Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


// Boost.Units - A C++ library for zero-overhead dimensional analysis and 
// unit/quantity manipulation and conversion
// Copyright (C) 2003-2008 Matthias Christian Schabel
// Copyright (C) 2007-2008 Steven Watanabe
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at

/// \file
/// \brief base dimensions (mass, length, time...).
/// \details base dimension definition registration.


#include <boost/units/config.hpp>
#include <boost/units/dim.hpp>
#include <boost/units/static_rational.hpp>
#include <boost/units/units_fwd.hpp>
#include <boost/units/detail/dimension_list.hpp>
#include <boost/units/detail/ordinal.hpp>
#include <boost/units/detail/prevent_redefinition.hpp>

namespace boost {

namespace units {

/// This must be in namespace boost::units so that ADL
/// will work with friend functions defined inline.
template<long N> struct base_dimension_ordinal { };

template<class T, long N> struct base_dimension_pair { };

template<class T, long N>
struct check_base_dimension {
    enum {
        value = 
            sizeof(boost_units_is_registered(units::base_dimension_ordinal<N>())) == sizeof(detail::yes) &&
            sizeof(boost_units_is_registered(units::base_dimension_pair<T, N>())) != sizeof(detail::yes)

/// Defines a base dimension.  To define a dimension you need to provide
/// the derived class (CRTP) and a unique integer.
/// @code
/// struct my_dimension : boost::units::base_dimension<my_dimension, 1> {};
/// @endcode
/// It is designed so that you will get an error message if you try
/// to use the same value in multiple definitions.
template<class Derived,
         long N
#if !defined(BOOST_UNITS_DOXYGEN) && !defined(__BORLANDC__)
         class = typename detail::ordinal_has_already_been_defined<
             check_base_dimension<Derived, N>::value
class base_dimension : 
    public ordinal<N> 
        /// INTERNAL ONLY
        typedef base_dimension                                                          this_type;
        /// A convenience typedef.  Equivalent to boost::units::derived_dimension<Derived,1>::type.
        typedef list<dim<Derived,static_rational<1> >, dimensionless_type>    dimension_type;
        typedef detail::unspecified dimension_type;
        /// Provided for mpl compatability.
        typedef Derived type;

        /// Check for C++0x.  In C++0x, we have to have identical
        /// arguments but a different return type to trigger an
        /// error.  Note that this is only needed for clang as
        /// check_base_dimension will trigger an error earlier
        /// for compilers with less strict name lookup.
        /// INTERNAL ONLY
        friend Derived* 
        check_double_register(const units::base_dimension_ordinal<N>&) 
        { return(0); }

        /// Register this ordinal
        /// INTERNAL ONLY
        friend detail::yes 
        boost_units_is_registered(const units::base_dimension_ordinal<N>&) 
        { detail::yes result; return(result); }
        /// But make sure we can identify the current instantiation!
        /// INTERNAL ONLY
        friend detail::yes 
        boost_units_is_registered(const units::base_dimension_pair<Derived, N>&) 
        { detail::yes result; return(result); }

} // namespace units

} // namespace boost