projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | .. Metafunctions/Composition and Argument Binding//_1,_2,..._n |10 |
| 2 | ||
| 3 | Placeholders | |
| 4 | ============ | |
| 5 | .. _`placeholder`: | |
| 6 | ||
| 7 | ||
| 8 | Synopsis | |
| 9 | -------- | |
| 10 | ||
| 11 | .. parsed-literal:: | |
| 12 | ||
| 13 | namespace placeholders { | |
| 14 | typedef |unspecified| _; | |
| 15 | typedef arg<1> _1; | |
| 16 | typedef arg<2> _2; | |
| 17 | |...| | |
| 18 | typedef arg<\ *n*\ > _\ *n*\ ; | |
| 19 | } | |
| 20 | ||
| 21 | using placeholders::_; | |
| 22 | using placeholders::_1; | |
| 23 | using placeholders::_2; | |
| 24 | |...| | |
| 25 | using placeholders::_\ *n*\ ; | |
| 26 | ||
| 27 | ||
| 28 | Description | |
| 29 | ----------- | |
| 30 | ||
| 31 | A placeholder in a form ``_``\ *n* is simply a synonym for the corresponding | |
| 32 | ``arg<n>`` specialization. The unnamed placeholder ``_`` (underscore) carries | |
| 33 | `special meaning`__ in bind and lambda expressions, and does not have | |
| 34 | defined semantics outside of these contexts. | |
| 35 | ||
| 36 | Placeholder names can be made available in the user namespace through | |
| 37 | ``using namespace mpl::placeholders;`` directive. | |
| 38 | ||
| 39 | __ `bind semantics`_ | |
| 40 | ||
| 41 | Header | |
| 42 | ------ | |
| 43 | ||
| 44 | .. parsed-literal:: | |
| 45 | ||
| 46 | #include <boost/mpl/placeholders.hpp> | |
| 47 | ||
| 48 | |Note:| The include might be omitted when using placeholders to construct a |Lambda | |
| 49 | Expression| for passing it to MPL's own algorithm or metafunction: any library | |
| 50 | component that is documented to accept a lambda expression makes the placeholders | |
| 51 | implicitly available for the user code |-- end note| | |
| 52 | ||
| 53 | ||
| 54 | Parameters | |
| 55 | ---------- | |
| 56 | ||
| 57 | None. | |
| 58 | ||
| 59 | ||
| 60 | Expression semantics | |
| 61 | -------------------- | |
| 62 | ||
| 63 | For any integral constant ``n`` in the range [1, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|\] and | |
| 64 | arbitrary types |a1...an|: | |
| 65 | ||
| 66 | ||
| 67 | .. parsed-literal:: | |
| 68 | ||
| 69 | typedef apply_wrap\ *n*\<_\ *n*\,a1,\ |...|\a\ *n*\ >::type x; | |
| 70 | ||
| 71 | :Return type: | |
| 72 | A type. | |
| 73 | ||
| 74 | :Semantics: | |
| 75 | Equivalent to | |
| 76 | ||
| 77 | .. parsed-literal:: | |
| 78 | ||
| 79 | typedef apply_wrap\ *n*\< arg<\ *n*\ >,a1,\ |...|\a\ *n* >::type x; | |
| 80 | ||
| 81 | ||
| 82 | Example | |
| 83 | ------- | |
| 84 | ||
| 85 | .. parsed-literal:: | |
| 86 | ||
| 87 | typedef apply_wrap\ ``5``\< _1,bool,char,short,int,long >::type t1; | |
| 88 | typedef apply_wrap\ ``5``\< _3,bool,char,short,int,long >::type t3; | |
| 89 | ||
| 90 | BOOST_MPL_ASSERT(( is_same< t1, bool > )); | |
| 91 | BOOST_MPL_ASSERT(( is_same< t3, short > )); | |
| 92 | ||
| 93 | ||
| 94 | See also | |
| 95 | -------- | |
| 96 | ||
| 97 | |Composition and Argument Binding|, |arg|, |lambda|, |bind|, |apply|, |apply_wrap| | |
| 98 | ||
| 99 | ||
| 100 | .. |placeholder| replace:: `placeholder`_ | |
| 101 | ||
| 102 | .. |_1| replace:: `_1`_ | |
| 103 | .. |_2| replace:: `_2`_ | |
| 104 | .. |_3| replace:: `_3`_ | |
| 105 | .. |_4| replace:: `_4`_ | |
| 106 | .. |_5| replace:: `_5`_ | |
| 107 | ||
| 108 | .. _`_1`: `Placeholders`_ | |
| 109 | .. _`_2`: `Placeholders`_ | |
| 110 | .. _`_3`: `Placeholders`_ | |
| 111 | .. _`_4`: `Placeholders`_ | |
| 112 | .. _`_5`: `Placeholders`_ | |
| 113 | ||
| 114 | .. |_1,_2,..._n| replace:: |_1|, |_2|, |_3|,\ |...| | |
| 115 | ||
| 116 | ||
| 117 |