1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Macro BOOST_VMD_IS_EMPTY</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="index.html" title="Chapter 1. The Variadic Macro Data Library 1.9"> 8<link rel="up" href="header/boost/vmd/is_empty_hpp.html" title="Header <boost/vmd/is_empty.hpp>"> 9<link rel="prev" href="header/boost/vmd/is_empty_hpp.html" title="Header <boost/vmd/is_empty.hpp>"> 10<link rel="next" href="BOOST_VMD_IS_EMPTY_NO_OPT.html" title="Macro BOOST_VMD_IS_EMPTY_NO_OPT"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 15<td align="center"><a href="../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="header/boost/vmd/is_empty_hpp.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/vmd/is_empty_hpp.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="BOOST_VMD_IS_EMPTY_NO_OPT.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="refentry"> 26<a name="BOOST_VMD_IS_EMPTY"></a><div class="titlepage"></div> 27<div class="refnamediv"> 28<h2><span class="refentrytitle">Macro BOOST_VMD_IS_EMPTY</span></h2> 29<p>BOOST_VMD_IS_EMPTY — Tests whether its input is empty or not. </p> 30</div> 31<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 32<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="header/boost/vmd/is_empty_hpp.html" title="Header <boost/vmd/is_empty.hpp>">boost/vmd/is_empty.hpp</a>> 33 34</span>BOOST_VMD_IS_EMPTY(...)</pre></div> 35<div class="refsect1"> 36<a name="idm46219107808720"></a><h2>Description</h2> 37<p>The macro checks to see if the input is empty or not. It returns 1 if the input is empty, else returns 0.</p> 38<p>The macro is a variadic macro taking any input. For the VC++8 compiler (VS2005) the macro takes a single parameter of input to check.</p> 39<p>For all levels of C++ prior to C++20 the macro is not perfect, and can not be so. The problem area is if the input to be checked is a function-like macro name, in which case either a compiler error can result or a false result can occur.</p> 40<p>For C++20, with its support for the new <span class="bold"><strong>VA_OPT</strong></span> preprocessor construct, the macro will always work correctly no matter what the variadic input, and is therefore 100% reliable.</p> 41<p>This macro is a replacement, using variadic macro support, for the undocumented macro BOOST_PP_IS_EMPTY in the Boost PP library. The code is taken from a posting by Paul Mensonides of a variadic version for BOOST_PP_IS_EMPTY, and changed in order to also support VC++. The code for the C++20 implementation of the macro, using the <span class="bold"><strong>VA_OPT</strong></span> preprocessor construct, is the author's own and reuses code added to the Boost preprocessor library by this author.</p> 42<p>... = variadic input, for VC++8 this must be a single parameter</p> 43<p>returns = 1 if the input is empty, 0 if it is not</p> 44<p>It is recommended to append BOOST_PP_EMPTY() to whatever input is being tested in order to avoid possible warning messages from some compilers about no parameters being passed to the macro when the input is truly empty. </p> 45</div> 46</div> 47<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 48<td align="left"></td> 49<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 50 East Inc</div></td> 51</tr></table> 52<hr> 53<div class="spirit-nav"> 54<a accesskey="p" href="header/boost/vmd/is_empty_hpp.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/vmd/is_empty_hpp.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="BOOST_VMD_IS_EMPTY_NO_OPT.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 55</div> 56</body> 57</html> 58
