| .. Metafunctions/Concepts//Placeholder Expression |40 |
| |
| Placeholder Expression |
| ====================== |
| |
| Description |
| ----------- |
| |
| A |Placeholder Expression| is a type that is either a |placeholder| or a class |
| template specialization with at least one argument that itself is a |
| |Placeholder Expression|. |
| |
| |
| Expression requirements |
| ----------------------- |
| |
| If ``X`` is a class template, and ``a1``,... ``an`` are arbitrary types, then |
| ``X<a1,...,an>`` is a |Placeholder Expression| if and only if all of the following |
| conditions hold: |
| |
| * At least one of the template arguments ``a1``,... ``an`` is a |placeholder| |
| or a |Placeholder Expression|. |
| |
| * All of ``X``\ 's template parameters, including the default ones, are types. |
| |
| * The number of ``X``\ 's template parameters, including the default ones, is |
| less or equal to the value of ``BOOST_MPL_LIMIT_METAFUNCTION_ARITY`` |
| `configuration macro`__. |
| |
| __ `Configuration`_ |
| |
| |
| Models |
| ------ |
| |
| * |_1| |
| * ``plus<_, int_<2> >`` |
| * ``if_< less<_1, int_<7> >, plus<_1,_2>, _1 >`` |
| |
| |
| See also |
| -------- |
| |
| |Lambda Expression|, |Placeholders|, |Metafunctions|, |apply|, |lambda| |
| |
| |
| .. copyright:: Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
