ceph/src/boost/libs/vmd/doc/vmd_variadic_macros.qbk

   1 [/
   2   (C) Copyright Edward Diener 2011-2015
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:vmd_vmacros Using variadic macros]
   9
  10 Variadic macros, as specified by C++11, is a feature taken from the C99
  11 specification. They are macros which take a final parameter denoted as
  12 '...' which represents one or more final arguments to the macro as a
  13 series of comma-separated tokens. In the macro expansion a special
  14 keyword of '\_\_VA\_ARGS\_\_' represents the comma-separated tokens. This
  15 information when passed to a variadic macro I call 'variadic macro data',
  16 which gives its name to this library. The more general term 'variadic data'
  17 is used in this documentation to specify data passed to a macro which can
  18 contain any number of macro tokens as a single macro parameter, such as is
  19 found in Boost PP data types.
  20
  21 [heading Boost support]
  22
  23 The Boost PP library has support for variadic macros and uses its
  24 own criteria to determine if a particular compiler has that support.
  25 Boost PP defines or uses the macro BOOST_PP_VARIADICS to denote whether
  26 the compiler being used supports variadic macros. When BOOST_PP_VARIADICS
  27 is set to 1 the compiler supports variadic macros, otherwise when
  28 BOOST_PP_VARIADICS is set to 0 the compiler does not support variadic macros.
  29 If a user of Boost PP sets this value, Boost PP uses the value the end-user
  30 sets, otherwise Boost PP defines the value of BOOST_PP_VARIADICS based on its
  31 own analysis of the compiler being used. This macro can also be checked to
  32 determine if a compiler has support for variadic macros.
  33
  34 [heading Determining variadic macro support]
  35
  36 The VMD library automatically determines whether variadic macro support
  37 is enabled for a particular compiler by also using the same BOOST_PP_VARIADICS
  38 macro from Boost PP. The end-user of VMD can also manually
  39 set the macro BOOST_PP_VARIADICS to turn on or off compiler support for
  40 variadic macros in the VMD library. When BOOST_PP_VARIADICS is set to 0
  41 variadic macros are not supported in the VMD library, otherwise when
  42 BOOST_PP_VARIADICS is set to non-zero they are supported in the VMD library.
  43 This same macro can be used to determine if VMD supports variadic macros for a
  44 particular compiler.
  45
  46 Since this library depends on variadic macro support, if BOOST_PP_VARIADICS
  47 is set to 0, using any of the macros in VMD will lead to a compiler error
  48 since the macro will not be defined. However just including any of the header
  49 files in VMD, even with no variadic macro support for the compiler, will not
  50 lead to any compiler errors.
  51
  52 [endsect]