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vmd_modifiers_splitting.html

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

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

  27. <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_splitting"></a><a class="link" href="vmd_modifiers_splitting.html" title="Splitting modifiers">Splitting
    28. 

  28.       modifiers</a>
    29. 

  29. </h3></div></div></div>
    30. 

  30. <p>
    31. 

  31.         The BOOST_VMD_ELEM macro, which by default just returns an element of a sequence,
    32. 

  32.         has a usage where you can have it return both the element and the remaining
    33. 

  33.         part of the sequence after the element, or even just the remaining part of
    34. 

  34.         the sequence after the element by itself. This offers a form of splitting
    35. 

  35.         the sequence on a particular element. When used to return the remaining part
    36. 

  36.         of a sequence the remaining data may subsequently be treated as a VMD sequence
    37. 

  37.         again.
    38. 

  38.       </p>
    39. 

  39. <p>
    40. 

  40.         To do this another set of optional modifiers are used which will be called
    41. 

  41.         'splitting modifers'. These modifiers are:
    42. 

  42.       </p>
    43. 

  43. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    44. 

  44. <li class="listitem">
    45. 

  45.             BOOST_VMD_RETURN_AFTER, which returns both the element information and
    46. 

  46.             the rest of the sequence after the element as a two-element tuple
    47. 

  47.           </li>
    48. 

  48. <li class="listitem">
    49. 

  49.             BOOST_VMD_RETURN_ONLY_AFTER, which returns only the rest of the sequence
    50. 

  50.             after the element specified
    51. 

  51.           </li>
    52. 

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

  53.             BOOST_VMD_RETURN_NO_AFTER, this is the internal default which only returns
    54. 

  54.             the element itself. It need never be specified but may be used to override
    55. 

  55.             a previous splitting modifier specified as an optional parameter.
    56. 

  56.           </li>
    57. 

  57. </ul></div>
    58. 

  58. <p>
    59. 

  59.         If more than one of the splitting modifiers are specified as optional parameters
    60. 

  60.         to BOOST_VMD_ELEM the last one specified is in effect.
    61. 

  61.       </p>
    62. 

  62. <p>
    63. 

  63.         The splitting modifiers BOOST_VMD_RETURN_NO_AFTER and BOOST_VMD_RETURN_AFTER
    64. 

  64.         work with either return type modifiers or filtering modifiers if they are
    65. 

  65.         used. The splitting modifier BOOST_VMD_RETURN_ONLY_AFTER works with filtering
    66. 

  66.         modifiers if it is used and any return type modifiers will be ignored. Optional
    67. 

  67.         modifiers may occur in any order after the required parameters to BOOST_VMD_ELEM.
    68. 

  68.       </p>
    69. 

  69. <p>
    70. 

  70.         If BOOST_VMD_RETURN_AFTER is in effect and an element is not found, either
    71. 

  71.         because the element number is out of range for the sequence or because filtering
    72. 

  72.         does not match the element type, a tuple will still be returned but both
    73. 

  73.         its elements will be empty.
    74. 

  74.       </p>
    75. 

  75. <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
    76. 

  76. 

  77. <span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANAME</span> <span class="special">(</span><span class="identifier">ANAME</span><span class="special">)</span> <span class="comment">// an identifier must always be registered to be found by VMD</span>
    78. 

  78. <span class="preprocessor">#define</span> <span class="identifier">A_SEQUENCE</span> <span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">)</span> <span class="number">46</span> <span class="special">(</span><span class="identifier">list_data1</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="identifier">ANAME</span>
    79. 

  79. 

  80. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(list_data1,BOOST_PP_NIL)'</span>
    81. 

  81. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_NO_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(list_data1,BOOST_PP_NIL)'</span>
    82. 

  82. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((list_data1,BOOST_PP_NIL),BOOST_VMD_TYPE_SEQ ANAME)'</span>
    83. 

  83. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'BOOST_VMD_TYPE_SEQ ANAME'</span>
    84. 

  84. 

  85. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
    86. 

  86. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_NO_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
    87. 

  87. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(,)'</span>
    88. 

  88. <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
    89. 

  89. </pre>
    90. 

  90. <p>
    91. 

  91.         Combining splitting modifiers with return type modifiers:
    92. 

  92.       </p>
    93. 

  93. <pre class="programlisting"><span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((BOOST_VMD_TYPE_LIST,(list_data1,BOOST_PP_NIL)),BOOST_VMD_TYPE_SEQ ANAME)'</span>
    94. 

  94. </pre>
    95. 

  95. <p>
    96. 

  96.         Combining splitting modifiers with filtering modifiers:
    97. 

  97.       </p>
    98. 

  98. <pre class="programlisting"><span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((list_data1,BOOST_PP_NIL),BOOST_VMD_TYPE_SEQ ANAME)'</span>
    99. 

  99. </pre>
    100. 

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