="Values">here.</a>
 62      </p>
 63</div>
 64<p>
 65        The second function call (the one with no arguments) calls the nullary function
 66        which then returns <code class="computeroutput"><span class="number">3</span></code>. The need
 67        for a second function call is the reason why the function is said to be
 68        <span class="bold"><strong><span class="emphasis"><em>Lazily Evaluated</em></span></strong></span>. The
 69        first call doesn't do anything. You need a second call to finally evaluate
 70        the thing. The first call lazily evaluates the function; i.e. doesn't do
 71        anything and defers the evaluation for later.
 72      </p>
 73<a name="phoenix.starter_kit.values.callbacks"></a><h5>
 74<a name="id801543"></a>
 75        <a class="link" href="values.html#phoenix.starter_kit.values.callbacks">Callbacks</a>
 76      </h5>
 77<p>
 78        It may not be immediately apparent how lazy evaluation can be useful by just
 79        looking at the example above. Putting the first and second function call
 80        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
 81        they are actually used that way), will make it clear. Example:
 82      </p>
 83<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">&gt;</span>
 84<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>
 85<span class="special">{</span>
 86    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 87<span class="special">}</span>
 88
 89<span class="keyword">int</span>
 90<span class="identifier">main</span><span class="special">()</span>
 91<span class="special">{</span>
 92    <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>
 93    <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>
 94    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 95<span class="special">}</span>
 96</pre>
 97<p>
 98        (See <a href="../../../../example/callback.cpp" target="_top">callback.cpp</a>)
 99      </p>
100</div>
101<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
102<td align="left"></td>
103<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
104        Distributed under the Boost Software License, Version 1.0. (See accompanying
105        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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