64 lines
2.7 KiB
HTML
64 lines
2.7 KiB
HTML
<html>
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<head>
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<title>BOOST_PP_VARIADIC_HAS_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_VARIADIC_HAS_OPT</b> macro
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determines whether or not the C++20 __VA_OPT__ construct is supported for the compiler's preprocessor being used.
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Expands to 1 if the __VA_OPT__ construct is supported, otherwise expands to 0.
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</div>
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<h4>Usage</h4>
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<div class="code"> <b>BOOST_PP_VARIADIC_HAS_OPT</b>() <br>
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</div>
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<h4>Arguments</h4>
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<dl>
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<dt>None<br>
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</dt>
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</dl>
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<h4>Remarks</h4>
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<div>
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This macro only returns 1 if the compiler is compiling at its own C++20 level and
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__VA_OPT__ is supported. It is possible for a compiler to support the __VA_OPT__
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construct when not compiling at its own C++20 level, but this macro will return
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0 in that case even if __VA_OPT__ is normally supported for that level. The reason
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for this is that such a compiler may have a compiler switch, enforcing a strict
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adherence to a particular C++ standard level, which gives a warning or an error
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if __VA_OPT__ is specified below the C++20 level, and the preprocessor library
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wants to avoid that happening. Therefore the macro will only test to see whether
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or not __VA_OPT__ is supported at the C++20 level, while otherwise always returning
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0 for all lesser C++ standard levels.
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</div>
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<h4>Requirements</h4>
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<div> <b>Header:</b> <a href="../headers/variadic/has_opt.html"><boost/preprocessor/variadic/has_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>
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#include <<a href="../headers/variadic/has_opt.html">boost/preprocessor/variadic/has_opt.hpp</a>><br>
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#if <a href="variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a>()
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// Preprocessor code which uses __VA_OPT__
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#else
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// Preprocessor code which does not use __VA_OPT__
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#endif
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</pre>
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<br>
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or within a macro definition<br><br>
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<pre>
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#include <boost/preprocessor/control/iif.hpp>
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#include <<a href="../headers/variadic/has_opt.html">boost/preprocessor/variadic/has_opt.hpp</a>><br>
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#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__)
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#define MACRO_USING_OPT(...) __VA_OPT__( preprocessor tokens ) anything
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#define MACRO_NOT_USING_OPT(...) anything
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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></i><i><EFBFBD> Copyright Edward Diener 2019</i> </div>
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<div style="margin-left: 0px;">
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<p><small>Distributed under the Boost Software License, Version 1.0.
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(See accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a>
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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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</div>
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</body>
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</html>
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