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Library/boost/libs/preprocessor/doc/ref/va_opt.html
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    <title>BOOST_PP_VA_OPT</title>
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    <div style="margin-left: 0px;"> The <b>BOOST_PP_VA_OPT</b> variadic
      macro is a more flexible alternative to the C++20 __VA_OPT__
      construct. It expands to either one of two inputs depending on
      whether the variadic data is empty or not, whereas the C++20
      __VA_OPT__ constructs expands to either its input or nothing
      depending on whether the variadic data is empty or not. This macro
      only exists when the compilation is at the C++20 level and the
      __VA_OPT__ construct is supported.</div>
    <h4>Usage</h4>
    <div class="code"> <b>BOOST_PP_VA_OPT</b>(x,y,<i>...</i>) <a
        href="../topics/variadic_macros.html#VNotation" target="_self"><sup>(v)</sup></a><br>
    </div>
    <h4>Arguments</h4>
    <p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; x<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; A tuple
      whose data is the macro expansion if the <i>variadic data</i> is
      <b>not</b> empty<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; y<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; A tuple
      whose data is the macro expansion if the <i>variadic data</i> is
      empty<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ,,,<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The <i>variadic
        data</i> to be checked for emptiness<br>
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    </dl>
    <h4>Remarks</h4>
    <div> When the macro invocation BOOST_PP_VARIADIC_HAS_OPT() expands
      to 1, then this macro exists and can be invoked, otherwise this
      macro does not exist and attempting to invoke it will lead to a
      preprocessor error that the macro can not be found. Because of
      this condition the header file for including this macro includes
      the header file for the BOOST_PP_VARIADIC_HAS_OPT macro.<br>
      <br>
      The difference between this macro and the __VA_OPT__ construct
      illustrates a limitation of the latter construct with a trade off
      of simpler syntax. The differences between the __VA_OPT__
      construct and this macro are:<br>
      <ul>
        <li>The __VA_OPT__ construct offers a choice as its expansion
          only between its input preprocessing tokens or nothing (
          called a "single placemarker token" ) depending on whether the
          implicit variadic data is empty or not. There is no way using
          the __VA_OPT__ construct to specify any alternative but the
          "single placemarker token" when the variadic data is empty
          whereas any preprocessing tokens can be specified when the
          variadic data is not empty. With the BOOST_PP_VA_OPT macro the
          user can specify as its expansion preprocessing tokens both
          when the variadic data is empty and when the variadic data is
          not empty.</li>
        <li>The __VA_OPT__ construct offers a simple syntax whereas this
          macro is more verbose. The BOOST_PP_VA_OPT macro's first and
          second parameters must be Boost PP tuples of data, in order to
          expand to normal or variadic data, and the third parameter
          must be the variadic data to check for emptiness, whereas the
          __VA_OPT__ construct has an implied variadic data as
          __VA_ARGS__ to check for emptiness and can specify its
          expansion directly in terms of its input.</li>
        <li>The __VA_OPT__ construct can only be specified in the
          replacement list of some macro, whereas the BOOST_PP_VA_OPT
          macro can be used both as an alternative to the __VA_OPT__
          construct in the replacement list of some macro and anywhere
          else a macro can be used.</li>
        <li>It is impossible to have a left parenthesis '(' or a right
          parenthesis ')' as preprocessing token data within the
          __VA_OPT__ construct whereas both are possible as part of the
          expanded data for the BOOST_PP_VA_OPT macro.<br>
        </li>
      </ul>
      <br>
      The exact BOOST_PP_VA_OPT equivalent to the construct of&nbsp;
      '__VA_OPT__ ( pp-tokens )' in the replacement list of a macro is
      'BOOST_PP_VA_OPT (( pp-tokens ),(),__VA_ARGS__)'.</div>
    <h4>See Also</h4>
    <ul>
      <li><a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a></li>
    </ul>
    <h4>Requirements</h4>
    <div> <b>Header:</b> &nbsp;<a
        href="../headers/facilities/va_opt.html">&lt;boost/preprocessor/facilities/va_opt.hpp&gt;</a>
    </div>
    <h4>Sample Code</h4>
    <div>
      <pre>#include &lt;<a href="../headers/facilities/va_opt.html">boost/preprocessor/facilities/va_opt.hpp</a>&gt;

# if <a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a>()

#define DATA
#define OBJECT OBJECT2
#define OBJECT2
#define FUNC(x) FUNC2(x)
#define FUNC2(x)
#define FUNC_GEN(x,y) (1,2,3)

<a href="va_opt.html">BOOST_PP_VA_OPT</a>((1),(2),DATA)                      // expands to 2
<a href="va_opt.html">BOOST_PP_VA_OPT</a>((3),(4),OBJECT)                    // expands to 4
<a href="va_opt.html">BOOST_PP_VA_OPT</a>((5),(6),FUNC(1))                   // expands to 6
<a href="va_opt.html">BOOST_PP_VA_OPT</a>((7,8),(9,10),FUNC)                 // expands to 7,8
<a href="va_opt.html">BOOST_PP_VA_OPT</a>((1,2,3,4,5),(6,7,8,9,10),FUNC_GEN) // expands to 1,2,3,4,5

#endif
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    <div style="margin-left: 0px;"> <i>� Copyright Edward Diener 2019</i>
    </div>
    <div style="margin-left: 0px;">
      <p><small>Distributed under the Boost Software License, Version
          1.0. (See accompanying file <a
            href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
          copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
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