.. Metafunctions/Composition and Argument Binding//_1,_2,..._n |10

Placeholders

.. _placeholder:

Synopsis

.. parsed-literal::

namespace placeholders {
typedef |unspecified| _;
typedef arg<1>      _1;
typedef arg<2>      _2;
|...|
typedef arg<\ *n*\ >      _\ *n*\ ;
}

using placeholders::_;
using placeholders::_1;
using placeholders::_2;
|...|
using placeholders::_\ *n*\ ;

Description

A placeholder in a form _\ n is simply a synonym for the corresponding arg<n> specialization. The unnamed placeholder _ (underscore) carries special meaning__ in bind and lambda expressions, and does not have defined semantics outside of these contexts.

Placeholder names can be made available in the user namespace through using namespace mpl::placeholders; directive.

__ bind semantics_

Header

.. parsed-literal::

#include <boost/mpl/placeholders.hpp>

|Note:| The include might be omitted when using placeholders to construct a |Lambda Expression| for passing it to MPL's own algorithm or metafunction: any library component that is documented to accept a lambda expression makes the placeholders implicitly available for the user code |-- end note|

Parameters

None.

Expression semantics

For any integral constant n in the range [1, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|] and arbitrary types |a1...an|:

.. parsed-literal::

typedef apply_wrap\ *n*\<_\ *n*\,a1,\ |...|\a\ *n*\ >::type x;

:Return type: A type.

:Semantics: Equivalent to

.. parsed-literal::

    typedef apply_wrap\ *n*\< arg<\ *n*\ >,a1,\ |...|\a\ *n* >::type x;

Example

.. parsed-literal::

typedef apply_wrap\ ``5``\< _1,bool,char,short,int,long >::type t1;
typedef apply_wrap\ ``5``\< _3,bool,char,short,int,long >::type t3;

BOOST_MPL_ASSERT(( is_same< t1, bool > ));
BOOST_MPL_ASSERT(( is_same< t3, short > ));

See also

|Composition and Argument Binding|, |arg|, |lambda|, |bind|, |apply|, |apply_wrap|

.. |placeholder| replace:: placeholder_

.. |1| replace:: _1 .. |2| replace:: _2 .. |3| replace:: _3 .. |4| replace:: _4 .. |5| replace:: _5

.. _1: Placeholders .. _2: Placeholders .. _3: Placeholders .. _4: Placeholders .. _5: Placeholders

.. |_1,_2,..._n| replace:: |_1|, |_2|, |_3|,\ |...|

.. 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)