projects / ceph.git / blame
| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
1 | .. Metafunctions/Composition and Argument Binding//arg |50 |
| 2 | ||
| 3 | arg | |
| 4 | === | |
| 5 | ||
| 6 | Synopsis | |
| 7 | -------- | |
| 8 | ||
| 9 | .. parsed-literal:: | |
| 10 | ||
| 11 | template< int n > struct arg; | |
| 12 | ||
| 13 | template<> struct arg<1> | |
| 14 | { | |
| 15 | template< typename A1,\ |...| typename A\ *n* = |unspecified| > | |
| 16 | struct apply | |
| 17 | { | |
| 18 | typedef A1 type; | |
| 19 | }; | |
| 20 | }; | |
| 21 | ||
| 22 | |...| | |
| 23 | ||
| 24 | template<> struct arg<\ *n*\> | |
| 25 | { | |
| 26 | template< typename A1,\ |...| typename A\ *n* > | |
| 27 | struct apply | |
| 28 | { | |
| 29 | typedef A\ *n* type; | |
| 30 | }; | |
| 31 | }; | |
| 32 | ||
| 33 | ||
| 34 | Description | |
| 35 | ----------- | |
| 36 | ||
| 37 | ``arg<n>`` specialization is a |Metafunction Class| that return the ``n``\ th of its arguments. | |
| 38 | ||
| 39 | ||
| 40 | Header | |
| 41 | ------ | |
| 42 | ||
| 43 | .. parsed-literal:: | |
| 44 | ||
| 45 | #include <boost/mpl/arg.hpp> | |
| 46 | ||
| 47 | ||
| 48 | Parameters | |
| 49 | ---------- | |
| 50 | ||
| 51 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 52 | | Parameter | Requirement | Description | | |
| 53 | +===============+===================================+===============================================+ | |
| 54 | | ``n`` | An integral constant | A number of argument to return. | | |
| 55 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 56 | ||
| 57 | ||
| 58 | Expression semantics | |
| 59 | -------------------- | |
| 60 | ||
| 61 | For any integral constant ``n`` in the range [1, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|\] and | |
| 62 | arbitrary types |a1...an|: | |
| 63 | ||
| 64 | .. parsed-literal:: | |
| 65 | ||
| 66 | typedef apply_wrap\ *n*\< arg<\ *n*\ >,a1,\ |...|\a\ *n* >::type x; | |
| 67 | ||
| 68 | :Return type: | |
| 69 | A type. | |
| 70 | ||
| 71 | :Semantics: | |
| 72 | ``x`` is identical to ``an``. | |
| 73 | ||
| 74 | ||
| 75 | Example | |
| 76 | ------- | |
| 77 | ||
| 78 | .. parsed-literal:: | |
| 79 | ||
| 80 | typedef apply_wrap\ ``5``\< arg<1>,bool,char,short,int,long >::type t1; | |
| 81 | typedef apply_wrap\ ``5``\< arg<3>,bool,char,short,int,long >::type t3; | |
| 82 | ||
| 83 | BOOST_MPL_ASSERT(( is_same< t1, bool > )); | |
| 84 | BOOST_MPL_ASSERT(( is_same< t3, short > )); | |
| 85 | ||
| 86 | ||
| 87 | See also | |
| 88 | -------- | |
| 89 | ||
| 90 | |Composition and Argument Binding|, |Placeholders|, |lambda|, |bind|, |apply|, |apply_wrap| | |
| 91 | ||
| 92 | ||
| 93 |