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| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
1 | .. Metafunctions/Type Selection//eval_if |30 |
| 2 | ||
| 3 | eval_if | |
| 4 | ======= | |
| 5 | ||
| 6 | Synopsis | |
| 7 | -------- | |
| 8 | ||
| 9 | .. parsed-literal:: | |
| 10 | ||
| 11 | template< | |
| 12 | typename C | |
| 13 | , typename F1 | |
| 14 | , typename F2 | |
| 15 | > | |
| 16 | struct eval_if | |
| 17 | { | |
| 18 | typedef |unspecified| type; | |
| 19 | }; | |
| 20 | ||
| 21 | ||
| 22 | ||
| 23 | Description | |
| 24 | ----------- | |
| 25 | ||
| 26 | Evaluates one of its two |nullary metafunction| arguments, ``F1`` or ``F2``, depending | |
| 27 | on the value ``C``. | |
| 28 | ||
| 29 | ||
| 30 | Header | |
| 31 | ------ | |
| 32 | ||
| 33 | .. parsed-literal:: | |
| 34 | ||
| 35 | #include <boost/mpl/eval_if.hpp> | |
| 36 | ||
| 37 | ||
| 38 | Parameters | |
| 39 | ---------- | |
| 40 | ||
| 41 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 42 | | Parameter | Requirement | Description | | |
| 43 | +===============+===================================+===============================================+ | |
| 44 | | ``C`` | |Integral Constant| | An evaluation condition. | | |
| 45 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 46 | | ``F1``, ``F2``| Nullary |Metafunction| | Metafunctions to select for evaluation from. | | |
| 47 | +---------------+-----------------------------------+-----------------------------------------------+ | |
| 48 | ||
| 49 | ||
| 50 | Expression semantics | |
| 51 | -------------------- | |
| 52 | ||
| 53 | For any |Integral Constant| ``c`` and nullary |Metafunction|\ s ``f1``, ``f2``: | |
| 54 | ||
| 55 | ||
| 56 | .. parsed-literal:: | |
| 57 | ||
| 58 | typedef eval_if<c,f1,f2>::type t; | |
| 59 | ||
| 60 | :Return type: | |
| 61 | Any type. | |
| 62 | ||
| 63 | :Semantics: | |
| 64 | If ``c::value == true``, ``t`` is identical to ``f1::type``; otherwise ``t`` is | |
| 65 | identical to ``f2::type``. | |
| 66 | ||
| 67 | ||
| 68 | Example | |
| 69 | ------- | |
| 70 | ||
| 71 | .. parsed-literal:: | |
| 72 | ||
| 73 | typedef eval_if< true\_, identity<char>, identity<long> >::type t1; | |
| 74 | typedef eval_if< false\_, identity<char>, identity<long> >::type t2; | |
| 75 | ||
| 76 | BOOST_MPL_ASSERT(( is_same<t1,char> )); | |
| 77 | BOOST_MPL_ASSERT(( is_same<t2,long> )); | |
| 78 | ||
| 79 | ||
| 80 | See also | |
| 81 | -------- | |
| 82 | ||
| 83 | |Metafunctions|, |Integral Constant|, |eval_if_c|, |if_| | |
| 84 | ||
| 85 | ||
| 86 |