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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.starter_kit.primitives.values"></a><a class="link" href="values.html" title="Values">Values</a> |
| </h4></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> |
| <a name="phoenix.starter_kit.primitives.values.lazy_evaluation"></a><h6> |
| <a name="id680696"></a> |
| <a class="link" href="values.html#phoenix.starter_kit.primitives.values.lazy_evaluation">Lazy |
| Evaluation</a> |
| </h6> |
| <p> |
| 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 |
| function, you say? Yes it is. 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" alt="tip"></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> |
| <a name="phoenix.starter_kit.primitives.values.callbacks"></a><h6> |
| <a name="id680921"></a> |
| <a class="link" href="values.html#phoenix.starter_kit.primitives.values.callbacks">Callbacks</a> |
| </h6> |
| <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> |
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| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<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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