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// (C) Copyright Edward Diener 2011-2015 // Use, modification and distribution are subject to the Boost Software License, // Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt). #if !defined(BOOST_VMD_IDENTITY_HPP) #define BOOST_VMD_IDENTITY_HPP #include <boost/vmd/detail/setup.hpp> #if BOOST_PP_VARIADICS #if BOOST_VMD_MSVC #include <boost/preprocessor/cat.hpp> #endif #include <boost/vmd/empty.hpp> /* The succeeding comments in this file are in doxygen format. */ /** \file */ /** \def BOOST_VMD_IDENTITY(item) \brief Macro which expands to its argument when invoked with any number of parameters. item = any single argument When BOOST_VMD_IDENTITY(item) is subsequently invoked with any number of parameters it expands to 'item'. Subsequently invoking the macro is done as 'BOOST_VMD_IDENTITY(item)(zero_or_more_arguments)'. The macro is equivalent to the Boost PP macro BOOST_PP_IDENTITY(item) with the difference being that BOOST_PP_IDENTITY(item) is always invoked with no arguments, as in 'BOOST_VMD_IDENTITY(item)()' whereas BOOST_VMD_IDENTITY can be invoked with any number of arguments. The macro is meant to be used in BOOST_PP_IF and BOOST_PP_IIF statements when only one of the clauses needs to be invoked with calling another macro and the other is meant to return an 'item'. returns = the macro as 'BOOST_VMD_IDENTITY(item)', when invoked with any number of parameters as in '(zero_or_more_arguments)', returns 'item'. The macro itself returns 'item BOOST_VMD_EMPTY'. */ #define BOOST_VMD_IDENTITY(item) item BOOST_VMD_EMPTY /** \def BOOST_VMD_IDENTITY_RESULT(result) \brief Macro which wraps any result which can return its value using BOOST_VMD_IDENTITY or 'item BOOST_VMD_EMPTY'. result = any single result returned when BOOST_VMD_IDENTITY is used or 'item BOOST_VMD_EMPTY'. The reason for this macro is to smooth over a problem when using VC++ with BOOST_VMD_IDENTITY. If your BOOST_VMD_IDENTITY macro can be used where VC++ is the compiler then you need to surround your macro code which could return a result with this macro in order that VC++ handles BOOST_VMD_IDENTITY correctly. If you are not using VC++ you do not have to use this macro, but doing so does no harm. */ #if BOOST_VMD_MSVC #define BOOST_VMD_IDENTITY_RESULT(result) BOOST_PP_CAT(result,) #else #define BOOST_VMD_IDENTITY_RESULT(result) result #endif #endif /* BOOST_PP_VARIADICS */ #endif /* BOOST_VMD_IDENTITY_HPP */
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