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   4 <title>Chapter&#160;1.&#160;The Variadic Macro Data Library 1.9</title>
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  21 <div class="chapter">
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  23 <div><h2 class="title">
  24 <a name="variadic_macro_data"></a>Chapter&#160;1.&#160;The Variadic Macro Data Library 1.9</h2></div>
  25 <div><div class="author"><h3 class="author">
  26 <span class="firstname">Edward</span> <span class="surname">Diener</span>
  27 </h3></div></div>
  28 <div><p class="copyright">Copyright &#169; 2010-2017 Tropic Software
  29       East Inc</p></div>
  30 </div></div>
  31 <div class="toc">
  32 <p><b>Table of Contents</b></p>
  33 <dl class="toc">
  34 <dt><span class="section"><a href="index.html#variadic_macro_data.vmd_intro">Introduction</a></span></dt>
  35 <dt><span class="section"><a href="variadic_macro_data/vmd_naming.html">Naming conventions</a></span></dt>
  36 <dt><span class="section"><a href="variadic_macro_data/vmd_whyhow.html">Why and how to use</a></span></dt>
  37 <dt><span class="section"><a href="variadic_macro_data/vmd_vmacros.html">Using variadic macros</a></span></dt>
  38 <dt><span class="section"><a href="variadic_macro_data/vmd_vc.html">Visual C++ define</a></span></dt>
  39 <dt><span class="section"><a href="variadic_macro_data/vmd_detail.html">Functional groups</a></span></dt>
  40 <dt><span class="section"><a href="variadic_macro_data/vmd_data_types.html">Data types</a></span></dt>
  41 <dt><span class="section"><a href="variadic_macro_data/vmd_specific.html">Specific macros for working
  42     with data types</a></span></dt>
  43 <dd><dl>
  44 <dt><span class="section"><a href="variadic_macro_data/vmd_specific.html#variadic_macro_data.vmd_specific.vmd_test_empty">Emptiness</a></span></dt>
  45 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_constraints.html">Macro
  46       constraints</a></span></dt>
  47 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_identifier.html">Identifiers</a></span></dt>
  48 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_number.html">Numbers</a></span></dt>
  49 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_type.html">Types</a></span></dt>
  50 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_pp_data_types.html">VMD
  51       and Boost PP data types</a></span></dt>
  52 <dt><span class="section"><a href="variadic_macro_data/vmd_specific/vmd_identifying.html">Identifying
  53       data types</a></span></dt>
  54 </dl></dd>
  55 <dt><span class="section"><a href="variadic_macro_data/vmd_generic.html">Generic macros for working
  56     with data types</a></span></dt>
  57 <dd><dl>
  58 <dt><span class="section"><a href="variadic_macro_data/vmd_generic.html#variadic_macro_data.vmd_generic.vmd_sequence">Parsing
  59       sequences</a></span></dt>
  60 <dd><dl>
  61 <dt><span class="section"><a href="variadic_macro_data/vmd_generic.html#variadic_macro_data.vmd_generic.vmd_sequence.vmd_sequence_convert">Converting
  62         sequences</a></span></dt>
  63 <dt><span class="section"><a href="variadic_macro_data/vmd_generic.html#variadic_macro_data.vmd_generic.vmd_sequence.vmd_sequence_access">Accessing
  64         a sequence element</a></span></dt>
  65 </dl></dd>
  66 <dt><span class="section"><a href="variadic_macro_data/vmd_generic/vmd_convert_sequence.html">Getting
  67       the type of data</a></span></dt>
  68 <dt><span class="section"><a href="variadic_macro_data/vmd_generic/vmd_assert.html">Testing for
  69       equality and inequality</a></span></dt>
  70 </dl></dd>
  71 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers.html">Macros with modifiers</a></span></dt>
  72 <dd><dl>
  73 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type">Return
  74       type modifiers</a></span></dt>
  75 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers/vmd_modifiers_filter.html">Filtering
  76       modifiers</a></span></dt>
  77 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers/vmd_modifiers_identifier.html">Identifier
  78       modifiers</a></span></dt>
  79 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers/vmd_modifiers_splitting.html">Splitting
  80       modifiers</a></span></dt>
  81 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers/vmd_modifiers_index.html">Index
  82       modifiers</a></span></dt>
  83 <dt><span class="section"><a href="variadic_macro_data/vmd_modifiers/vmd_modifiers_single.html">Modifiers
  84       and the single-element sequence</a></span></dt>
  85 </dl></dd>
  86 <dt><span class="section"><a href="variadic_macro_data/vmd_identifier_subtype.html">Identifier
  87     subtypes</a></span></dt>
  88 <dt><span class="section"><a href="variadic_macro_data/vmd_useful.html">Useful variadic macros
  89     not in Boost PP</a></span></dt>
  90 <dd><dl>
  91 <dt><span class="section"><a href="variadic_macro_data/vmd_useful.html#variadic_macro_data.vmd_useful.vmd_assert">Asserting
  92       and data types</a></span></dt>
  93 <dt><span class="section"><a href="variadic_macro_data/vmd_useful/vmd_identity.html">Generating
  94       emptiness and identity</a></span></dt>
  95 <dt><span class="section"><a href="variadic_macro_data/vmd_useful/vmd_empty_ppdata.html">Functionality
  96       for "empty" seqs and tuples</a></span></dt>
  97 </dl></dd>
  98 <dt><span class="section"><a href="variadic_macro_data/vmd_internal_macros.html">Controlling internal
  99     usage</a></span></dt>
 100 <dt><span class="section"><a href="variadic_macro_data/vmd_reentrant.html">Boost PP re-entrant
 101     versions</a></span></dt>
 102 <dt><span class="section"><a href="variadic_macro_data/vmd_dynamic_typing.html">Input as dynamic
 103     types</a></span></dt>
 104 <dt><span class="section"><a href="variadic_macro_data/vmd_vc_isms.html">Visual C++ gotchas in
 105     VMD</a></span></dt>
 106 <dt><span class="section"><a href="variadic_macro_data/vmd_conv.html">Version 1.7 to 1.8 conversion</a></span></dt>
 107 <dt><span class="section"><a href="variadic_macro_data/vmd_examples.html">Examples using VMD functionality</a></span></dt>
 108 <dt><span class="section"><a href="variadic_macro_data_reference.html">Variadic Macro Data Reference</a></span></dt>
 109 <dd><dl>
 110 <dt><span class="section"><a href="variadic_macro_data_reference.html#header.boost.vmd.array.to_seq_hpp">Header &lt;boost/vmd/array/to_seq.hpp&gt;</a></span></dt>
 111 <dd><dl></dl></dd>
 112 <dt><span class="section"><a href="header/boost/vmd/list/to_seq_hpp.html">Header &lt;boost/vmd/list/to_seq.hpp&gt;</a></span></dt>
 113 <dd><dl></dl></dd>
 114 <dt><span class="section"><a href="header/boost/vmd/to_seq_hpp.html">Header &lt;boost/vmd/to_seq.hpp&gt;</a></span></dt>
 115 <dd><dl></dl></dd>
 116 <dt><span class="section"><a href="header/boost/vmd/tuple/to_seq_hpp.html">Header &lt;boost/vmd/tuple/to_seq.hpp&gt;</a></span></dt>
 117 <dd><dl></dl></dd>
 118 <dt><span class="section"><a href="header/boost/vmd/array/to_tuple_hpp.html">Header &lt;boost/vmd/array/to_tuple.hpp&gt;</a></span></dt>
 119 <dd><dl></dl></dd>
 120 <dt><span class="section"><a href="header/boost/vmd/list/to_tuple_hpp.html">Header &lt;boost/vmd/list/to_tuple.hpp&gt;</a></span></dt>
 121 <dd><dl></dl></dd>
 122 <dt><span class="section"><a href="header/boost/vmd/seq/to_tuple_hpp.html">Header &lt;boost/vmd/seq/to_tuple.hpp&gt;</a></span></dt>
 123 <dd><dl></dl></dd>
 124 <dt><span class="section"><a href="header/boost/vmd/to_tuple_hpp.html">Header &lt;boost/vmd/to_tuple.hpp&gt;</a></span></dt>
 125 <dd><dl></dl></dd>
 126 <dt><span class="section"><a href="header/boost/vmd/assert_hpp.html">Header &lt;boost/vmd/assert.hpp&gt;</a></span></dt>
 127 <dd><dl></dl></dd>
 128 <dt><span class="section"><a href="header/boost/vmd/assert_is_array_hpp.html">Header &lt;boost/vmd/assert_is_array.hpp&gt;</a></span></dt>
 129 <dd><dl></dl></dd>
 130 <dt><span class="section"><a href="header/boost/vmd/assert_is_empty_hpp.html">Header &lt;boost/vmd/assert_is_empty.hpp&gt;</a></span></dt>
 131 <dd><dl></dl></dd>
 132 <dt><span class="section"><a href="header/boost/vmd/assert_is_identifier_hpp.html">Header &lt;boost/vmd/assert_is_identifier.hpp&gt;</a></span></dt>
 133 <dd><dl></dl></dd>
 134 <dt><span class="section"><a href="header/boost/vmd/assert_is_list_hpp.html">Header &lt;boost/vmd/assert_is_list.hpp&gt;</a></span></dt>
 135 <dd><dl></dl></dd>
 136 <dt><span class="section"><a href="header/boost/vmd/assert_is_number_hpp.html">Header &lt;boost/vmd/assert_is_number.hpp&gt;</a></span></dt>
 137 <dd><dl></dl></dd>
 138 <dt><span class="section"><a href="header/boost/vmd/assert_is_seq_hpp.html">Header &lt;boost/vmd/assert_is_seq.hpp&gt;</a></span></dt>
 139 <dd><dl></dl></dd>
 140 <dt><span class="section"><a href="header/boost/vmd/assert_is_tuple_hpp.html">Header &lt;boost/vmd/assert_is_tuple.hpp&gt;</a></span></dt>
 141 <dd><dl></dl></dd>
 142 <dt><span class="section"><a href="header/boost/vmd/assert_is_type_hpp.html">Header &lt;boost/vmd/assert_is_type.hpp&gt;</a></span></dt>
 143 <dd><dl></dl></dd>
 144 <dt><span class="section"><a href="header/boost/vmd/elem_hpp.html">Header &lt;boost/vmd/elem.hpp&gt;</a></span></dt>
 145 <dd><dl></dl></dd>
 146 <dt><span class="section"><a href="header/boost/vmd/empty_hpp.html">Header &lt;boost/vmd/empty.hpp&gt;</a></span></dt>
 147 <dd><dl></dl></dd>
 148 <dt><span class="section"><a href="header/boost/vmd/enum_hpp.html">Header &lt;boost/vmd/enum.hpp&gt;</a></span></dt>
 149 <dd><dl></dl></dd>
 150 <dt><span class="section"><a href="header/boost/vmd/equal_hpp.html">Header &lt;boost/vmd/equal.hpp&gt;</a></span></dt>
 151 <dd><dl></dl></dd>
 152 <dt><span class="section"><a href="header/boost/vmd/get_type_hpp.html">Header &lt;boost/vmd/get_type.hpp&gt;</a></span></dt>
 153 <dd><dl></dl></dd>
 154 <dt><span class="section"><a href="header/boost/vmd/identity_hpp.html">Header &lt;boost/vmd/identity.hpp&gt;</a></span></dt>
 155 <dd><dl></dl></dd>
 156 <dt><span class="section"><a href="header/boost/vmd/is_array_hpp.html">Header &lt;boost/vmd/is_array.hpp&gt;</a></span></dt>
 157 <dd><dl></dl></dd>
 158 <dt><span class="section"><a href="header/boost/vmd/is_empty_hpp.html">Header &lt;boost/vmd/is_empty.hpp&gt;</a></span></dt>
 159 <dd><dl></dl></dd>
 160 <dt><span class="section"><a href="header/boost/vmd/is_empty_array_hpp.html">Header &lt;boost/vmd/is_empty_array.hpp&gt;</a></span></dt>
 161 <dd><dl></dl></dd>
 162 <dt><span class="section"><a href="header/boost/vmd/is_empty_list_hpp.html">Header &lt;boost/vmd/is_empty_list.hpp&gt;</a></span></dt>
 163 <dd><dl></dl></dd>
 164 <dt><span class="section"><a href="header/boost/vmd/is_identifier_hpp.html">Header &lt;boost/vmd/is_identifier.hpp&gt;</a></span></dt>
 165 <dd><dl></dl></dd>
 166 <dt><span class="section"><a href="header/boost/vmd/is_list_hpp.html">Header &lt;boost/vmd/is_list.hpp&gt;</a></span></dt>
 167 <dd><dl></dl></dd>
 168 <dt><span class="section"><a href="header/boost/vmd/is_multi_hpp.html">Header &lt;boost/vmd/is_multi.hpp&gt;</a></span></dt>
 169 <dd><dl></dl></dd>
 170 <dt><span class="section"><a href="header/boost/vmd/is_number_hpp.html">Header &lt;boost/vmd/is_number.hpp&gt;</a></span></dt>
 171 <dd><dl></dl></dd>
 172 <dt><span class="section"><a href="header/boost/vmd/is_parens_empty_hpp.html">Header &lt;boost/vmd/is_parens_empty.hpp&gt;</a></span></dt>
 173 <dd><dl></dl></dd>
 174 <dt><span class="section"><a href="header/boost/vmd/is_seq_hpp.html">Header &lt;boost/vmd/is_seq.hpp&gt;</a></span></dt>
 175 <dd><dl></dl></dd>
 176 <dt><span class="section"><a href="header/boost/vmd/is_tuple_hpp.html">Header &lt;boost/vmd/is_tuple.hpp&gt;</a></span></dt>
 177 <dd><dl></dl></dd>
 178 <dt><span class="section"><a href="header/boost/vmd/is_type_hpp.html">Header &lt;boost/vmd/is_type.hpp&gt;</a></span></dt>
 179 <dd><dl></dl></dd>
 180 <dt><span class="section"><a href="header/boost/vmd/is_unary_hpp.html">Header &lt;boost/vmd/is_unary.hpp&gt;</a></span></dt>
 181 <dd><dl></dl></dd>
 182 <dt><span class="section"><a href="header/boost/vmd/not_equal_hpp.html">Header &lt;boost/vmd/not_equal.hpp&gt;</a></span></dt>
 183 <dd><dl></dl></dd>
 184 <dt><span class="section"><a href="header/boost/vmd/seq/is_vmd_seq_hpp.html">Header &lt;boost/vmd/seq/is_vmd_seq.hpp&gt;</a></span></dt>
 185 <dd><dl></dl></dd>
 186 <dt><span class="section"><a href="header/boost/vmd/seq/pop_back_hpp.html">Header &lt;boost/vmd/seq/pop_back.hpp&gt;</a></span></dt>
 187 <dd><dl></dl></dd>
 188 <dt><span class="section"><a href="header/boost/vmd/tuple/pop_back_hpp.html">Header &lt;boost/vmd/tuple/pop_back.hpp&gt;</a></span></dt>
 189 <dd><dl></dl></dd>
 190 <dt><span class="section"><a href="header/boost/vmd/seq/pop_front_hpp.html">Header &lt;boost/vmd/seq/pop_front.hpp&gt;</a></span></dt>
 191 <dd><dl></dl></dd>
 192 <dt><span class="section"><a href="header/boost/vmd/tuple/pop_front_hpp.html">Header &lt;boost/vmd/tuple/pop_front.hpp&gt;</a></span></dt>
 193 <dd><dl></dl></dd>
 194 <dt><span class="section"><a href="header/boost/vmd/seq/push_back_hpp.html">Header &lt;boost/vmd/seq/push_back.hpp&gt;</a></span></dt>
 195 <dd><dl></dl></dd>
 196 <dt><span class="section"><a href="header/boost/vmd/tuple/push_back_hpp.html">Header &lt;boost/vmd/tuple/push_back.hpp&gt;</a></span></dt>
 197 <dd><dl></dl></dd>
 198 <dt><span class="section"><a href="header/boost/vmd/seq/push_front_hpp.html">Header &lt;boost/vmd/seq/push_front.hpp&gt;</a></span></dt>
 199 <dd><dl></dl></dd>
 200 <dt><span class="section"><a href="header/boost/vmd/tuple/push_front_hpp.html">Header &lt;boost/vmd/tuple/push_front.hpp&gt;</a></span></dt>
 201 <dd><dl></dl></dd>
 202 <dt><span class="section"><a href="header/boost/vmd/seq/remove_hpp.html">Header &lt;boost/vmd/seq/remove.hpp&gt;</a></span></dt>
 203 <dd><dl></dl></dd>
 204 <dt><span class="section"><a href="header/boost/vmd/tuple/remove_hpp.html">Header &lt;boost/vmd/tuple/remove.hpp&gt;</a></span></dt>
 205 <dd><dl></dl></dd>
 206 <dt><span class="section"><a href="header/boost/vmd/seq/size_hpp.html">Header &lt;boost/vmd/seq/size.hpp&gt;</a></span></dt>
 207 <dd><dl></dl></dd>
 208 <dt><span class="section"><a href="header/boost/vmd/size_hpp.html">Header &lt;boost/vmd/size.hpp&gt;</a></span></dt>
 209 <dd><dl></dl></dd>
 210 <dt><span class="section"><a href="header/boost/vmd/tuple/size_hpp.html">Header &lt;boost/vmd/tuple/size.hpp&gt;</a></span></dt>
 211 <dd><dl></dl></dd>
 212 <dt><span class="section"><a href="header/boost/vmd/seq/to_array_hpp.html">Header &lt;boost/vmd/seq/to_array.hpp&gt;</a></span></dt>
 213 <dd><dl></dl></dd>
 214 <dt><span class="section"><a href="header/boost/vmd/to_array_hpp.html">Header &lt;boost/vmd/to_array.hpp&gt;</a></span></dt>
 215 <dd><dl></dl></dd>
 216 <dt><span class="section"><a href="header/boost/vmd/tuple/to_array_hpp.html">Header &lt;boost/vmd/tuple/to_array.hpp&gt;</a></span></dt>
 217 <dd><dl></dl></dd>
 218 <dt><span class="section"><a href="header/boost/vmd/seq/to_list_hpp.html">Header &lt;boost/vmd/seq/to_list.hpp&gt;</a></span></dt>
 219 <dd><dl></dl></dd>
 220 <dt><span class="section"><a href="header/boost/vmd/to_list_hpp.html">Header &lt;boost/vmd/to_list.hpp&gt;</a></span></dt>
 221 <dd><dl></dl></dd>
 222 <dt><span class="section"><a href="header/boost/vmd/tuple/to_list_hpp.html">Header &lt;boost/vmd/tuple/to_list.hpp&gt;</a></span></dt>
 223 <dd><dl></dl></dd>
 224 <dt><span class="section"><a href="header/boost/vmd/tuple/is_vmd_tuple_hpp.html">Header &lt;boost/vmd/tuple/is_vmd_tuple.hpp&gt;</a></span></dt>
 225 <dd><dl></dl></dd>
 226 </dl></dd>
 227 <dt><span class="section"><a href="variadic_macro_data/vmd_design.html">Design</a></span></dt>
 228 <dt><span class="section"><a href="variadic_macro_data/vmd_compilers.html">Compilers</a></span></dt>
 229 <dt><span class="section"><a href="variadic_macro_data/vmd_history.html">History</a></span></dt>
 230 <dt><span class="section"><a href="variadic_macro_data/vmd_ack.html">Acknowledgements</a></span></dt>
 231 <dt><span class="section"><a href="index/s24.html">Index</a></span></dt>
 232 </dl>
 233 </div>
 234 <div class="section">
 235 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 236 <a name="variadic_macro_data.vmd_intro"></a><a class="link" href="index.html#variadic_macro_data.vmd_intro" title="Introduction">Introduction</a>
 237 </h2></div></div></div>
 238 <p>
 239       Welcome to the Variadic Macro Data library.
 240     </p>
 241 <p>
 242       The Variadic Macro Data library, referred to hereafter as VMD for short, is
 243       a library of variadic macros which provide enhancements to the functionality
 244       in the Boost preprocessor library ( Boost PP ), especially as it relates to
 245       preprocessor data types.
 246     </p>
 247 <p>
 248       The preprocessor data types with which VMD has specific functionality are emptiness,
 249       identifiers, numbers ( a subset of identifiers ), types ( a subset of identifiers
 250       ), Boost PP arrays, Boost PP lists, Boost PP seqs, Boost PP tuples, and sequences.
 251       The first four are basic preprocessor data types while the latter five are
 252       composite preprocessor data types. A sequence is zero or more of the other
 253       preprocessor data types following each other.
 254     </p>
 255 <h4>
 256 <a name="variadic_macro_data.vmd_intro.h0"></a>
 257       <span class="phrase"><a name="variadic_macro_data.vmd_intro.data_type_examples"></a></span><a class="link" href="index.html#variadic_macro_data.vmd_intro.data_type_examples">Data
 258       type examples</a>
 259     </h4>
 260 <div class="table">
 261 <a name="variadic_macro_data.vmd_intro.dwe"></a><p class="title"><b>Table&#160;1.1.&#160;Data types with examples</b></p>
 262 <div class="table-contents"><table class="table" summary="Data types with examples">
 263 <colgroup>
 264 <col>
 265 <col>
 266 </colgroup>
 267 <thead><tr>
 268 <th>
 269               <p>
 270                 Type
 271               </p>
 272             </th>
 273 <th>
 274               <p>
 275                 Example
 276               </p>
 277             </th>
 278 </tr></thead>
 279 <tbody>
 280 <tr>
 281 <td>
 282               <p>
 283                 identifier
 284               </p>
 285             </td>
 286 <td>
 287               <p>
 288                 anyname
 289               </p>
 290             </td>
 291 </tr>
 292 <tr>
 293 <td>
 294               <p>
 295                 number
 296               </p>
 297             </td>
 298 <td>
 299               <p>
 300                 47
 301               </p>
 302             </td>
 303 </tr>
 304 <tr>
 305 <td>
 306               <p>
 307                 type
 308               </p>
 309             </td>
 310 <td>
 311               <p>
 312                 BOOST_VMD_TYPE_NUMBER
 313               </p>
 314             </td>
 315 </tr>
 316 <tr>
 317 <td>
 318               <p>
 319                 array
 320               </p>
 321             </td>
 322 <td>
 323               <p>
 324                 (4,(an_identifier,156,BOOST_VMD_TYPE_IDENTIFIER))
 325               </p>
 326             </td>
 327 </tr>
 328 <tr>
 329 <td>
 330               <p>
 331                 list
 332               </p>
 333             </td>
 334 <td>
 335               <p>
 336                 (78,(some_identifier,(BOOST_VMD_TYPE_TYPE,BOOST_PP_NIL)))
 337               </p>
 338             </td>
 339 </tr>
 340 <tr>
 341 <td>
 342               <p>
 343                 seq
 344               </p>
 345             </td>
 346 <td>
 347               <p>
 348                 (identifier)(89)(245)
 349               </p>
 350             </td>
 351 </tr>
 352 <tr>
 353 <td>
 354               <p>
 355                 tuple
 356               </p>
 357             </td>
 358 <td>
 359               <p>
 360                 (any_id,175,BOOST_VMD_TYPE_LIST,happy,21)
 361               </p>
 362             </td>
 363 </tr>
 364 <tr>
 365 <td>
 366               <p>
 367                 sequence
 368               </p>
 369             </td>
 370 <td>
 371               <p>
 372                 tree 59 (56,BOOST_VMD_TYPE_SEQ) (128)(fire)(clown) (47,(BOOST_VMD_TYPE_TUPLE,BOOST_PP_NIL))
 373               </p>
 374             </td>
 375 </tr>
 376 </tbody>
 377 </table></div>
 378 </div>
 379 <br class="table-break"><p>
 380       Emptiness is the lack of any preprocessing tokens. A macro which expands to
 381       nothing, as in:
 382     </p>
 383 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">RETURN_NOTHING</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span>
 384 </pre>
 385 <p>
 386       is said to return emptiness. Conversely a macro could accept nothing when invoked,
 387       such as in:
 388     </p>
 389 <pre class="programlisting"><span class="identifier">RETURN_NOTHING</span><span class="special">()</span>
 390 </pre>
 391 <p>
 392       Finally emptiness can be part of any composite data type as in:
 393     </p>
 394 <pre class="programlisting"><span class="special">(</span><span class="number">45</span><span class="special">,,</span><span class="identifier">some_name</span><span class="special">)</span>
 395 </pre>
 396 <p>
 397       where the second tuple element is empty.
 398     </p>
 399 <h4>
 400 <a name="variadic_macro_data.vmd_intro.h1"></a>
 401       <span class="phrase"><a name="variadic_macro_data.vmd_intro.what_is_the_advantage"></a></span><a class="link" href="index.html#variadic_macro_data.vmd_intro.what_is_the_advantage">What
 402       is the advantage ?</a>
 403     </h4>
 404 <p>
 405       VMD can identify any of the preprocessor data types previously mentioned, and
 406       can parse sequences into their individual preprocessor data types. You may
 407       well ask why that is important.
 408     </p>
 409 <p>
 410       In Boost PP macro programming a great deal of the control logic of designing
 411       a macro is based on the support Boost PP has for numbers and testing for the
 412       value of a number, in particular 0 and 1 to represent boolean choices. Essentially
 413       Boost PP often uses the value of a number to control the logic in a macro's
 414       design.
 415     </p>
 416 <p>
 417       VMD does not attempt, in any way, to duplicate Boost PP's support for testing
 418       the value of numbers or of the boolean 0 or 1 values, but just reuses that
 419       functionality. What VMD offers, which goes beyond Boost PP, is a system for
 420       deciphering the preprocessor data types, as well as comparing any of the preprocessor
 421       data types for equality to any given value. This allows macro logic to be designed
 422       in a more flexible way, relying on the type of data and/or the value of the
 423       data. If this intrigues you, continue reading to understand how you can use
 424       VMD to do macro programming.
 425     </p>
 426 <h4>
 427 <a name="variadic_macro_data.vmd_intro.h2"></a>
 428       <span class="phrase"><a name="variadic_macro_data.vmd_intro.functionality_areas"></a></span><a class="link" href="index.html#variadic_macro_data.vmd_intro.functionality_areas">Functionality
 429       areas</a>
 430     </h4>
 431 <p>
 432       The functionality of the library may be summed up as:
 433     </p>
 434 <div class="orderedlist"><ol class="orderedlist" type="1">
 435 <li class="listitem">
 436           Provide a better way of testing for and using empty parameters and empty
 437           preprocessor data.
 438         </li>
 439 <li class="listitem">
 440           Provide ways for testing/parsing for identifiers, numbers, types, tuples,
 441           arrays, lists, and seqs.
 442         </li>
 443 <li class="listitem">
 444           Provide ways for testing/parsing sequences of identifiers, numbers, types,
 445           tuples, arrays, lists. and seqs.
 446         </li>
 447 <li class="listitem">
 448           Provide some useful variadic macros not in Boost PP.
 449         </li>
 450 </ol></div>
 451 <p>
 452       The library is a header only library and all macros in the library are included
 453       by a single header, whose name is 'vmd.hpp'. Individual headers may be used
 454       for different functionality in the library and will be denoted when that functionality
 455       is explained.
 456     </p>
 457 <p>
 458       All the macros in the library begin with the sequence 'BOOST_VMD_' to distinguish
 459       them from other macros the end-user might use. Therefore the end-user should
 460       not use any C++ identifiers, whether in macros or otherwise, which being with
 461       the sequence 'BOOST_VMD_'.
 462     </p>
 463 <p>
 464       Use of the library is only dependent on Boost PP. The library also uses Boost
 465       detail lightweight_test.hpp for its own tests.
 466     </p>
 467 </div>
 468 </div>
 469 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 470 <td align="left"><p><small>Last revised: December 10, 2019 at 00:21:47 GMT</small></p></td>
 471 <td align="right"><div class="copyright-footer"></div></td>
 472 </tr></table>
 473 <hr>
 474 <div class="spirit-nav"><a accesskey="n" href="variadic_macro_data/vmd_naming.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div>
 475 </body>
 476 </html>