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  25. <div class="section">
    26. 

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

  27. <a name="phoenix.starter_kit.lazy_operators"></a><a class="link" href="lazy_operators.html" title="Lazy Operators">Lazy Operators</a>
    28. 

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

  29. <p>
    30. 

  30.         You can use the usual set of operators to form expressions. Examples:
    31. 

  31.       </p>
    32. 

  32. <pre class="programlisting"><span class="identifier">arg1</span> <span class="special">*</span> <span class="identifier">arg1</span>
    33. 

  33. <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">arg1</span> <span class="special">+</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">z</span><span class="special">)</span>
    34. 

  34. <span class="identifier">arg1</span> <span class="special">=</span> <span class="identifier">arg2</span> <span class="special">+</span> <span class="special">(</span><span class="number">3</span> <span class="special">*</span> <span class="identifier">arg3</span><span class="special">)</span>
    35. 

  35. <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">[</span><span class="identifier">arg2</span><span class="special">]</span> <span class="comment">// assuming arg1 is indexable and arg2 is a valid index
    36. 

  36. </span></pre>
    37. 

  37. <p>
    38. 

  38.         Note the expression: <code class="computeroutput"><span class="number">3</span> <span class="special">*</span>
    39. 

  39.         <span class="identifier">arg3</span></code>. This expression is actually
    40. 

  40.         a short-hand equivalent to: <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span>
    41. 

  41.         <span class="special">*</span> <span class="identifier">arg3</span></code>.
    42. 

  42.         In most cases, like above, you can get away with it. But in some cases, you
    43. 

  43.         will have to explicitly wrap your values in <code class="computeroutput"><span class="identifier">val</span></code>.
    44. 

  44.         Rules of thumb:
    45. 

  45.       </p>
    46. 

  46. <div class="itemizedlist"><ul class="itemizedlist" type="disc">
    47. 

  47. <li class="listitem">
    48. 

  48.             In a binary expression (e.g. <code class="computeroutput"><span class="number">3</span>
    49. 

  49.             <span class="special">*</span> <span class="identifier">arg3</span></code>),
    50. 

  50.             at least one of the operands must be a phoenix primitive or expression.
    51. 

  51.           </li>
    52. 

  52. <li class="listitem">
    53. 

  53.             In a unary expression (e.g. <code class="computeroutput"><span class="identifier">arg1</span><span class="special">++</span></code>), the single operand must be a phoenix
    54. 

  54.             primitive or expression.
    55. 

  55.           </li>
    56. 

  56. </ul></div>
    57. 

  57. <p>
    58. 

  58.         If these basic rules are not followed, the result is either an error, or
    59. 

  59.         is immediately evaluated. Some examples:
    60. 

  60.       </p>
    61. 

  61. <pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="number">123</span>    <span class="comment">// lazy
    62. 

  62. </span><span class="identifier">x</span> <span class="special">=</span> <span class="number">123</span>         <span class="comment">// immediate
    63. 

  63. </span>
    64. 

  64. <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span>       <span class="comment">// lazy
    65. 

  65. </span><span class="identifier">x</span><span class="special">[</span><span class="number">0</span><span class="special">]</span>            <span class="comment">// immediate
    66. 

  66. </span>
    67. 

  67. <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</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="comment">// lazy
    68. 

  68. </span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span>       <span class="comment">// lazy (equivalent to ref(x)[val(i)])
    69. 

  69. </span><span class="identifier">x</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="comment">// illegal (x is not a phoenix primitive or expression)
    70. 

  70. </span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</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="comment">// illegal (x is not a phoenix primitive or expression)
    71. 

  71. </span></pre>
    72. 

  72. <div class="sidebar">
    73. 

  73. <div class="titlepage"></div>
    74. 

  74. <p>
    75. 

  75.         <span class="inlinemediaobject"><img src="../../images/tip.png" alt="tip"></span> Learn more about operators <a class="link" href="../modules/operator.html" title="Operator">here.</a>
    76. 

  76.       </p>
    77. 

  77. </div>
    78. 

  78. <a name="phoenix.starter_kit.lazy_operators.first_practical_example"></a><h5>
    79. 

  79. <a name="id802885"></a>
    80. 

  80.         <a class="link" href="lazy_operators.html#phoenix.starter_kit.lazy_operators.first_practical_example">First
    81. 

  81.         Practical Example</a>
    82. 

  82.       </h5>
    83. 

  83. <p>
    84. 

  84.         We've covered enough ground to present a real world example. We want to find
    85. 

  85.         the first odd number in an STL container. Normally we use a functor (function
    86. 

  86.         object) or a function pointer and pass that in to STL's <code class="computeroutput"><span class="identifier">find_if</span></code>
    87. 

  87.         generic function:
    88. 

  88.       </p>
    89. 

  89. <p>
    90. 

  90.         Write a function:
    91. 

  91.       </p>
    92. 

  92. <pre class="programlisting"><span class="keyword">bool</span>
    93. 

  93. <span class="identifier">is_odd</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">arg1</span><span class="special">)</span>
    94. 

  94. <span class="special">{</span>
    95. 

  95.     <span class="keyword">return</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">;</span>
    96. 

  96. <span class="special">}</span>
    97. 

  97. </pre>
    98. 

  98. <p>
    99. 

  99.         Pass a pointer to the function to STL's <code class="computeroutput"><span class="identifier">find_if</span></code>
    100. 

  100.         algorithm:
    101. 

  101.       </p>
    102. 

  102. <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="special">&amp;</span><span class="identifier">is_odd</span><span class="special">)</span>
    103. 

  103. </pre>
    104. 

  104. <p>
    105. 

  105.         Using Phoenix, the same can be achieved directly with a one-liner:
    106. 

  106.       </p>
    107. 

  107. <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)</span>
    108. 

  108. </pre>
    109. 

  109. <p>
    110. 

  110.         The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span>
    111. 

  111.         <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> automagically creates a functor with the
    112. 

  112.         expected behavior. In FP, this unnamed function is called a lambda function.
    113. 

  113.         Unlike the function pointer version, which is monomorphic (expects and works
    114. 

  114.         only with a fixed type int argument), the Phoenix version is fully polymorphic
    115. 

  115.         and works with any container (of ints, of longs, of bignum, etc.) as long
    116. 

  116.         as its elements can handle the <code class="computeroutput"><span class="identifier">arg1</span>
    117. 

  117.         <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> expression.
    118. 

  118.       </p>
    119. 

  119. <p>
    120. 

  120.         (See <a href="../../../../example/find_if.cpp" target="_top">find_if.cpp</a>)
    121. 

  121.       </p>
    122. 

  122. <div class="sidebar">
    123. 

  123. <div class="titlepage"></div>
    124. 

  124. <p>
    125. 

  125.         <span class="inlinemediaobject"><img src="../../images/tip.png" alt="tip"></span> ...<span class="bold"><strong>That's it, we're done</strong></span>.
    126. 

  126.         Well if you wish to know a little bit more, read on...
    127. 

  127.       </p>
    128. 

  128. </div>
    129. 

  129. </div>
    130. 

  130. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    131. 

  131. <td align="left"></td>
    132. 

  132. <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
    133. 

  133.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    134. 

  134.         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>)
    135. 

  135.       </p>
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