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<title>Data types</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="variadic_macro_data.vmd_data_types"></a><a class="link" href="vmd_data_types.html" title="Data types">Data types</a>
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<p>
      The VMD library has functionality for testing and parsing preprocessor data.
    </p>
<p>
      The C++ preprocessor defines preprocessor data as preprocessing tokens. The
      types of preprocessing tokens can be seen in section 2.5 of the C++ standard
      document.
    </p>
<p>
      The VMD library works with a subset of two of these types of preprocessor tokens
      as "data types". These are the "identifier" and "pp-number"
      preprocessor tokens. The preprocessor token types which VMD cannot parse are:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          header-name
        </li>
<li class="listitem">
          character-literal
        </li>
<li class="listitem">
          user-defined-characteral-literal
        </li>
<li class="listitem">
          string-literal
        </li>
<li class="listitem">
          user-defined-string-literal
        </li>
<li class="listitem">
          preprocessing-op-or-punc
        </li>
</ul></div>
<p>
      Even though VMD cannot parse these preprocessor token types, it is still a
      very useful library since a large part of macro programming works with 'identifier'
      and 'pp-number' tokens.
    </p>
<p>
      VMD identifiers are preprocessing tokens consisting of alphanumeric characters
      and the underscore ( _ ) character. This is very similar to a preprocessor
      token "identifier" with the difference being that a VMD identifier
      can start with a numeric character, allowing VMD identifiers to also be positive
      integral literals. VMD offers functionality for parsing VMD identifiers both
      as a separate element or in a sequence of preprocessing tokens.
    </p>
<p>
      VMD numbers are Boost PP numbers, ie. preprocessing tokens of whole numbers
      between 0 and 256 inclusive. These are a small subset of preprocessor token
      "pp-number". VMD offers functionality for parsing numbers both as
      a separate element or in a sequence of preprocessing tokens. A VMD number is
      really a subset of VMD identifiers for which VMD offers specific functionality.
      The Boost PP library has it own extensive support for numbers, which VMD does
      not duplicate.
    </p>
<p>
      VMD v-types are, like numbers, a subset of VMD identifiers consisting of identifiers
      beginning with BOOST_VMD_TYPE_ followed by a data type mnemonic. Each v-type
      can be recognized by VMD functionality and therefore passed or returned by
      macros. Like any identifier a v-type can be parsed both as a separate element
      or in a sequence of preprocessing tokens.
    </p>
<p>
      VMD can also test for emptiness, or the absence of any preprocessing tokens
      when passed as macro input.
    </p>
<p>
      The Boost PP library supports four individual high-level data types. These
      are arrays, lists, seqs, and tuples. When using variadic macros arrays are
      really obsolete since tuples have all the functionality of arrays with a simpler
      syntax. Nonetheless arrays are fully supported by VMD. A further data type
      supported by Boost PP is variadic data, which is a comma separated grouping
      of preprocessor elements. VMD has no special support for variadic data outside
      of what is already in Boost PP.
    </p>
<p>
      VMD has functionality to work with the four Boost PP high-level data types.
      VMD can test the Boost PP data types and parse them in a sequence of preprocessor
      tokens.
    </p>
<p>
      VMD can also parse sequences. A sequence consists of zero or more other top-level
      data types already mentioned represented consecutively. As such a sequence
      represents any data type which VMD can parse since it can consist of emptiness,
      a single data type, or multiple data types represented consecutively.
    </p>
<p>
      Emptiness, the three identifier types, the four Boost PP composite data types,
      and VMD sequences are the data types which VMD understands. Other low-level
      preprocessor data types can of course be used in macro programming but VMD
      cannot parse such preprocessor data.
    </p>
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