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 Hu Chunming
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/*!
@file
Defines `boost::hana::always`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
#define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/detail/create.hpp>
#include <utility>
namespace boost { namespace hana {
//! @ingroup group-functional
//! Return a constant function returning `x` regardless of the
//! argument(s) it is invoked with.
//!
//! Specifically, `always(x)` is a function such that
//! @code
//! always(x)(y...) == x
//! @endcode
//! for any `y...`. A copy of `x` is made and it is owned by the
//! `always(x)` function. When `always(x)` is called, it will return
//! a reference to the `x` it owns. This reference is valid as long
//! as `always(x)` is in scope.
//!
//!
//! ### Example
//! @include example/functional/always.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto always = [](auto&& x) {
return [perfect-capture](auto const& ...y) -> decltype(auto) {
return forwarded(x);
};
};
#else
template <typename T>
struct _always {
T val_;
template <typename ...Args>
constexpr T const& operator()(Args const& ...) const&
{ return val_; }
template <typename ...Args>
constexpr T& operator()(Args const& ...) &
{ return val_; }
template <typename ...Args>
constexpr T operator()(Args const& ...) &&
{ return std::move(val_); }
};
BOOST_HANA_INLINE_VARIABLE constexpr detail::create<_always> always{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
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