1 2 // (C) Copyright Edward Diener 2011-2015,2019 3 // Use, modification and distribution are subject to the Boost Software License, 4 // Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 5 // http://www.boost.org/LICENSE_1_0.txt). 6 7 #if !defined(BOOST_VMD_IS_EMPTY_HPP) 8 #define BOOST_VMD_IS_EMPTY_HPP 9 10 #include <boost/vmd/detail/setup.hpp> 11 12 #if BOOST_PP_VARIADICS 13 14 #include <boost/preprocessor/punctuation/is_begin_parens.hpp> 15 #include <boost/vmd/detail/is_empty.hpp> 16 17 /* 18 19 The succeeding comments in this file are in doxygen format. 20 21 */ 22 23 /** \file 24 */ 25 26 /** \def BOOST_VMD_IS_EMPTY(...) 27 28 \brief Tests whether its input is empty or not. 29 30 The macro checks to see if the input is empty or not. 31 It returns 1 if the input is empty, else returns 0. 32 33 The macro is a variadic macro taking any input. 34 For the VC++8 compiler (VS2005) the macro takes a single parameter of input to check. 35 36 For all levels of C++ prior to C++20 the macro is not perfect, 37 and can not be so. The problem area is if the input to be 38 checked is a function-like macro name, in which case either 39 a compiler error can result or a false result can occur. 40 41 For C++20, with its support for the new __VA_OPT__ preprocessor 42 construct, the macro will always work correctly no matter what 43 the variadic input, and is therefore 100% reliable. 44 45 This macro is a replacement, using variadic macro support, 46 for the undocumented macro BOOST_PP_IS_EMPTY in the Boost 47 PP library. The code is taken from a posting by Paul Mensonides 48 of a variadic version for BOOST_PP_IS_EMPTY, and changed 49 in order to also support VC++. The code for the C++20 50 implementation of the macro, using the __VA_OPT__ preprocessor 51 construct, is the author's own and reuses code added to the 52 Boost preprocessor library by this author. 53 54 ... = variadic input, for VC++8 this must be a single parameter 55 56 returns = 1 if the input is empty, 0 if it is not 57 58 It is recommended to append BOOST_PP_EMPTY() to whatever input 59 is being tested in order to avoid possible warning messages 60 from some compilers about no parameters being passed to the macro 61 when the input is truly empty. 62 63 */ 64 65 #if BOOST_VMD_MSVC_V8 66 67 #define BOOST_VMD_IS_EMPTY(sequence) \ 68 BOOST_VMD_DETAIL_IS_EMPTY_IIF \ 69 ( \ 70 BOOST_PP_IS_BEGIN_PARENS \ 71 ( \ 72 sequence \ 73 ) \ 74 ) \ 75 ( \ 76 BOOST_VMD_DETAIL_IS_EMPTY_GEN_ZERO, \ 77 BOOST_VMD_DETAIL_IS_EMPTY_PROCESS \ 78 ) \ 79 (sequence) \ 80 /**/ 81 82 #else 83 84 # if defined(__cplusplus) && __cplusplus > 201703L 85 #include <boost/preprocessor/variadic/has_opt.hpp> 86 #include <boost/preprocessor/facilities/is_empty.hpp> 87 #define BOOST_VMD_IS_EMPTY(...) \ 88 BOOST_VMD_DETAIL_IS_EMPTY_IIF \ 89 ( \ 90 BOOST_PP_VARIADIC_HAS_OPT() \ 91 ) \ 92 ( \ 93 BOOST_PP_IS_EMPTY_OPT, \ 94 BOOST_VMD_IS_EMPTY_NO_OPT \ 95 ) \ 96 (__VA_ARGS__) \ 97 /**/ 98 # else 99 #define BOOST_VMD_IS_EMPTY(...) \ 100 BOOST_VMD_IS_EMPTY_NO_OPT(__VA_ARGS__) \ 101 /**/ 102 # endif 103 #define BOOST_VMD_IS_EMPTY_NO_OPT(...) \ 104 BOOST_VMD_DETAIL_IS_EMPTY_IIF \ 105 ( \ 106 BOOST_PP_IS_BEGIN_PARENS \ 107 ( \ 108 __VA_ARGS__ \ 109 ) \ 110 ) \ 111 ( \ 112 BOOST_VMD_DETAIL_IS_EMPTY_GEN_ZERO, \ 113 BOOST_VMD_DETAIL_IS_EMPTY_PROCESS \ 114 ) \ 115 (__VA_ARGS__) \ 116 /**/ 117 #endif /* BOOST_VMD_MSVC_V8 */ 118 #endif /* BOOST_PP_VARIADICS */ 119 #endif /* BOOST_VMD_IS_EMPTY_HPP */ 120