boost_preprocessor/include/boost/preprocessor/empty.hpp

#ifndef BOOST_PREPROCESSOR_EMPTY_HPP
#define BOOST_PREPROCESSOR_EMPTY_HPP

// Copyright (C) 2001
// Housemarque Oy
// http://www.housemarque.com
//
// Permission to copy, use, modify, sell and distribute this software is
// granted provided this copyright notice appears in all copies. This
// software is provided "as is" without express or implied warranty, and
// with no claim as to its suitability for any purpose.

// See http://www.boost.org for most recent version.

/*! \file

<a href="../../../../boost/preprocessor/empty.hpp">Click here to see the header.</a>
*/

//! Expands to nothing. Used with BOOST_PP_IF() and as an unused parameter.
/*!
Example usage as the implementation of BOOST_PP_COMMA_IF(C):

<PRE>\verbatim
  #define BOOST_PP_COMMA_IF(C)\
    BOOST_PP_IF(C,BOOST_PP_COMMA,BOOST_PP_EMPTY)()
\endverbatim</PRE>

Example usage as an unused macro parameter:

<PRE>\verbatim
  #define BOOST_PP_DEF(CV)\
    template<class base>            \
    CV typename implement_subscript_using_begin_subscript<base>::value_type&\
      implement_subscript_using_begin_subscript<base>::operator[]\
      ( index_type                  \
          i                         \
      ) CV                          \
    { return base::begin()[i];      \
    }

  BOOST_PP_DEF(BOOST_PP_EMPTY())
  BOOST_PP_DEF(const)
  #undef BOOST_PP_DEF
\endverbatim</PRE>

The above expands to:

<PRE>\verbatim
  template<class base>
  typename implement_subscript_using_begin_subscript<base>::value_type&
    implement_subscript_using_begin_subscript<base>::operator[]
    ( index_type
        i
    )
  { return base::begin()[i];
  }

  template<class base>
  const typename implement_subscript_using_begin_subscript<base>::value_type&
    implement_subscript_using_begin_subscript<base>::operator[]
    ( index_type
        i
    ) const
  { return base::begin()[i];
  }
\endverbatim</PRE>

In case you wonder, the above code is part of a generalized layer for
implementing the subscripting operators of a random access container.
*/
#define BOOST_PP_EMPTY()

//! Obsolete. Use BOOST_PP_EMPTY().
#define BOOST_PREPROCESSOR_EMPTY()
#endif