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  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  27. <a name="phoenix.starter_kit.values"></a><a class="link" href="values.html" title="Values">Values</a>
    28. 

  28. </h3></div></div></div>
    29. 

  29. <p>
    30. 

  30.         Values are functions! Examples:
    31. 

  31.       </p>
    32. 

  32. <pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span>
    33. 

  33. <span class="identifier">val</span><span class="special">(</span><span class="string">"Hello, World"</span><span class="special">)</span>
    34. 

  34. </pre>
    35. 

  35. <p>
    36. 

  36.         The first evaluates to a nullary function (a function taking no arguments)
    37. 

  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. 

  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. 

  39.         World"</span></code>.
    40. 

  40.       </p>
    41. 

  41. <a name="phoenix.starter_kit.values.lazy_evaluation"></a><h5>
    42. 

  42. <a name="id801318"></a>
    43. 

  43.         <a class="link" href="values.html#phoenix.starter_kit.values.lazy_evaluation">Lazy Evaluation</a>
    44. 

  44.       </h5>
    45. 

  45. <p>
    46. 

  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. 

  47.         function, you say? Yes it is. However, read carefully: <span class="emphasis"><em>"evaluates
    48. 

  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. 

  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. 

  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. 

  51.         returns a function, you can invoke it. Example:
    52. 

  52.       </p>
    53. 

  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. 

  54. </pre>
    55. 

  55. <p>
    56. 

  56.         (See <a href="../../../../example/values.cpp" target="_top">values.cpp</a>)
    57. 

  57.       </p>
    58. 

  58. <div class="sidebar">
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  59. <div class="titlepage"></div>
    60. 

  60. <p>
    61. 

  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. 

  62.       </p>
    63. 

  63. </div>
    64. 

  64. <p>
    65. 

  65.         The second function call (the one with no arguments) calls the nullary function
    66. 

  66.         which then returns <code class="computeroutput"><span class="number">3</span></code>. The need
    67. 

  67.         for a second function call is the reason why the function is said to be
    68. 

  68.         <span class="bold"><strong><span class="emphasis"><em>Lazily Evaluated</em></span></strong></span>. The
    69. 

  69.         first call doesn't do anything. You need a second call to finally evaluate
    70. 

  70.         the thing. The first call lazily evaluates the function; i.e. doesn't do
    71. 

  71.         anything and defers the evaluation for later.
    72. 

  72.       </p>
    73. 

  73. <a name="phoenix.starter_kit.values.callbacks"></a><h5>
    74. 

  74. <a name="id801543"></a>
    75. 

  75.         <a class="link" href="values.html#phoenix.starter_kit.values.callbacks">Callbacks</a>
    76. 

  76.       </h5>
    77. 

  77. <p>
    78. 

  78.         It may not be immediately apparent how lazy evaluation can be useful by just
    79. 

  79.         looking at the example above. Putting the first and second function call
    80. 

  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. 

  81.         they are actually used that way), will make it clear. Example:
    82. 

  82.       </p>
    83. 

  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. 

  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. 

  85. <span class="special">{</span>
    86. 

  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. 

  87. <span class="special">}</span>
    88. 

  88. 

  89. <span class="keyword">int</span>
    90. 

  90. <span class="identifier">main</span><span class="special">()</span>
    91. 

  91. <span class="special">{</span>
    92. 

  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. 

  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. 

  94.     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
    95. 

  95. <span class="special">}</span>
    96. 

  96. </pre>
    97. 

  97. <p>
    98. 

  98.         (See <a href="../../../../example/callback.cpp" target="_top">callback.cpp</a>)
    99. 

  99.       </p>
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  104.         Distributed under the Boost Software License, Version 1.0. (See accompanying
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  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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