/*!
|
@file
|
Defines `boost::hana::always`.
|
|
@copyright Louis Dionne 2013-2017
|
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>
|
|
|
BOOST_HANA_NAMESPACE_BEGIN
|
//! @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_); }
|
};
|
|
constexpr detail::create<_always> always{};
|
#endif
|
BOOST_HANA_NAMESPACE_END
|
|
#endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
|
