boost/vmd/identity.hpp

0001
0002 //  (C) Copyright Edward Diener 2011-2015
0003 //  Use, modification and distribution are subject to the Boost Software License,
0004 //  Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
0005 //  http://www.boost.org/LICENSE_1_0.txt).
0006
0007 #if !defined(BOOST_VMD_IDENTITY_HPP)
0008 #define BOOST_VMD_IDENTITY_HPP
0009
0010 #include <boost/vmd/detail/setup.hpp>
0011
0012 #if BOOST_PP_VARIADICS
0013
0014 #if BOOST_VMD_MSVC
0015 #include <boost/preprocessor/cat.hpp>
0016 #endif
0017 #include <boost/vmd/empty.hpp>
0018
0019 /*
0020
0021   The succeeding comments in this file are in doxygen format.
0022
0023 */
0024
0025 /** \file
0026 */
0027
0028 /** \def BOOST_VMD_IDENTITY(item)
0029
0030     \brief Macro which expands to its argument when invoked with any number of parameters.
0031
0032     item = any single argument
0033
0034     When BOOST_VMD_IDENTITY(item) is subsequently invoked with any number of parameters it expands
0035     to 'item'. Subsequently invoking the macro is done as 'BOOST_VMD_IDENTITY(item)(zero_or_more_arguments)'.
0036
0037     The macro is equivalent to the Boost PP macro BOOST_PP_IDENTITY(item) with the difference
0038     being that BOOST_PP_IDENTITY(item) is always invoked with no arguments, as in
0039     'BOOST_VMD_IDENTITY(item)()' whereas BOOST_VMD_IDENTITY can be invoked with any number of
0040     arguments.
0041
0042     The macro is meant to be used in BOOST_PP_IF and BOOST_PP_IIF statements when only one
0043     of the clauses needs to be invoked with calling another macro and the other is meant to
0044     return an 'item'.
0045
0046     returns = the macro as 'BOOST_VMD_IDENTITY(item)', when invoked with any number of parameters
0047               as in '(zero_or_more_arguments)', returns 'item'. The macro itself returns
0048               'item BOOST_VMD_EMPTY'.
0049
0050 */
0051
0052 #define BOOST_VMD_IDENTITY(item) item BOOST_VMD_EMPTY
0053
0054 /** \def BOOST_VMD_IDENTITY_RESULT(result)
0055
0056     \brief Macro which wraps any result which can return its value using BOOST_VMD_IDENTITY or 'item BOOST_VMD_EMPTY'.
0057
0058     result = any single result returned when BOOST_VMD_IDENTITY is used or 'item BOOST_VMD_EMPTY'.
0059
0060     The reason for this macro is to smooth over a problem when using VC++ with BOOST_VMD_IDENTITY.
0061     If your BOOST_VMD_IDENTITY macro can be used where VC++ is the compiler then you need to
0062     surround your macro code which could return a result with this macro in order that VC++ handles
0063     BOOST_VMD_IDENTITY correctly.
0064
0065     If you are not using VC++ you do not have to use this macro, but doing so does no harm.
0066
0067 */
0068
0069 #if BOOST_VMD_MSVC
0070 #define BOOST_VMD_IDENTITY_RESULT(result) BOOST_PP_CAT(result,)
0071 #else
0072 #define BOOST_VMD_IDENTITY_RESULT(result) result
0073 #endif
0074
0075 #endif /* BOOST_PP_VARIADICS */
0076 #endif /* BOOST_VMD_IDENTITY_HPP */