1 2 // (C) Copyright Edward Diener 2011-2015 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_IDENTITY_HPP) 8 #define BOOST_VMD_IDENTITY_HPP 9 10 #include <boost/vmd/detail/setup.hpp> 11 12 #if BOOST_PP_VARIADICS 13 14 #if BOOST_VMD_MSVC 15 #include <boost/preprocessor/cat.hpp> 16 #endif 17 #include <boost/vmd/empty.hpp> 18 19 /* 20 21 The succeeding comments in this file are in doxygen format. 22 23 */ 24 25 /** \file 26 */ 27 28 /** \def BOOST_VMD_IDENTITY(item) 29 30 \brief Macro which expands to its argument when invoked with any number of parameters. 31 32 item = any single argument 33 34 When BOOST_VMD_IDENTITY(item) is subsequently invoked with any number of parameters it expands 35 to 'item'. Subsequently invoking the macro is done as 'BOOST_VMD_IDENTITY(item)(zero_or_more_arguments)'. 36 37 The macro is equivalent to the Boost PP macro BOOST_PP_IDENTITY(item) with the difference 38 being that BOOST_PP_IDENTITY(item) is always invoked with no arguments, as in 39 'BOOST_VMD_IDENTITY(item)()' whereas BOOST_VMD_IDENTITY can be invoked with any number of 40 arguments. 41 42 The macro is meant to be used in BOOST_PP_IF and BOOST_PP_IIF statements when only one 43 of the clauses needs to be invoked with calling another macro and the other is meant to 44 return an 'item'. 45 46 returns = the macro as 'BOOST_VMD_IDENTITY(item)', when invoked with any number of parameters 47 as in '(zero_or_more_arguments)', returns 'item'. The macro itself returns 48 'item BOOST_VMD_EMPTY'. 49 50 */ 51 52 #define BOOST_VMD_IDENTITY(item) item BOOST_VMD_EMPTY 53 54 /** \def BOOST_VMD_IDENTITY_RESULT(result) 55 56 \brief Macro which wraps any result which can return its value using BOOST_VMD_IDENTITY or 'item BOOST_VMD_EMPTY'. 57 58 result = any single result returned when BOOST_VMD_IDENTITY is used or 'item BOOST_VMD_EMPTY'. 59 60 The reason for this macro is to smooth over a problem when using VC++ with BOOST_VMD_IDENTITY. 61 If your BOOST_VMD_IDENTITY macro can be used where VC++ is the compiler then you need to 62 surround your macro code which could return a result with this macro in order that VC++ handles 63 BOOST_VMD_IDENTITY correctly. 64 65 If you are not using VC++ you do not have to use this macro, but doing so does no harm. 66 67 */ 68 69 #if BOOST_VMD_MSVC 70 #define BOOST_VMD_IDENTITY_RESULT(result) BOOST_PP_CAT(result,) 71 #else 72 #define BOOST_VMD_IDENTITY_RESULT(result) result 73 #endif 74 75 #endif /* BOOST_PP_VARIADICS */ 76 #endif /* BOOST_VMD_IDENTITY_HPP */ 77