Synopsis

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.

Header

#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|:

typedef apply_wrapn<_n,a1,|...|an>::type x;

typedef apply_wrapn< arg<n>,a1,|...|an >::type x;

Example

typedef apply_wrap5< _1,bool,char,short,int,long >::type t1;
typedef apply_wrap5< _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|