projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | .. Metafunctions/Concepts//Placeholder Expression |40 |
| 2 | ||
| 3 | Placeholder Expression | |
| 4 | ====================== | |
| 5 | ||
| 6 | Description | |
| 7 | ----------- | |
| 8 | ||
| 9 | A |Placeholder Expression| is a type that is either a |placeholder| or a class | |
| 10 | template specialization with at least one argument that itself is a | |
| 11 | |Placeholder Expression|. | |
| 12 | ||
| 13 | ||
| 14 | Expression requirements | |
| 15 | ----------------------- | |
| 16 | ||
| 17 | If ``X`` is a class template, and ``a1``,... ``an`` are arbitrary types, then | |
| 18 | ``X<a1,...,an>`` is a |Placeholder Expression| if and only if all of the following | |
| 19 | conditions hold: | |
| 20 | ||
| 21 | * At least one of the template arguments ``a1``,... ``an`` is a |placeholder| | |
| 22 | or a |Placeholder Expression|. | |
| 23 | ||
| 24 | * All of ``X``\ 's template parameters, including the default ones, are types. | |
| 25 | ||
| 26 | * The number of ``X``\ 's template parameters, including the default ones, is | |
| 27 | less or equal to the value of ``BOOST_MPL_LIMIT_METAFUNCTION_ARITY`` | |
| 28 | `configuration macro`__. | |
| 29 | ||
| 30 | __ `Configuration`_ | |
| 31 | ||
| 32 | ||
| 33 | Models | |
| 34 | ------ | |
| 35 | ||
| 36 | * |_1| | |
| 37 | * ``plus<_, int_<2> >`` | |
| 38 | * ``if_< less<_1, int_<7> >, plus<_1,_2>, _1 >`` | |
| 39 | ||
| 40 | ||
| 41 | See also | |
| 42 | -------- | |
| 43 | ||
| 44 | |Lambda Expression|, |Placeholders|, |Metafunctions|, |apply|, |lambda| | |
| 45 | ||
| 46 | ||
| 47 |