[/
  Copyright 2014, 2025 Peter Dimov

  Distributed under the Boost Software License, Version 1.0.

  See accompanying file LICENSE_1_0.txt
  or copy at http://boost.org/LICENSE_1_0.txt
]

[section:is_placeholder is_placeholder]

[simplesect Authors]

* Peter Dimov

[endsimplesect]

[section Header <boost/is_placeholder.hpp>]

The header `<boost/is_placeholder.hpp>` defines the class template
`boost::is_placeholder<T>`. It defines a nested integral constant
`value` which is `0` when `T` is not a `boost::bind` placeholder
type, and a positive value corresponding to the placeholder index
otherwise.

That is, `is_placeholder<_1>::value` is `1`,
`is_placeholder<_2>::value` is `2`, and so on.

`is_placeholder` can be specialized for user types. If it is,
`boost::bind` will recognize these types as placeholders.

[section Synopsis]

``
namespace boost
{
    template<class T> struct is_placeholder;
}
``

[endsect]

[section Example]

``
#include <boost/is_placeholder.hpp>
#include <boost/bind.hpp>

struct arg1_type {};
constexpr arg1_type arg1{};

template<> struct boost::is_placeholder<arg1_type>
{
    static constexpr int value = 1;
};

int f( int x ) { return x + 1; }

int main()
{
    return boost::bind( f, arg1 )( -1 );
}
``

[endsect]

[endsect]

[endsect]