.. 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)