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Library/boost/libs/preprocessor/doc/ref/check_empty.html
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19 дек 2024, 19:11
19 дек 2024, 19:11
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<html> <head> <title>BOOST_PP_CHECK_EMPTY</title> <link rel="stylesheet" type="text/css" href="../styles.css"> </head> <body> <div style="margin-left: 0px;"> The <b>BOOST_PP_CHECK_EMPTY</b> variadic macro checks to see if its variadic input is empty or not. It expands to 1 if its input is empty and expands to 0 if its input is not empty. The 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_CHECK_EMPTY</b>(<i>...</i>) <a href="../topics/variadic_macros.html#VNotation" target="_self"><sup>(v)</sup></a><br> </div> <h4>Arguments</h4> <dl> <dt>...<br> </dt> <dd> The <i>variadic data</i> to be checked for emptiness. </dd> </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> It is possible to pass data to this macro which expands to nothing, in which case this macro will expand to 1 just as if nothing has been passed. </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> <a href="../headers/facilities/check_empty.html"><boost/preprocessor/facilities/check_empty.hpp></a> </div> <h4>Sample Code</h4> <div> <pre> #include <<a href="../headers/facilities/check_empty.html">boost/preprocessor/facilities/check_empty.hpp</a>> # 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="check_empty.html">BOOST_PP_CHECK_EMPTY</a>(DATA) // expands to 1 <a href="check_empty.html">BOOST_PP_CHECK_EMPTY</a>(OBJECT) // expands to 1 <a href="check_empty.html">BOOST_PP_CHECK_EMPTY</a>(FUNC(1)) // expands to 1 <a href="check_empty.html">BOOST_PP_CHECK_EMPTY</a>(FUNC) // expands to 0 <a href="check_empty.html">BOOST_PP_CHECK_EMPTY</a>(FUNC_GEN) // expands to 0 #endif </pre> </div> <hr size="1"> <div style="margin-left: 0px;"> <i></i><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> </div> </body> </html>