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