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125 lines
5.9 KiB
HTML
125 lines
5.9 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html;
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charset=windows-1252">
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<title>BOOST_PP_VA_OPT</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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</head>
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<body>
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<div style="margin-left: 0px;"> The <b>BOOST_PP_VA_OPT</b> variadic
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macro is a more flexible alternative to the C++20 __VA_OPT__
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construct. It expands to either one of two inputs depending on
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whether the variadic data is empty or not, whereas the C++20
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__VA_OPT__ constructs expands to either its input or nothing
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depending on whether the variadic data is empty or not. This macro
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only exists when the compilation is at the C++20 level and the
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__VA_OPT__ construct is supported.</div>
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<h4>Usage</h4>
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<div class="code"> <b>BOOST_PP_VA_OPT</b>(x,y,<i>...</i>) <a
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href="../topics/variadic_macros.html#VNotation" target="_self"><sup>(v)</sup></a><br>
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</div>
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<h4>Arguments</h4>
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<p> x<br>
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A tuple
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whose data is the macro expansion if the <i>variadic data</i> is
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<b>not</b> empty<br>
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y<br>
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A tuple
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whose data is the macro expansion if the <i>variadic data</i> is
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empty<br>
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,,,<br>
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The <i>variadic
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data</i> to be checked for emptiness<br>
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</p>
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<dl>
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</dl>
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<h4>Remarks</h4>
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<div> When the macro invocation BOOST_PP_VARIADIC_HAS_OPT() expands
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to 1, then this macro exists and can be invoked, otherwise this
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macro does not exist and attempting to invoke it will lead to a
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preprocessor error that the macro can not be found. Because of
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this condition the header file for including this macro includes
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the header file for the BOOST_PP_VARIADIC_HAS_OPT macro.<br>
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<br>
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The difference between this macro and the __VA_OPT__ construct
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illustrates a limitation of the latter construct with a trade off
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of simpler syntax. The differences between the __VA_OPT__
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construct and this macro are:<br>
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<ul>
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<li>The __VA_OPT__ construct offers a choice as its expansion
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only between its input preprocessing tokens or nothing (
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called a "single placemarker token" ) depending on whether the
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implicit variadic data is empty or not. There is no way using
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the __VA_OPT__ construct to specify any alternative but the
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"single placemarker token" when the variadic data is empty
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whereas any preprocessing tokens can be specified when the
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variadic data is not empty. With the BOOST_PP_VA_OPT macro the
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user can specify as its expansion preprocessing tokens both
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when the variadic data is empty and when the variadic data is
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not empty.</li>
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<li>The __VA_OPT__ construct offers a simple syntax whereas this
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macro is more verbose. The BOOST_PP_VA_OPT macro's first and
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second parameters must be Boost PP tuples of data, in order to
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expand to normal or variadic data, and the third parameter
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must be the variadic data to check for emptiness, whereas the
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__VA_OPT__ construct has an implied variadic data as
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__VA_ARGS__ to check for emptiness and can specify its
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expansion directly in terms of its input.</li>
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<li>The __VA_OPT__ construct can only be specified in the
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replacement list of some macro, whereas the BOOST_PP_VA_OPT
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macro can be used both as an alternative to the __VA_OPT__
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construct in the replacement list of some macro and anywhere
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else a macro can be used.</li>
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<li>It is impossible to have a left parenthesis '(' or a right
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parenthesis ')' as preprocessing token data within the
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__VA_OPT__ construct whereas both are possible as part of the
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expanded data for the BOOST_PP_VA_OPT macro.<br>
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</li>
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</ul>
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<br>
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The exact BOOST_PP_VA_OPT equivalent to the construct of
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'__VA_OPT__ ( pp-tokens )' in the replacement list of a macro is
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'BOOST_PP_VA_OPT (( pp-tokens ),(),__VA_ARGS__)'.</div>
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<h4>See Also</h4>
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<ul>
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<li><a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a></li>
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</ul>
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<h4>Requirements</h4>
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<div> <b>Header:</b> <a
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href="../headers/facilities/va_opt.html"><boost/preprocessor/facilities/va_opt.hpp></a>
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</div>
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<h4>Sample Code</h4>
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<div>
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<pre>#include <<a href="../headers/facilities/va_opt.html">boost/preprocessor/facilities/va_opt.hpp</a>>
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# if <a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a>()
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#define DATA
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#define OBJECT OBJECT2
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#define OBJECT2
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#define FUNC(x) FUNC2(x)
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#define FUNC2(x)
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#define FUNC_GEN(x,y) (1,2,3)
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<a href="va_opt.html">BOOST_PP_VA_OPT</a>((1),(2),DATA) // expands to 2
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<a href="va_opt.html">BOOST_PP_VA_OPT</a>((3),(4),OBJECT) // expands to 4
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<a href="va_opt.html">BOOST_PP_VA_OPT</a>((5),(6),FUNC(1)) // expands to 6
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<a href="va_opt.html">BOOST_PP_VA_OPT</a>((7,8),(9,10),FUNC) // expands to 7,8
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<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
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#endif
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</pre>
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</div>
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<hr size="1">
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<div style="margin-left: 0px;"> <i><EFBFBD> Copyright Edward Diener 2019</i>
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</div>
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<div style="margin-left: 0px;">
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<p><small>Distributed under the Boost Software License, Version
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1.0. (See accompanying file <a
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href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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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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</div>
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</body>
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</html>
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