ceph/src/boost/libs/hana/include/boost/hana/functional/always.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::always`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
  11 #define BOOST_HANA_FUNCTIONAL_ALWAYS_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/detail/create.hpp>
  15
  16 #include <utility>
  17
  18
  19 BOOST_HANA_NAMESPACE_BEGIN
  20     //! @ingroup group-functional
  21     //! Return a constant function returning `x` regardless of the
  22     //! argument(s) it is invoked with.
  23     //!
  24     //! Specifically, `always(x)` is a function such that
  25     //! @code
  26     //!     always(x)(y...) == x
  27     //! @endcode
  28     //! for any `y...`. A copy of `x` is made and it is owned by the
  29     //! `always(x)` function. When `always(x)` is called, it will return
  30     //! a reference to the `x` it owns. This reference is valid as long
  31     //! as `always(x)` is in scope.
  32     //!
  33     //!
  34     //! ### Example
  35     //! @include example/functional/always.cpp
  36 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  37     constexpr auto always = [](auto&& x) {
  38         return [perfect-capture](auto const& ...y) -> decltype(auto) {
  39             return forwarded(x);
  40         };
  41     };
  42 #else
  43     template <typename T>
  44     struct _always {
  45         T val_;
  46
  47         template <typename ...Args>
  48         constexpr T const& operator()(Args const& ...) const&
  49         { return val_; }
  50
  51         template <typename ...Args>
  52         constexpr T& operator()(Args const& ...) &
  53         { return val_; }
  54
  55         template <typename ...Args>
  56         constexpr T operator()(Args const& ...) &&
  57         { return std::move(val_); }
  58     };
  59
  60     constexpr detail::create<_always> always{};
  61 #endif
  62 BOOST_HANA_NAMESPACE_END
  63
  64 #endif // !BOOST_HANA_FUNCTIONAL_ALWAYS_HPP