projects / ceph.git / blame
| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
1 | .. Metafunctions/Type Selection//if_ |10 |
| 2 | ||
| 3 | if\_ | |
| 4 | ==== | |
| 5 | ||
| 6 | Synopsis | |
| 7 | -------- | |
| 8 | ||
| 9 | .. parsed-literal:: | |
| 10 | ||
| 11 | template< | |
| 12 | typename C | |
| 13 | , typename T1 | |
| 14 | , typename T2 | |
| 15 | > | |
| 16 | struct if\_ | |
| 17 | { | |
| 18 | typedef |unspecified| type; | |
| 19 | }; | |
| 20 | ||
| 21 | ||
| 22 | ||
| 23 | Description | |
| 24 | ----------- | |
| 25 | ||
| 26 | Returns one of its two arguments, ``T1`` or ``T2``, depending on the value ``C``. | |
| 27 | ||
| 28 | ||
| 29 | Header | |
| 30 | ------ | |
| 31 | ||
| 32 | .. parsed-literal:: | |
| 33 | ||
| 34 | #include <boost/mpl/if.hpp> | |
| 35 | ||
| 36 | ||
| 37 | Parameters | |
| 38 | ---------- | |
| 39 | ||
| 40 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 41 | | Parameter | Requirement | Description | | |
| 42 | +===============+===================================+===============================================+ | |
| 43 | | ``C`` | |Integral Constant| | A selection condition. | | |
| 44 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 45 | | ``T1``, ``T2``| Any type | Types to select from. | | |
| 46 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 47 | ||
| 48 | ||
| 49 | Expression semantics | |
| 50 | -------------------- | |
| 51 | ||
| 52 | For any |Integral Constant| ``c`` and arbitrary types ``t1``, ``t2``: | |
| 53 | ||
| 54 | ||
| 55 | .. parsed-literal:: | |
| 56 | ||
| 57 | typedef if_<c,t1,t2>::type t; | |
| 58 | ||
| 59 | :Return type: | |
| 60 | Any type. | |
| 61 | ||
| 62 | :Semantics: | |
| 63 | If ``c::value == true``, ``t`` is identical to ``t1``; otherwise ``t`` is | |
| 64 | identical to ``t2``. | |
| 65 | ||
| 66 | ||
| 67 | Example | |
| 68 | ------- | |
| 69 | ||
| 70 | .. parsed-literal:: | |
| 71 | ||
| 72 | typedef if\_<true\_,char,long>::type t1; | |
| 73 | typedef if\_<false\_,char,long>::type t2; | |
| 74 | ||
| 75 | BOOST_MPL_ASSERT(( is_same<t1, char> )); | |
| 76 | BOOST_MPL_ASSERT(( is_same<t2, long> )); | |
| 77 | ||
| 78 | ||
| 79 | .. parsed-literal:: | |
| 80 | ||
| 81 | // allocates space for an object of class T on heap or "inplace" | |
| 82 | // depending on its size | |
| 83 | template< typename T > struct lightweight | |
| 84 | { | |
| 85 | // ... | |
| 86 | typedef typename if\_< | |
| 87 | less_equal< sizeof\_<T>, sizeof\_<T*> > | |
| 88 | , inplace_storage<T> | |
| 89 | , heap_storage<T> | |
| 90 | >::type impl_t; | |
| 91 | ||
| 92 | impl_t impl; | |
| 93 | }; | |
| 94 | ||
| 95 | ||
| 96 | See also | |
| 97 | -------- | |
| 98 | ||
| 99 | |Metafunctions|, |Integral Constant|, |if_c|, |eval_if| | |
| 100 | ||
| 101 | ||
| 102 |