65 lines
1.8 KiB
C++
65 lines
1.8 KiB
C++
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/*!
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@file
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Defines `boost::hana::always`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
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#define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/detail/create.hpp>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @ingroup group-functional
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//! Return a constant function returning `x` regardless of the
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//! argument(s) it is invoked with.
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//!
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//! Specifically, `always(x)` is a function such that
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//! @code
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//! always(x)(y...) == x
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//! @endcode
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//! for any `y...`. A copy of `x` is made and it is owned by the
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//! `always(x)` function. When `always(x)` is called, it will return
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//! a reference to the `x` it owns. This reference is valid as long
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//! as `always(x)` is in scope.
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//!
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//!
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//! ### Example
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//! @include example/functional/always.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto always = [](auto&& x) {
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return [perfect-capture](auto const& ...y) -> decltype(auto) {
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return forwarded(x);
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};
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};
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#else
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template <typename T>
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struct _always {
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T val_;
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template <typename ...Args>
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constexpr T const& operator()(Args const& ...) const&
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{ return val_; }
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template <typename ...Args>
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constexpr T& operator()(Args const& ...) &
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{ return val_; }
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template <typename ...Args>
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constexpr T operator()(Args const& ...) &&
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{ return std::move(val_); }
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};
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constexpr detail::create<_always> always{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
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