ceph/src/boost/boost/vmd/identity.hpp

   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 */