# Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world.

### boost/vmd/identity.hpp


//  (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

#if !defined(BOOST_VMD_IDENTITY_HPP)
#define BOOST_VMD_IDENTITY_HPP

#include <boost/vmd/detail/setup.hpp>

#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