| .. Metafunctions/Composition and Argument Binding//protect |60 |
| |
| protect |
| ======= |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename F |
| > |
| struct protect |
| { |
| // |unspecified| |
| // |...| |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| ``protect`` is an identity wrapper for a |Metafunction Class| that prevents |
| its argument from being recognized as a |bind expression|. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/protect.hpp> |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+---------------------------+---------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===========================+=======================================+ |
| | ``F`` | |Metafunction Class| | A metafunction class to wrap. | |
| +---------------+---------------------------+---------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any |Metafunction Class| ``f``: |
| |
| |
| .. parsed-literal:: |
| |
| typedef protect<f> g; |
| |
| :Return type: |
| |Metafunction Class|. |
| |
| :Semantics: |
| If ``f`` is a |bind expression|, equivalent to |
| |
| .. parsed-literal:: |
| |
| struct g |
| { |
| template< |
| typename U1 = |unspecified|\,\ |...| typename U\ *n* = |unspecified| |
| > |
| struct apply |
| : apply_wrap\ *n*\<f,U1,\ |...|\ U\ *n*\ > |
| { |
| }; |
| }; |
| |
| otherwise equivalent to ``typedef f g;``. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| struct f |
| { |
| template< typename T1, typename T2 > struct apply |
| { |
| typedef T2 type; |
| }; |
| }; |
| |
| typedef bind< quote\ ``3``\<if\_>,_1,_2,bind<f,_1,_2> > b1; |
| typedef bind< quote\ ``3``\<if\_>,_1,_2,protect< bind<f,_1,_2> > > b2; |
| |
| typedef apply_wrap\ ``2``\< b1,false\_,char >::type r1; |
| typedef apply_wrap\ ``2``\< b2,false\_,char >::type r2; |
| |
| BOOST_MPL_ASSERT(( is_same<r1,char> )); |
| BOOST_MPL_ASSERT(( is_same<r2,protect< bind<f,_1,_2> > > )); |
| |
| |
| See also |
| -------- |
| |
| |Composition and Argument Binding|, |Invocation|, |bind|, |quote|, |apply_wrap| |
| |
| |
| .. 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) |
