| .. Metafunctions/Concepts//Metafunction |10 |
| |
| Metafunction |
| ============ |
| |
| Description |
| ----------- |
| |
| .. _`nullary-metafunction`: |
| |
| A *metafunction* is a class or a class template that represents a |
| function invocable at compile-time. An non-nullary metafunction is |
| invoked by instantiating the class template with particular |
| template parameters (metafunction arguments); the result of the |
| metafunction application is accessible through the instantiation's |
| nested ``type`` typedef. All metafunction's arguments must be types |
| (i.e. only *type template parameters* are allowed). A metafunction |
| can have a variable number of parameters. A *nullary metafunction* is |
| represented as a (template) class with a nested ``type`` typename |
| member. |
| |
| .. |nullary metafunction| replace:: `nullary-metafunction`_ |
| |
| |
| Expression requirements |
| ----------------------- |
| |
| |In the following table...| ``f`` is a |Metafunction|. |
| |
| +-------------------------------+-----------------------+---------------------------+ |
| | Expression | Type | Complexity | |
| +===============================+=======================+===========================+ |
| | ``f::type`` | Any type | Unspecified. | |
| +-------------------------------+-----------------------+---------------------------+ |
| | ``f<>::type`` | Any type | Unspecified. | |
| +-------------------------------+-----------------------+---------------------------+ |
| | ``f<a1,..,an>::type`` | Any type | Unspecified. | |
| +-------------------------------+-----------------------+---------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| .. parsed-literal:: |
| |
| typedef f::type x; |
| |
| :Precondition: |
| ``f`` is a nullary |Metafunction|; ``f::type`` is a *type-name*. |
| |
| :Semantics: |
| ``x`` is the result of the metafunction invocation. |
| |
| |
| .. ................................................................................... |
| |
| .. parsed-literal:: |
| |
| typedef f<>::type x; |
| |
| :Precondition: |
| ``f`` is a nullary |Metafunction|; ``f<>::type`` is a *type-name*. |
| |
| :Semantics: |
| ``x`` is the result of the metafunction invocation. |
| |
| |
| .. ................................................................................... |
| |
| .. parsed-literal:: |
| |
| typedef f<a1,\ |...| \a\ *n*\>::type x; |
| |
| :Precondition: |
| ``f`` is an *n*-ary |Metafunction|; |a1...an| are types; |
| ``f<a1,...an>::type`` is a *type-name*. |
| |
| :Semantics: |
| ``x`` is the result of the metafunction invocation |
| with the actual arguments |a1...an|. |
| |
| |
| Models |
| ------ |
| |
| * |identity| |
| * |plus| |
| * |begin| |
| * |insert| |
| * |fold| |
| |
| |
| See also |
| -------- |
| |
| |Metafunctions|, |Metafunction Class|, |Lambda Expression|, |Invocation|, |apply|, |lambda|, |bind| |
| |
| |
| .. 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) |
