ceph/src/boost/libs/type_erasure/include/boost/type_erasure/placeholder.hpp

   1 // Boost.TypeErasure library
   2 //
   3 // Copyright 2011 Steven Watanabe
   4 //
   5 // Distributed under the Boost Software License Version 1.0. (See
   6 // accompanying file LICENSE_1_0.txt or copy at
   7 // http://www.boost.org/LICENSE_1_0.txt)
   8 //
   9 // $Id$
  10
  11 #ifndef BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED
  12 #define BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED
  13
  14 namespace boost {
  15 namespace type_erasure {
  16
  17 /**
  18  * Placeholders are used heavily throughout the library.
  19  * Every placeholder must derive from @ref placeholder.
  20  * The library provides a number of placeholders,
  21  * out of the box, but you are welcome to define your own,
  22  * if you want more descriptive names.  The placeholder
  23  * @ref _self is special in that it is used as the default
  24  * wherever possible.
  25  *
  26  * What exactly is a placeholder?  Placeholders act as
  27  * a substitute for template parameters in concepts.
  28  * The library automatically replaces all the placeholders
  29  * used in a concept with the actual types involved when
  30  * it stores an object in an @ref any.
  31  *
  32  * For example, in the following,
  33  *
  34  * @code
  35  * any<copy_constructible<_a>, _a> x(1);
  36  * @endcode
  37  *
  38  * The library sees that we're constructing an @ref any
  39  * that uses the @ref _a placeholder with an @c int.
  40  * Thus it binds @ref _a to int and instantiates
  41  * @ref copy_constructible "copy_constructible<int>".
  42  *
  43  * When there are multiple placeholders involved, you
  44  * will have to use @ref tuple, or pass the bindings
  45  * explicitly, but the substitution still works the
  46  * same way.
  47  */
  48 struct placeholder {};
  49
  50 struct _a : placeholder {};
  51 struct _b : placeholder {};
  52 struct _c : placeholder {};
  53 struct _d : placeholder {};
  54 struct _e : placeholder {};
  55 struct _f : placeholder {};
  56 struct _g : placeholder {};
  57
  58 /**
  59  * \brief The default placeholder
  60  *
  61  * @ref _self is the default @ref placeholder used
  62  * by @ref any.  It should be used as a default
  63  * by most concepts, so using concepts with no
  64  * explicit arguments will "just work" as much as
  65  * possible.
  66  */
  67 struct _self : placeholder {};
  68
  69 }
  70 }
  71
  72 #endif