| 1 | // Boost.TypeErasure library |
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| 2 | // |
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| 3 | // Copyright 2011 Steven Watanabe |
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| 4 | // |
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| 5 | // Distributed under the Boost Software License Version 1.0. (See |
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| 6 | // accompanying file LICENSE_1_0.txt or copy at |
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| 7 | // http://www.boost.org/LICENSE_1_0.txt) |
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| 8 | // |
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| 9 | // $Id$ |
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| 10 | |
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| 11 | #ifndef BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED |
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| 12 | #define BOOST_TYPE_ERASURE_PLACEHOLDERS_HPP_INCLUDED |
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| 13 | |
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| 14 | namespace boost { |
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| 15 | namespace type_erasure { |
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| 16 | |
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| 17 | /** |
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| 18 | * Placeholders are used heavily throughout the library. |
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| 19 | * Every placeholder must derive from @ref placeholder. |
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| 20 | * The library provides a number of placeholders, |
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| 21 | * out of the box, but you are welcome to define your own, |
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| 22 | * if you want more descriptive names. The placeholder |
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| 23 | * @ref _self is special in that it is used as the default |
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| 24 | * wherever possible. |
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| 25 | * |
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| 26 | * What exactly is a placeholder? Placeholders act as |
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| 27 | * a substitute for template parameters in concepts. |
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| 28 | * The library automatically replaces all the placeholders |
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| 29 | * used in a concept with the actual types involved when |
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| 30 | * it stores an object in an @ref any. |
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| 31 | * |
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| 32 | * For example, in the following, |
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| 33 | * |
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| 34 | * @code |
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| 35 | * any<copy_constructible<_a>, _a> x(1); |
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| 36 | * @endcode |
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| 37 | * |
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| 38 | * The library sees that we're constructing an @ref any |
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| 39 | * that uses the @ref _a placeholder with an @c int. |
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| 40 | * Thus it binds @ref _a to int and instantiates |
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| 41 | * @ref copy_constructible "copy_constructible<int>". |
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| 42 | * |
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| 43 | * When there are multiple placeholders involved, you |
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| 44 | * will have to use @ref tuple, or pass the bindings |
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| 45 | * explicitly, but the substitution still works the |
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| 46 | * same way. |
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| 47 | */ |
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| 48 | struct placeholder { |
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| 49 | /// INTERNAL ONLY |
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| 50 | typedef void _boost_type_erasure_is_placeholder; |
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| 51 | }; |
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| 52 | |
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| 53 | struct _a : placeholder {}; |
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| 54 | struct _b : placeholder {}; |
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| 55 | struct _c : placeholder {}; |
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| 56 | struct _d : placeholder {}; |
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| 57 | struct _e : placeholder {}; |
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| 58 | struct _f : placeholder {}; |
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| 59 | struct _g : placeholder {}; |
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| 60 | |
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| 61 | /** |
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| 62 | * \brief The default placeholder |
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| 63 | * |
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| 64 | * @ref _self is the default @ref placeholder used |
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| 65 | * by @ref any. It should be used as a default |
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| 66 | * by most concepts, so using concepts with no |
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| 67 | * explicit arguments will "just work" as much as |
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| 68 | * possible. |
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| 69 | */ |
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| 70 | struct _self : placeholder {}; |
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| 71 | |
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| 72 | } |
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| 73 | } |
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| 74 | |
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| 75 | #endif |
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| 76 | |
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