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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="variadic_macro_data.vmd_modifiers"></a><a class="link" href="vmd_modifiers.html" title="Macros with modifiers">Macros with modifiers</a>
  28 </h2></div></div></div>
  29 <div class="toc"><dl class="toc">
  30 <dt><span class="section"><a href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type">Return
  31       type modifiers</a></span></dt>
  32 <dt><span class="section"><a href="vmd_modifiers/vmd_modifiers_filter.html">Filtering
  33       modifiers</a></span></dt>
  34 <dt><span class="section"><a href="vmd_modifiers/vmd_modifiers_identifier.html">Identifier
  35       modifiers</a></span></dt>
  36 <dt><span class="section"><a href="vmd_modifiers/vmd_modifiers_splitting.html">Splitting
  37       modifiers</a></span></dt>
  38 <dt><span class="section"><a href="vmd_modifiers/vmd_modifiers_index.html">Index
  39       modifiers</a></span></dt>
  40 <dt><span class="section"><a href="vmd_modifiers/vmd_modifiers_single.html">Modifiers
  41       and the single-element sequence</a></span></dt>
  42 </dl></div>
  43 <p>
  44       The basic functionality for VMD macros parsing data types has been given using
  45       the required parameters of those macros. This basic functionality may be perfectly
  46       adequate for macro programmers to use VMD effectively in their programming
  47       efforts.
  48     </p>
  49 <p>
  50       A number of those macros take optional parameters, called in general "modifiers",
  51       which enhance or change the functionality of those macros in various ways.
  52       All modifiers are VMD identifiers.
  53     </p>
  54 <p>
  55       In all situations modifiers are optional parameters which are parsed by VMD
  56       to provide enhanced functionality for some of its macros. They are never required
  57       as part of the basic functionality of a macro.
  58     </p>
  59 <p>
  60       When modifiers are used as optional arguments to a macro they can be input
  61       after the required parameters in any order and VMD will still handle the optional
  62       parameters correctly.
  63     </p>
  64 <p>
  65       There are two general types of modifiers, 'specific modifiers' and 'user-defined
  66       modifiers'. Specific modifers start with BOOST_VMD_ and are both registered
  67       and pre-detected identifiers known to VMD. The specific modifiers change the
  68       expansion of particular macros in various ways which will be subsequently explained.
  69       User-defined modifiers are user-defined identifiers which the end-user of specific
  70       macros must register and pre-detect for himself. They also change the expansion
  71       of particular macros in various ways which will be subsequently explained.
  72     </p>
  73 <p>
  74       For any particular macro if a specific modifier is not appropriate it is just
  75       ignored. This means that VMD never generates a preprocessing error or gives
  76       an incorrect result just because a specific modifier does not apply for a particular
  77       macro. Any modifier which is not recognized as a specific modifier is treated
  78       as a user-defined modifier. In cases where a user-defined modifier is not appropriate
  79       it is also just ignored.
  80     </p>
  81 <p>
  82       The situations where modifiers can be used to enhance the basic functionality
  83       of VMD macros can be divided by particular types of specific modifiers. Each
  84       particular type of a specific modifier has a name given to it, functionality,
  85       and set of identifiers associated with that particular type. Each particular
  86       type of a specific modifier may be used as optional parameters in one or more
  87       designated macros depending on the specific modifier type.
  88     </p>
  89 <p>
  90       When more than one mutually exclusive specific modifier from a particular type
  91       of modifier is specified as an optional parameter the last specified takes
  92       effect. This allows the programmer to override a specific modifier by adding
  93       the overridden identifier as an optional argument to the end of the macro's
  94       invocation.
  95     </p>
  96 <p>
  97       Header files for specific modifiers are automatically included when the header
  98       files for macros taking those specific modifiers are included.
  99     </p>
 100 <p>
 101       Header files for user-defined modifiers, which register and pre-detect those
 102       user-defined modifiers, must be included as needed by the programmer using
 103       those modifiers.
 104     </p>
 105 <p>
 106       The following topics will explain each particular type of modifier and where
 107       it may be used.
 108     </p>
 109 <div class="section">
 110 <div class="titlepage"><div><div><h3 class="title">
 111 <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type"></a><a class="link" href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type" title="Return type modifiers">Return
 112       type modifiers</a>
 113 </h3></div></div></div>
 114 <p>
 115         A number of macros are capable of returning the type of data as a v-type
 116         rather than, or along with, the data itself. The most obvious of these is
 117         BOOST_VMD_GET_TYPE which generically returns the type of the input.
 118       </p>
 119 <p>
 120         Return type modifiers turn on, turn off, or change the type of the data returned
 121         in some way.
 122       </p>
 123 <p>
 124         These modifiers are:
 125       </p>
 126 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 127 <li class="listitem">
 128             BOOST_VMD_RETURN_NO_TYPE, do not return the type of data.
 129           </li>
 130 <li class="listitem">
 131             BOOST_VMD_RETURN_TYPE, return the type of data parsing any tuple-like
 132             syntactical construct as its most specific type. This means that any
 133             tuple-like construct is parsed first as a possible list, next as a possible
 134             array if it is not a list, and finally as a tuple if it is not a list
 135             or an array.
 136           </li>
 137 <li class="listitem">
 138             BOOST_VMD_RETURN_TYPE_LIST, parse any tuple-like syntactical construct
 139             first as a possible list and only then as a tuple if it is not a list.
 140           </li>
 141 <li class="listitem">
 142             BOOST_VMD_RETURN_TYPE_ARRAY, parse any tuple-like syntactical construct
 143             first as a possible array and only then as a tuple if it is not an array.
 144           </li>
 145 <li class="listitem">
 146             BOOST_VMD_RETURN_TYPE_TUPLE, parse any tuple-like syntactical construct
 147             only as a tuple.
 148           </li>
 149 </ul></div>
 150 <p>
 151         When VMD parses input generically it must determine the type of each data
 152         element of the input. For nearly all of the VMD data types this is never
 153         a problem. For the array or list data types this can be a problem, as explained
 154         when discussing parsing arrays and lists respectively using the specific
 155         macros BOOST_VMD_IS_ARRAY and BOOST_VMD_IS_LIST. The problem is that a valid
 156         tuple can be an invalid list or an invalid array, whose parsing as the more
 157         specific type will lead to UB. Because of this when VMD parses input generically,
 158         and only the data of an element is needed to continue parsing correctly,
 159         it parses all tuple-like data as a tuple and never as a list or an array.
 160       </p>
 161 <p>
 162         When VMD parses input generically, and the type of the data is required in
 163         some way as part of the return of a macro, VMD by default parses for the
 164         most specific type of each data element in order to return the most accurate
 165         type. In this situation by default the BOOST_VMD_RETURN_TYPE modifier is
 166         internally in effect without having to be specified.
 167       </p>
 168 <p>
 169         If more than one of the return type modifiers are specified as optional parameters
 170         the last one specified is in effect.
 171       </p>
 172 <h5>
 173 <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.h0"></a>
 174         <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_boost_vmd_get_type"></a></span><a class="link" href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_boost_vmd_get_type">Usage
 175         with BOOST_VMD_GET_TYPE</a>
 176       </h5>
 177 <p>
 178         The only macro in which VMD without the use of modifiers is being asked to
 179         return the type of data is BOOST_VMD_GET_TYPE. For this macro the BOOST_VMD_RETURN_TYPE
 180         modifier is internally in effect so if no return type modifiers are input
 181         as optional parameters BOOST_VMD_GET_TYPE looks for the most specific type.
 182       </p>
 183 <p>
 184         For the BOOST_VMD_GET_TYPE macro the optional return type modifier BOOST_VMD_RETURN_NO_TYPE,
 185         if specified, is always ignored since the purpose of BOOST_VMD_GET_TYPE is
 186         solely to return the v-type.
 187       </p>
 188 <p>
 189         Let's look at how this works with BOOST_VMD_GET_TYPE by specifying VMD sequences
 190         that have tuples which may or may not be valid lists or arrays.
 191       </p>
 192 <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">get_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 193
 194 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_ARRAY</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">4</span><span class="special">))</span>
 195 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_LIST</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 196 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_LIST_OR_ARRAY</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="identifier">BOOST_PP_NIL</span><span class="special">))</span>
 197 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY</span> <span class="special">(&amp;</span><span class="number">2</span><span class="special">,(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span>
 198 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_LIST</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,^</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 199 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_EMPTY</span>
 200 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY</span> <span class="identifier">TUPLE_BUT_INVALID_LIST</span>
 201
 202 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_ARRAY</span><span class="special">,</span> <span class="identifier">the</span> <span class="identifier">most</span> <span class="identifier">specific</span> <span class="identifier">type</span>
 203 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 204 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_ARRAY</span>
 205 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 206
 207 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">,</span> <span class="identifier">the</span> <span class="identifier">most</span> <span class="identifier">specific</span> <span class="identifier">type</span>
 208 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 209 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 210 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_LIST</span>
 211
 212 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST_OR_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">,</span> <span class="identifier">the</span> <span class="identifier">most</span> <span class="identifier">specific</span> <span class="identifier">type</span>
 213 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST_OR_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 214 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST_OR_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_ARRAY</span>
 215 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_IS_LIST_OR_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_LIST</span>
 216
 217 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="identifier">give</span> <span class="identifier">UB</span>
 218 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 219 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="identifier">give</span> <span class="identifier">UB</span>
 220 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 221
 222 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="identifier">give</span> <span class="identifier">UB</span>
 223 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 224 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_TUPLE</span>
 225 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">TUPLE_BUT_INVALID_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="identifier">give</span> <span class="identifier">UB</span>
 226
 227 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">SEQUENCE_EMPTY</span><span class="special">)</span>
 228    <span class="identifier">will</span> <span class="identifier">always</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_EMPTY</span> <span class="identifier">even</span> <span class="keyword">if</span> <span class="identifier">we</span> <span class="identifier">add</span> <span class="identifier">any</span> <span class="keyword">return</span> <span class="identifier">type</span> <span class="identifier">modifiers</span>
 229 <span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI</span><span class="special">)</span>
 230    <span class="identifier">will</span> <span class="identifier">always</span> <span class="keyword">return</span> <span class="identifier">BOOST_VMD_TYPE_SEQUENCE</span> <span class="identifier">even</span> <span class="keyword">if</span> <span class="identifier">we</span> <span class="identifier">add</span> <span class="identifier">any</span> <span class="keyword">return</span> <span class="identifier">type</span> <span class="identifier">modifiers</span>
 231 </pre>
 232 <h5>
 233 <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.h1"></a>
 234         <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_sequence_converting_m"></a></span><a class="link" href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_sequence_converting_m">Usage
 235         with sequence converting macros</a>
 236       </h5>
 237 <p>
 238         The sequence converting macros converts a sequence to a composite Boost PP
 239         data type or to variadic data, where each element's data in the sequence
 240         becomes an element in the destination composite type. The macros are:
 241       </p>
 242 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 243 <li class="listitem">
 244             BOOST_VMD_TO_ARRAY, converts the sequence to an array
 245           </li>
 246 <li class="listitem">
 247             BOOST_VMD_TO_LIST, converts the sequence to a list
 248           </li>
 249 <li class="listitem">
 250             BOOST_VMD_TO_SEQ, converts the sequence to a seq
 251           </li>
 252 <li class="listitem">
 253             BOOST_VMD_TO_TUPLE, converts the sequence to a tuple
 254           </li>
 255 <li class="listitem">
 256             BOOST_VMD_ENUM, converts the sequence to variadic data
 257           </li>
 258 </ul></div>
 259 <p>
 260         When it does the conversion, using just the required parameter of the sequence
 261         itself, it converts only the data value of each sequence element to the elements
 262         of a composite Boost PP data type or variadic data. Because it needs only
 263         the data value of each sequence element it determines the type of each sequence
 264         element as the most general type that it can be. This means that all tuple-like
 265         data are parsed as tuples rather than as possible lists or arrays.
 266       </p>
 267 <p>
 268         Using a return type modifier we can convert from a VMD sequence to a Boost
 269         PP composite data type or variadic data and retain the type of data of each
 270         element in the sequence as part of the conversion. When doing this each of
 271         the converted elements of the composite data type becomes a two-element tuple
 272         where the first element is the type of the data and the second element is
 273         the data itself.
 274       </p>
 275 <p>
 276         For the sequence conversion macros the default return type modifier internally
 277         set is BOOST_VMD_RETURN_NO_TYPE, which means that the type is not retained.
 278         By specifying another optional return type modifier we tell the conversion
 279         to preserve the type in the conversion output.
 280       </p>
 281 <p>
 282         If the sequence is empty, since there are no sequence elements, any return
 283         type modifier we use accomplishes nothing but is fine to use.
 284       </p>
 285 <p>
 286         First we show how sequence conversion macros work with the BOOST_VMD_RETURN_TYPE
 287         modifier, which always parses for the most specific type.
 288       </p>
 289 <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="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 290 <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">to_array</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 291 <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">to_list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 292 <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">to_seq</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 293 <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">to_tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 294
 295 <span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANID</span> <span class="special">(</span><span class="identifier">ANID</span><span class="special">)</span>
 296 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_EMPTY_1</span>
 297 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE</span> <span class="number">35</span>
 298 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE_ID</span> <span class="identifier">ANID</span>
 299 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE_ARRAY</span> <span class="special">(</span><span class="number">3</span><span class="special">,(</span><span class="number">0</span><span class="special">,</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">))</span>
 300 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE_LIST</span> <span class="special">(</span><span class="identifier">data</span><span class="special">,(</span><span class="identifier">more_data</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">))</span>
 301 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_1</span> <span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="number">1</span><span class="special">)</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">4</span><span class="special">)</span>
 302 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_2</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="special">(</span><span class="number">2</span><span class="special">,(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">))</span>
 303
 304 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_EMPTY_1</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">an</span> <span class="identifier">empty</span> <span class="identifier">array</span> <span class="char">'(0,())'</span>
 305 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_EMPTY_1</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="identifier">an</span> <span class="identifier">empty</span> <span class="identifier">array</span> <span class="char">'(0,())'</span>
 306
 307 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">one</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">list</span> <span class="char">'(35,BOOST_PP_NIL)'</span>
 308 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 309    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">one</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">list</span> <span class="char">'((BOOST_VMD_TYPE_NUMBER,35),BOOST_PP_NIL)'</span>
 310
 311 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_ID</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">one</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">seq</span> <span class="char">'(ANID)'</span>
 312 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_ID</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 313    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">one</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">seq</span> <span class="char">'((BOOST_VMD_TYPE_IDENTIFIER,ANID))'</span>
 314
 315 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">single</span> <span class="identifier">element</span> <span class="identifier">tuple</span> <span class="char">'((3,(0,1,2)))'</span>
 316 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 317    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">single</span> <span class="identifier">element</span> <span class="identifier">tuple</span> <span class="char">'((BOOST_VMD_TYPE_ARRAY,(3,(0,1,2))))'</span>
 318
 319 <span class="identifier">BOOST_VMD_ENUM</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">single</span><span class="special">-</span><span class="identifier">element</span> <span class="char">'(data,(more_data,BOOST_PP_NIL))'</span>
 320 <span class="identifier">BOOST_VMD_ENUM</span><span class="special">(</span><span class="identifier">SEQUENCE_SINGLE_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 321    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">single</span> <span class="identifier">element</span> <span class="char">'(BOOST_VMD_TYPE_LIST,(data,(more_data,BOOST_PP_NIL)))'</span>
 322
 323 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_1</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">multi</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">tuple</span> <span class="char">'((0,1),(2)(3)(4))'</span>
 324 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 325    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">multi</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">tuple</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(0,1)),(BOOST_VMD_TYPE_SEQ,(2)(3)(4)))'</span>
 326
 327 <span class="identifier">BOOST_VMD_ENUM</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_2</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">multi</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">variadic</span> <span class="identifier">data</span> <span class="char">'BOOST_VMD_TYPE_SEQ,(2,(5,6))'</span>
 328 <span class="identifier">BOOST_VMD_ENUM</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 329    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">multi</span><span class="special">-</span><span class="identifier">element</span> <span class="identifier">variadic</span> <span class="identifier">data</span> <span class="char">'(BOOST_VMD_TYPE_TYPE,BOOST_VMD_TYPE_SEQ),(BOOST_VMD_TYPE_ARRAY,(2,(5,6)))'</span>
 330 </pre>
 331 <p>
 332         Lets look at how we might use other return type modifiers when doing conversions
 333         to avoid UB if we want the type as part of the conversion but the type might
 334         be a valid tuple while being an invalid list or array.
 335       </p>
 336 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_VALID_ARRAY</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">4</span><span class="special">))</span>
 337 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_VALID_LIST</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 338 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_2</span> <span class="special">(&amp;</span><span class="number">2</span><span class="special">,(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span>
 339 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_2</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,^</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 340
 341 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_T1</span> <span class="identifier">TUPLE_IS_VALID_ARRAY</span> <span class="identifier">TUPLE_IS_VALID_LIST</span>
 342 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_T2</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_2</span> <span class="identifier">TUPLE_IS_VALID_LIST</span>
 343 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_T3</span> <span class="identifier">TUPLE_IS_VALID_ARRAY</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_2</span>
 344 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_T4</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_2</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_2</span>
 345 </pre>
 346 <p>
 347         We present a number of uses of various sequence conversions with each of
 348         our four different sequences, and show their results.
 349       </p>
 350 <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">to_seq</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 351
 352 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 353    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">seq</span> <span class="char">'((BOOST_VMD_TYPE_ARRAY,(2,(3,4)))) ((BOOST_VMD_TYPE_LIST,(anydata,BOOST_PP_NIL)))'</span>
 354 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 355    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">seq</span> <span class="char">'((BOOST_VMD_TYPE_ARRAY,(2,(3,4)))) ((BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL)))'</span>
 356 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 357    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">seq</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(2,(3,4)))) ((BOOST_VMD_TYPE_LIST,(anydata,BOOST_PP_NIL)))'</span>
 358 <span class="identifier">BOOST_VMD_TO_SEQ</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span>
 359    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">seq</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(2,(3,4)))) ((BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL)))'</span>
 360 </pre>
 361 <p>
 362         The SEQUENCE_MULTI_T1 is a valid array followed by a valid list. All return
 363         type modifiers produce their results without any UBs.
 364       </p>
 365 <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">to_tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 366
 367 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 368    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span> <span class="identifier">as</span> <span class="identifier">an</span> <span class="identifier">array</span>
 369 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 370    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span> <span class="identifier">as</span> <span class="identifier">an</span> <span class="identifier">array</span>
 371 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 372    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">tuple</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(&amp;2,(3,4))),(BOOST_VMD_TYPE_LIST,(anydata,BOOST_PP_NIL)))'</span>
 373 <span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span>
 374    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">tuple</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(&amp;2,(3,4))),(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL)))'</span>
 375 </pre>
 376 <p>
 377         The SEQUENCE_MULTI_T2 is an invalid array, but valid tuple, followed by a
 378         valid list.
 379       </p>
 380 <p>
 381         In sequence conversion we will get UB whenever we use a return type modifier
 382         that parses the data type of the invalid array as an array. But if we use
 383         the return type modifiers BOOST_VMD_RETURN_TYPE_LIST or BOOST_VMD_RETURN_TYPE_TUPLE
 384         we are never parsing the invalid array as an array but as a tuple instead
 385         and therefore we successfully return the type of the invalid array as a BOOST_VMD_TYPE_TUPLE.
 386       </p>
 387 <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">to_array</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 388
 389 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 390    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span> <span class="identifier">as</span> <span class="identifier">a</span> <span class="identifier">list</span>
 391 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 392    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span> <span class="identifier">as</span> <span class="identifier">a</span> <span class="identifier">list</span>
 393 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 394    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">array</span> <span class="char">'(2,((BOOST_VMD_TYPE_ARRAY,(2,(3,4))),(BOOST_VMD_TYPE_TUPLE,(anydata,^BOOST_PP_NIL))))'</span>
 395 <span class="identifier">BOOST_VMD_TO_ARRAY</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span>
 396    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">array</span> <span class="char">'(2,((BOOST_VMD_TYPE_TUPLE,(2,(3,4))),(BOOST_VMD_TYPE_TUPLE,(anydata,^BOOST_PP_NIL))))'</span>
 397 </pre>
 398 <p>
 399         The SEQUENCE_MULTI_T3 is a valid array followed by an invalid list, but a
 400         valid tuple.
 401       </p>
 402 <p>
 403         In sequence conversion we will get UBs whenever we use a return type modifier
 404         that parses the data type of the invalid list as a list. But if we use the
 405         return type modifiers BOOST_VMD_RETURN_TYPE_ARRAY or BOOST_VMD_RETURN_TYPE_TUPLE
 406         we are never parsing the invalid list as a list but as a tuple instead and
 407         therefore we successfully return the type of the invalid list as a BOOST_VMD_TYPE_TUPLE.
 408       </p>
 409 <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">to_list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 410
 411 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 412    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span> <span class="keyword">or</span> <span class="identifier">invalid</span> <span class="identifier">list</span>
 413 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 414    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span>
 415 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 416    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span>
 417 <span class="identifier">BOOST_VMD_TO_LIST</span><span class="special">(</span><span class="identifier">SEQUENCE_MULTI_T4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span>
 418    <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">the</span> <span class="identifier">list</span> <span class="char">'((BOOST_VMD_TYPE_TUPLE,(&amp;2,(3,4))),((BOOST_VMD_TYPE_TUPLE,(anydata,^BOOST_PP_NIL)),BOOST_PP_NIL))'</span>
 419 </pre>
 420 <p>
 421         The SEQUENCE_MULTI_T4 is an invalid array, but valid tuple, followed by an
 422         invalid list, but valid tuple.
 423       </p>
 424 <p>
 425         In sequence conversion we will get UBs whenever we use a return type modifier
 426         that parses the sequence other than looking for only valid tuples. So here
 427         we must use the return type modifier BOOST_VMD_RETURN_TYPE_TUPLE for a sequence
 428         conversion without generating UBs.
 429       </p>
 430 <h5>
 431 <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.h2"></a>
 432         <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_boost_vmd_elem"></a></span><a class="link" href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_boost_vmd_elem">Usage
 433         with BOOST_VMD_ELEM</a>
 434       </h5>
 435 <p>
 436         The default functionality of BOOST_VMD_ELEM when the required parameters
 437         are used is to return the particular element's data. When BOOST_VMD_ELEM
 438         does this it parses all elements of the sequence by determining the most
 439         general type of data for each element. The parsing algorithm stops when it
 440         reaches the element number whose data is returned. This means that all tuple-like
 441         data are parsed as tuples rather than as possible lists or arrays.
 442       </p>
 443 <p>
 444         Using return type modifiers as optional parameters we can tell BOOST_VMD_ELEM
 445         to return the type of the element found, as well as its data, in the form
 446         of a two element tuple. The first element of the tuple is the type of the
 447         data and the second element of the tuple is the data itself.
 448       </p>
 449 <p>
 450         When we use a return type modifier with BOOST_VMD_ELEM, which tells us to
 451         return the type of the data along with the data, the particular modifier
 452         only tells BOOST_VMD_ELEM how to parse the type of data for the element found.
 453         BOOST_VMD_ELEM will continue to parse elements in the sequence preceding
 454         the element found by determining the most general type of the data since
 455         this is the safest way of parsing the data itself.
 456       </p>
 457 <p>
 458         Using the return type modifier BOOST_VMD_RETURN_TYPE with BOOST_VMD_ELEM
 459         is perfectly safe as long as the particular element found is not an invalid
 460         list or array, but a valid tuple. It is when the element found may be an
 461         invalid list or invalid array that we must use other return type modifiers
 462         in order to parse the type of the element correctly.
 463       </p>
 464 <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>
 465
 466 <span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANID_E</span> <span class="special">(</span><span class="identifier">ANID_E</span><span class="special">)</span>
 467 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE_E</span> <span class="number">35</span>
 468 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_SINGLE_E2</span> <span class="identifier">ANID_E</span>
 469 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E</span> <span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="number">1</span><span class="special">)</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">4</span><span class="special">)</span>
 470 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E_2</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="special">(</span><span class="number">2</span><span class="special">,(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">))</span>
 471
 472 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_SINGLE_E</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'35'</span>
 473 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_SINGLE_E</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_NUMBER,35)'</span>
 474
 475 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_SINGLE_E2</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANID_E'</span>
 476 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_SINGLE_E2</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_IDENTIFIER,ANID_E)'</span>
 477
 478 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(2)(3)(4)'</span>
 479 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E</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_SEQ,(2)(3)(4))'</span>
 480
 481 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E_2</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'BOOST_VMD_TYPE_SEQ'</span>
 482 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E_2</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_TYPE,BOOST_VMD_TYPE_SEQ)'</span>
 483 </pre>
 484 <p>
 485         When we use the other return type modifiers with BOOST_VMD_ELEM we do so
 486         because the element we want may be an invalid list or an invalid array but
 487         a valid tuple and yet we still want its type returned as part of the result.
 488       </p>
 489 <p>
 490         Let's look at how this works with BOOST_VMD_ELEM by specifying VMD sequences
 491         that have tuples which are in the form of arrays or lists which cannot be
 492         parsed as such by VMD without generating UBs.
 493       </p>
 494 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_VALID_ARRAY_E</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">4</span><span class="special">))</span>
 495 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_IS_VALID_LIST_E</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 496 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_E</span> <span class="special">(&amp;</span><span class="number">2</span><span class="special">,(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span>
 497 <span class="preprocessor">#define</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_E</span> <span class="special">(</span><span class="identifier">anydata</span><span class="special">,^</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
 498
 499 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E1</span> <span class="identifier">TUPLE_IS_VALID_ARRAY_E</span> <span class="identifier">TUPLE_IS_VALID_LIST_E</span>
 500 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E2</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_E</span> <span class="identifier">TUPLE_IS_VALID_LIST_E</span>
 501 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E3</span> <span class="identifier">TUPLE_IS_VALID_ARRAY_E</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_E</span>
 502 <span class="preprocessor">#define</span> <span class="identifier">SEQUENCE_MULTI_E4</span> <span class="identifier">TUPLE_BUT_INVALID_ARRAY_E</span> <span class="identifier">TUPLE_BUT_INVALID_LIST_E</span>
 503 </pre>
 504 <p>
 505         We present a number of uses of BOOST_VMD_ELEM with each of our four different
 506         sequences, and show their results.
 507       </p>
 508 <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>
 509
 510 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</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_ARRAY,(2,(3,4)))'</span>
 511 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_ARRAY,(2,(3,4)))'</span>
 512 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 513 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 514 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</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,(anydata,BOOST_PP_NIL))'</span>
 515 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 516 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_LIST,(anydata,BOOST_PP_NIL))'</span>
 517 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E1</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 518 </pre>
 519 <p>
 520         The SEQUENCE_MULTI_E1 is a valid array followed by a valid list. All return
 521         type modifiers produce their results without any UBs.
 522       </p>
 523 <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>
 524
 525 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 526    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span> <span class="identifier">as</span> <span class="identifier">an</span> <span class="identifier">array</span>
 527 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 528    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span> <span class="identifier">as</span> <span class="identifier">an</span> <span class="identifier">array</span>
 529 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(&amp;2,(3,4)))'</span>
 530 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(&amp;2,(3,4)))'</span>
 531 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</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,(anydata,BOOST_PP_NIL))'</span>
 532 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 533 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_LIST,(anydata,BOOST_PP_NIL))'</span>
 534 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E2</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 535 </pre>
 536 <p>
 537         The SEQUENCE_MULTI_E2 is an invalid array, but valid tuple, followed by a
 538         valid list.
 539       </p>
 540 <p>
 541         When we access element 0 of our sequence, and use a return type modifier
 542         that parses its data type as an array we will get UB. But if we use the return
 543         type modifiers BOOST_VMD_RETURN_TYPE_LIST or BOOST_VMD_RETURN_TYPE_TUPLE
 544         we are never parsing the invalid array as an array but as a tuple instead
 545         and therefore we successfully return the type of the invalid array as a BOOST_VMD_TYPE_TUPLE.
 546       </p>
 547 <p>
 548         When we access element 1 of our sequence, which is both a valid list and
 549         a valid tuple, we will never get UB. We will get the return type of the element
 550         based on which return type modifier we use.
 551       </p>
 552 <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>
 553
 554 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</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_ARRAY,(2,(3,4)))'</span>
 555 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_ARRAY,(2,(3,4)))'</span>
 556 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 557 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 558 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 559    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span> <span class="identifier">as</span> <span class="identifier">a</span> <span class="identifier">list</span>
 560 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 561 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 562    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span> <span class="identifier">as</span> <span class="identifier">a</span> <span class="identifier">list</span>
 563 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E3</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 564 </pre>
 565 <p>
 566         The SEQUENCE_MULTI_E3 is a valid array followed by an invalid list, but valid
 567         tuple.
 568       </p>
 569 <p>
 570         When we access element 0 of our sequence, which is both a valid array and
 571         a valid tuple, we will never get UB. We will get the return type of the element
 572         based on which return type modifier we use.
 573       </p>
 574 <p>
 575         When we access element 1 of our sequence, and use a return type modifier
 576         that parses its data type as a list we will get UB. But if we use the return
 577         type modifiers BOOST_VMD_RETURN_TYPE_ARRAY or BOOST_VMD_RETURN_TYPE_TUPLE
 578         we are never parsing the invalid list as a list but as a tuple instead and
 579         therefore we successfully return the type of the invalid list as a BOOST_VMD_TYPE_TUPLE.
 580       </p>
 581 <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>
 582
 583 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 584    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span>
 585 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span>
 586    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">array</span>
 587 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 588 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(2,(3,4)))'</span>
 589 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span>
 590    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span>
 591 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_ARRAY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 592 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_LIST</span><span class="special">)</span>
 593    <span class="identifier">will</span> <span class="identifier">produce</span> <span class="identifier">UB</span> <span class="identifier">when</span> <span class="identifier">attempting</span> <span class="identifier">to</span> <span class="identifier">parse</span> <span class="identifier">the</span> <span class="identifier">invalid</span> <span class="identifier">list</span>
 594 <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">SEQUENCE_MULTI_E4</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE_TUPLE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(BOOST_VMD_TYPE_TUPLE,(anydata,BOOST_PP_NIL))'</span>
 595 </pre>
 596 <p>
 597         The SEQUENCE_MULTI_E4 is an invalid array, but valid tuple, followed by an
 598         invalid list, but valid tuple.
 599       </p>
 600 <p>
 601         When we access element 0 of our sequence, which is an invalid array but a
 602         valid tuple, we must use a return type modifier which does not parse element
 603         0 as an array, else we will get UB. This means we must use the return type
 604         modifiers BOOST_VMD_RETURN_TYPE_LIST or BOOST_VMD_RETURN_TYPE_TUPLE so we
 605         are never parsing the invalid array as an array but as a tuple instead and
 606         therefore we successfully return the type of the invalid array as a BOOST_VMD_TYPE_TUPLE.
 607       </p>
 608 <p>
 609         When we access element 1 of our sequence, which is an invalid list but a
 610         valid tuple, we must use a return type modifier which does not parse element
 611         1 as a list, else we will get UB. This means we must use the return type
 612         modifiers BOOST_VMD_RETURN_TYPE_ARRAY or BOOST_VMD_RETURN_TYPE_TUPLE so we
 613         are never parsing the invalid list as a list but as a tuple instead and therefore
 614         we successfully return the type of the invalid list as a BOOST_VMD_TYPE_TUPLE.
 615       </p>
 616 <h5>
 617 <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.h3"></a>
 618         <span class="phrase"><a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_other_modifiers_of_bo"></a></span><a class="link" href="vmd_modifiers.html#variadic_macro_data.vmd_modifiers.vmd_modifiers_return_type.usage_with_other_modifiers_of_bo">Usage
 619         with other modifiers of BOOST_VMD_ELEM</a>
 620       </h5>
 621 <p>
 622         We have not yet discussed the rest of the modifiers which may be used with
 623         BOOST_VMD_ELEM, but return type modifiers are completely independent of any
 624         of them. This means they can be combined with other modifiers and whenever
 625         the element of the sequence is returned the return type modifiers determine
 626         of what the value of that element consists; whether it be just the element
 627         data or the element as a type/data tuple with the type parsed according to
 628         our return type modifier. When we subsequently discuss the use of other modifiers
 629         with BOOST_VMD_ELEM and refer to the element being returned, we are referring
 630         to that element as it is determined by the return type modifiers, which by
 631         default only returns the element's data.
 632       </p>
 633 </div>
 634 </div>
 635 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 636 <td align="left"></td>
 637 <td align="right"><div class="copyright-footer">Copyright &#169; 2010-2017 Tropic Software
 638       East Inc</div></td>
 639 </tr></table>
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