1<html> 2<head> 3 <title>BOOST_PP_VARIADIC_HAS_OPT</title> 4 <link rel="stylesheet" type="text/css" href="../styles.css"> 5</head> 6<body> 7<div style="margin-left: 0px;"> The <b>BOOST_PP_VARIADIC_HAS_OPT</b> macro 8determines whether or not the C++20 __VA_OPT__ construct is supported for the compiler's preprocessor being used. 9Expands to 1 if the __VA_OPT__ construct is supported, otherwise expands to 0. 10</div> 11<h4>Usage</h4> 12<div class="code"> <b>BOOST_PP_VARIADIC_HAS_OPT</b>() <br> 13 </div> 14<h4>Arguments</h4> 15<dl> 16 <dt>None<br> 17</dt> 18</dl> 19 <h4>Remarks</h4> 20 <div> 21 This macro only returns 1 if the compiler is compiling at its own C++20 level and 22 __VA_OPT__ is supported. It is possible for a compiler to support the __VA_OPT__ 23 construct when not compiling at its own C++20 level, but this macro will return 24 0 in that case even if __VA_OPT__ is normally supported for that level. The reason 25 for this is that such a compiler may have a compiler switch, enforcing a strict 26 adherence to a particular C++ standard level, which gives a warning or an error 27 if __VA_OPT__ is specified below the C++20 level, and the preprocessor library 28 wants to avoid that happening. Therefore the macro will only test to see whether 29 or not __VA_OPT__ is supported at the C++20 level, while otherwise always returning 30 0 for all lesser C++ standard levels. 31 </div> 32<h4>Requirements</h4> 33<div> <b>Header:</b> <a href="../headers/variadic/has_opt.html"><boost/preprocessor/variadic/has_opt.hpp></a> 34</div> 35<h4>Sample Code</h4> 36<div> 37<pre> 38#include <<a href="../headers/variadic/has_opt.html">boost/preprocessor/variadic/has_opt.hpp</a>><br> 39#if <a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a>() 40// Preprocessor code which uses __VA_OPT__ 41#else 42// Preprocessor code which does not use __VA_OPT__ 43#endif 44</pre> 45<br> 46or within a macro definition<br><br> 47<pre> 48#include <boost/preprocessor/control/iif.hpp> 49#include <<a href="../headers/variadic/has_opt.html">boost/preprocessor/variadic/has_opt.hpp</a>><br> 50#define USE_OPT(...) BOOST_PP_IIF(<a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a>(),MACRO_USING_OPT,MACRO_NOT_USING_OPT)(__VA_ARGS__) 51#define MACRO_USING_OPT(...) __VA_OPT__( preprocessor tokens ) anything 52#define MACRO_NOT_USING_OPT(...) anything 53</pre> 54</div> 55<hr size="1"> 56<div style="margin-left: 0px;"> <i></i><i>� Copyright Edward Diener 2019</i> </div> 57<div style="margin-left: 0px;"> 58<p><small>Distributed under the Boost Software License, Version 1.0. 59(See accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> 60or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p> 61</div> 62</body> 63</html> 64