4 // Fraunhofer Institut fuer offene Kommunikationssysteme (FOKUS)
5 // Kompetenzzentrum fuer Satelitenkommunikation (SatCom)
6 // Stefan Bund <g0dil@berlios.de>
8 // This program is free software; you can redistribute it and/or modify
9 // it under the terms of the GNU General Public License as published by
10 // the Free Software Foundation; either version 2 of the License, or
11 // (at your option) any later version.
13 // This program is distributed in the hope that it will be useful,
14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 // GNU General Public License for more details.
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the
20 // Free Software Foundation, Inc.,
21 // 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
24 \brief mpl public header */
33 ///////////////////////////////hh.p////////////////////////////////////////
38 /** \defgroup senfmpl Low-level template meta programming helpers
41 # ifndef SENF_MPL_RV_ALIGNMENT
42 # define SENF_MPL_RV_ALIGNMENT 16
45 /** \brief Return-value type used to implement overload selection
47 The senf::mpl::rv type is used together with \ref SENF_MPL_RV() to select template
48 specializations based on a set of overloads:
51 template <unsigned _> struct select {};
56 static bool const has_int_value = true;
60 senf::mpl::rv<0> select_(int, senf::mpl::take_int<T::value> * = 0);
65 static bool const has_int_value = false;
69 senf::mpl::rv<1> select_(...);
72 struct choice : public select<SENF_MPL_RV( select_<T>(0) )> {};
74 struct A { static const int value = 0; };
77 choice<A> a; a.frobble();
78 choice<B> b; b.dazzle();
81 The selection is always based on two components: A selector class specialized for each of
82 the possible choices and an overloaded function (only signatures, no implementation needed)
83 to provide the conditions.
85 When instantiatinv <tt>choice<T></tt>, we forward \a T to the <tt>select_</tt> set of
86 overloads. Because of <a href="http://en.wikipedia.org/wiki/SFINAE">SFINAE</a>, the overload
87 set will only contain those instantiations, for which template expansion does not fail.
89 So, if \a T has an integer \c value member, both \c select_ overloads are ok and the call
90 <tt>select_<T>(0)</tt> will choose the first (case 0) variant, since the argument \c 0 is
91 better matched by \c int than by <tt>...</tt>.
93 However, if \a T does not have an integer \c value member, expansion for the first overload
94 fails and the overload set only contains the second case.
96 \ref SENF_MPL_RV() internally uses \c sizeof to find out, \e which overload was selected
97 and returns the senf::mpl::rv-argument of that overloads return type. For this to work, the
98 \c select_ functions need not be implemented since no code is generated and \c select_ is
101 This number is than forwarded as template argument to \c select which is specialized for
102 each case. Therefore, <tt>choice<A></tt> has a \c frobble() member whereas
103 <tt>choice<B></tt> has a \c dazzle() member.
105 \see \ref SENF_MPL_RV
108 template <unsigned n>
110 char _[SENF_MPL_RV_ALIGNMENT][n+1];
113 /** \brief Get return value of overload selector
115 Used together with senf::mpl::rv to implement overload selection.
117 \see \ref senf::mpl::rv
121 # define SENF_MPL_RV(expr) (sizeof(expr)/SENF_MPL_RV_ALIGNMENT-1)
123 /** \brief Take an arbitrary unsigned integer template argument
125 Used together with <a href="http://en.wikipedia.org/wiki/SFINAE">SFINAE</a>: The expression
126 <tt>take_uint<</tt> \a expr <tt>></tt> is only valid if \a expr is valid and returns a value
127 convertible to an unsigned integer.
131 template <unsigned long _> struct take_uint {};
133 /** \brief Take an arbitrary integer template argument
135 Used together with <a href="http://en.wikipedia.org/wiki/SFINAE">SFINAE</a>: The expression
136 <tt>take_int<</tt> \a expr <tt>></tt> is only valid if \a expr is valid and returns a value
137 convertible to an integer.
141 template <long _> struct take_int {};
143 /** \brief Take an arbitrary type template argument
145 Used together with <a href="http://en.wikipedia.org/wiki/SFINAE">SFINAE</a>: The expression
146 <tt>take_class<</tt> \a expr <tt>></tt> is only valid if \a expr is valid and is a type.
150 template <class _> struct take_class {};
152 /** \brief Define 'previous value' slot
154 The 'previous value' macros \ref SENF_MPL_PREVV_DEF(), \ref SENF_MPL_PREVV_SET() and \ref
155 SENF_MPL_PREVV_GET() provide a facility to get the last unsigned integer value assigned to
156 the slot before the current point in the current class.
160 // Define PREVV slot named 'accum' initialized to 0
161 SENF_MPL_PREVV_DEF(accum, 0);
164 SENF_MPL_PREVV_SET(accum, SENF_MPL_PREVV_GET(accum) + 2);
166 // Multiply 'accum' by 3
167 SENF_MPL_PREVV_SET(accum, SENF_MPL_PREVV_GET(accum) * 3);
169 // Define the result as a constant expression. result is now 6
170 static unsigned result = SENF_MPL_PREVV_GET(accum);
173 Of course, it does not make sense to use these macros for simple arithmetic as in the
174 example. The SENF_MPL_PREVV macros allow to define macros which pass information from one
175 macro invocation to the next.
177 \implementation The implementation is based on __LINE__: We check backwards for a value
178 defined on a previous line. The check is limited to 80 lines backwards.
183 # define SENF_MPL_PREVV_DEF(name,value) \
184 template <unsigned _> \
185 static senf::mpl::rv<0> _SENF_MPL_PREVV_ ## name (senf::mpl::rv<_> *); \
186 SENF_MPL_PREVV_SET(name,value)
188 /** \brief Set 'prevision value' slot
189 \see \ref SENF_MPL_PREVV_DEF()
193 # define SENF_MPL_PREVV_SET(name,value) \
194 static senf::mpl::rv<unsigned(value)+1> _SENF_MPL_PREVV_ ## name (senf::mpl::rv<__LINE__>*)
196 /** \brief Get current 'previous value' slot value
197 \see \ref SENF_MPL_PREVV_DEF()
201 # define SENF_MPL_PREVV_GET(name) \
202 SENF_MPL_PREVV_I_GET(name)
206 ///////////////////////////////hh.e////////////////////////////////////////
216 // comment-column: 40
217 // c-file-style: "senf"
218 // indent-tabs-mode: nil
219 // ispell-local-dictionary: "american"
220 // compile-command: "scons -u test"