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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="variadic_macro_data.vmd_whyhow"></a><a class="link" href="vmd_whyhow.html" title="Why and how to use">Why and how to use</a>
</h2></div></div></div>
<p>
      The VMD library provides the ability to create a macro which takes different
      types of parameters and can therefore generate different output depending on
      the parameter types as well as their values.
    </p>
<p>
      This is equivalent to the way that overloaded functions provide the ability
      for a singularly named function to provide different functionality depending
      on the parameter types.
    </p>
<p>
      In the case of macros, where more than one macro of the same name but different
      macro expansion is not allowed, a single macro name can create different expansions.
    </p>
<p>
      As a simple example:
    </p>
<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">is_seq</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<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">is_tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#define</span> <span class="identifier">AMACRO</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span>                 <span class="special">\</span>
  <span class="identifier">BOOST_PP_IIF</span>                        <span class="special">\</span>
    <span class="special">(</span>                                 <span class="special">\</span>
    <span class="identifier">BOOST_VMD_IS_SEQ</span><span class="special">(</span><span class="identifier">param</span><span class="special">),</span>          <span class="special">\</span>
    <span class="identifier">Seq</span><span class="special">,</span>                              <span class="special">\</span>
    <span class="identifier">BOOST_PP_IIF</span>                      <span class="special">\</span>
      <span class="special">(</span>                               <span class="special">\</span>
      <span class="identifier">BOOST_VMD_IS_TUPLE</span><span class="special">(</span><span class="identifier">param</span><span class="special">),</span>      <span class="special">\</span>
      <span class="identifier">Tuple</span><span class="special">,</span>                          <span class="special">\</span>
      <span class="identifier">Unknown</span>                         <span class="special">\</span>
      <span class="special">)</span>                               <span class="special">\</span>
    <span class="special">)</span>
</pre>
<p>
      If the param passed is a seq the output of the macro is 'Seq'. If the param
      passed is a tuple the output of the macro is 'Tuple'. Otherwise the output
      of the macro is 'Unknown'.
    </p>
<p>
      Obviously much more complicated cases can be created in which the types and
      values of various parameters are parsed in order to produce variable macro
      output depending on the input. Using variadic macros, macros with variable
      numbers and types of arguments give the macro programmer even greater freedom
      to design macros with flexibility.
    </p>
<p>
      Another feature of the VMD library is the ability to parse identifiers. A system
      of registering identifiers which VMD can recognize has been created. Once an
      identifier is registered VMD can recognize it as part of macro input as an
      identifier and return the identifier. Furthermore VMD can compare identifiers
      for equality or inequality once an identifier has been pre-detected using VMD's
      system for pre-detecting identifiers.
    </p>
<p>
      As another simple example:
    </p>
<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">is_identifier</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_NAME</span> <span class="special">(</span><span class="identifier">NAME</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ADDRESS</span> <span class="special">(</span><span class="identifier">ADDRESS</span><span class="special">)</span>

<span class="preprocessor">#define</span> <span class="identifier">AMACRO1</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span>              <span class="special">\</span>
  <span class="identifier">BOOST_PP_IIF</span>                      <span class="special">\</span>
    <span class="special">(</span>                               <span class="special">\</span>
    <span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">),</span> <span class="special">\</span>
    <span class="identifier">AMACRO1_IDENTIFIER</span><span class="special">,</span>             <span class="special">\</span>
    <span class="identifier">AMACRO1_NO_IDENTIFIER</span>           <span class="special">\</span>
    <span class="special">)</span>                               <span class="special">\</span>
  <span class="special">(</span><span class="identifier">param</span><span class="special">)</span>

<span class="preprocessor">#define</span> <span class="identifier">AMACRO1_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span> <span class="identifier">AMACRO1_</span> <span class="special">##</span> <span class="identifier">param</span>
<span class="preprocessor">#define</span> <span class="identifier">AMACRO1_NO_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span> <span class="identifier">Parameter</span> <span class="identifier">is</span> <span class="keyword">not</span> <span class="identifier">an</span> <span class="identifier">identifier</span>
<span class="preprocessor">#define</span> <span class="identifier">AMACRO1_NAME</span> <span class="identifier">Identifier</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">NAME</span>
<span class="preprocessor">#define</span> <span class="identifier">AMACRO1_ADDRESS</span> <span class="identifier">Identifier</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">ADDRESS</span>
</pre>
<p>
      Here we use VMD's identifier registration system to determine and handle a
      particular identifier we may be expecting as a macro parameter. If the input
      to 'AMACRO1' is 'NAME' the output is 'Identifier is a NAME'. If the input to
      'AMACRO1' is 'ADDRESS' the output is 'Identifier is an ADDRESS'. Otherwise
      the output is 'Parameter is not an identifier'.
    </p>
<p>
      Identifier pre-detection makes things clearer, allowing us to detect within
      VMD whether macro input matches a particular identifier. Using the same setup
      as our previous example, but with identifier pre-detection:
    </p>
<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">is_identifier</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_NAME</span> <span class="special">(</span><span class="identifier">NAME</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_NAME_NAME</span>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ADDRESS</span> <span class="special">(</span><span class="identifier">ADDRESS</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_ADDRESS_ADDRESS</span>

<span class="preprocessor">#define</span> <span class="identifier">AMACRO2</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span>                        <span class="special">\</span>
  <span class="identifier">BOOST_PP_IIF</span>                                <span class="special">\</span>
    <span class="special">(</span>                                         <span class="special">\</span>
    <span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">,</span><span class="identifier">NAME</span><span class="special">),</span>      <span class="special">\</span>
    <span class="identifier">AMACRO2_NAME</span><span class="special">,</span>                             <span class="special">\</span>
    <span class="identifier">BOOST_PP_IIF</span>                              <span class="special">\</span>
      <span class="special">(</span>                                       <span class="special">\</span>
      <span class="identifier">BOOST_VMD_IS_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">,</span><span class="identifier">ADDRESS</span><span class="special">),</span> <span class="special">\</span>
      <span class="identifier">AMACRO2_ADDRESS</span><span class="special">,</span>                        <span class="special">\</span>
      <span class="identifier">AMACRO2_NO_IDENTIFIER</span>                   <span class="special">\</span>
      <span class="special">)</span>                                       <span class="special">\</span>
    <span class="special">)</span>                                         <span class="special">\</span>
  <span class="special">(</span><span class="identifier">param</span><span class="special">)</span>

<span class="preprocessor">#define</span> <span class="identifier">AMACRO2_NO_IDENTIFIER</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span> <span class="identifier">Parameter</span> <span class="identifier">is</span> <span class="keyword">not</span> <span class="identifier">a</span> <span class="identifier">NAME</span> <span class="keyword">or</span> <span class="identifier">ADDRESS</span> <span class="identifier">identifier</span>
<span class="preprocessor">#define</span> <span class="identifier">AMACRO2_NAME</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span> <span class="identifier">Identifier</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">NAME</span>
<span class="preprocessor">#define</span> <span class="identifier">AMACRO2_ADDRESS</span><span class="special">(</span><span class="identifier">param</span><span class="special">)</span> <span class="identifier">Identifier</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">ADDRESS</span>
</pre>
<p>
      If the input to 'AMACRO2' is 'NAME' the output is 'Identifier is a NAME'. If
      the input to 'AMACRO2' is 'ADDRESS' the output is 'Identifier is an ADDRESS'.
      Otherwise the output is 'Parameter is not a NAME or ADDRESS identifier'.
    </p>
<p>
      The VMD library also has 2 different subtypes of identifiers which can always
      be recognized. The first are numbers, equivalent to the number in Boost PP,
      numeric values with a range of 0-256. The second are v-types, which are identifiers
      starting with BOOST_VMD_TYPE_ followed by a name for the type of data. As an
      example, the v-type of a Boost PP tuple is BOOST_VMD_TYPE_TUPLE and the v-type
      of a v-type itself is BOOST_VMD_TYPE_TYPE. All data types have their own v-type
      identifier; types are recognized by the VMD macros and may be passed as input
      data just like any other of the types of data VMD recognizes.
    </p>
<p>
      The VMD identifier system even has a way, to be explained later, for the end-user
      to create his own subtype identifiers.
    </p>
<p>
      Another reason to use VMD is that VMD understands 'sequences' of the VMD data
      types. You can have a sequence of data types and VMD can convert the sequence
      to any of the Boost PP data types, or access any individual data type in a
      sequence.
    </p>
<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>
<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>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_NAME</span> <span class="special">(</span><span class="identifier">NAME</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">ASEQUENCE</span> <span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">)</span> <span class="identifier">NAME</span> <span class="number">147</span> <span class="identifier">BOOST_VMD_TYPE_NUMBER</span> <span class="special">(</span><span class="identifier">a</span><span class="special">)(</span><span class="identifier">b</span><span class="special">)</span>

<span class="identifier">BOOST_VMD_TO_TUPLE</span><span class="special">(</span><span class="identifier">ASEQUENCE</span><span class="special">)</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">ASEQUENCE</span><span class="special">)</span>
</pre>
<p>
      Our first expansion returns the tuple:
    </p>
<pre class="programlisting"><span class="special">((</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">),</span><span class="identifier">NAME</span><span class="special">,</span><span class="number">147</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_NUMBER</span><span class="special">,(</span><span class="identifier">a</span><span class="special">)(</span><span class="identifier">b</span><span class="special">))</span>
</pre>
<p>
      Our second expansion returns the sequence element:
    </p>
<pre class="programlisting"><span class="number">147</span>
</pre>
<p>
      Sequences give the macro programmer the ability to accept input data from the
      user which may more closely mimic C++ constructs.
    </p>
<p>
      Another reason to use VMD is that VMD understands data types. Besides specifically
      asking if a particular input is a particular data type, you can use the macro
      BOOST_VMD_GET_TYPE to retrieve the type of any VMD data.
    </p>
<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>

<span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">((</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">))</span> <span class="comment">// expands to BOOST_VMD_TYPE_TUPLE</span>
<span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="number">235</span><span class="special">)</span>   <span class="comment">// expands to BOOST_VMD_TYPE_NUMBER</span>
</pre>
<p>
      etc.
    </p>
<p>
      There is still much more of VMD functionality but hopefully this brief introduction
      of what VMD can do will interest you so that you will read on to understand
      VMD's functionality for the macro programmer.
    </p>
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