boost/type_erasure/placeholder.hpp

0001 // Boost.TypeErasure library
0002 //
0003 // Copyright 2011 Steven Watanabe
0004 //
0005 // Distributed under the Boost Software License Version 1.0. (See
0006 // accompanying file LICENSE_1_0.txt or copy at
0007 // http://www.boost.org/LICENSE_1_0.txt)
0008 //
0009 // $Id$
0010
0011 #ifndef BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED
0012 #define BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED
0013
0014 namespace boost {
0015 namespace type_erasure {
0016
0017 /**
0018  * Placeholders are used heavily throughout the library.
0019  * Every placeholder must derive from @ref placeholder.
0020  * The library provides a number of placeholders,
0021  * out of the box, but you are welcome to define your own,
0022  * if you want more descriptive names.  The placeholder
0023  * @ref _self is special in that it is used as the default
0024  * wherever possible.
0025  *
0026  * What exactly is a placeholder?  Placeholders act as
0027  * a substitute for template parameters in concepts.
0028  * The library automatically replaces all the placeholders
0029  * used in a concept with the actual types involved when
0030  * it stores an object in an @ref any.
0031  *
0032  * For example, in the following,
0033  *
0034  * @code
0035  * any<copy_constructible<_a>, _a> x(1);
0036  * @endcode
0037  *
0038  * The library sees that we're constructing an @ref any
0039  * that uses the @ref _a placeholder with an @c int.
0040  * Thus it binds @ref _a to int and instantiates
0041  * @ref copy_constructible "copy_constructible<int>".
0042  *
0043  * When there are multiple placeholders involved, you
0044  * will have to use @ref tuple, or pass the bindings
0045  * explicitly, but the substitution still works the
0046  * same way.
0047  */
0048 struct placeholder {
0049     /// INTERNAL ONLY
0050     typedef void _boost_type_erasure_is_placeholder;
0051 };
0052
0053 struct _a : placeholder {};
0054 struct _b : placeholder {};
0055 struct _c : placeholder {};
0056 struct _d : placeholder {};
0057 struct _e : placeholder {};
0058 struct _f : placeholder {};
0059 struct _g : placeholder {};
0060
0061 /**
0062  * \brief The default placeholder
0063  *
0064  * @ref _self is the default @ref placeholder used
0065  * by @ref any.  It should be used as a default
0066  * by most concepts, so using concepts with no
0067  * explicit arguments will "just work" as much as
0068  * possible.
0069  */
0070 struct _self : placeholder {};
0071
0072 }
0073 }
0074
0075 #endif