ceph/src/boost/libs/hana/include/boost/hana/fwd/if.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::if_`.
   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_FWD_IF_HPP
  11 #define BOOST_HANA_FWD_IF_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Conditionally return one of two values based on a condition.
  19     //! @ingroup group-Logical
  20     //!
  21     //! Specifically, `then` is returned iff `cond` is true-valued, and
  22     //! `else_` is returned otherwise. Note that some `Logical` models may
  23     //! allow `then` and `else_` to have different types, while others may
  24     //! require both values to have the same type.
  25     //!
  26     //!
  27     //! @param cond
  28     //! The condition determining which of the two values is returned.
  29     //!
  30     //! @param then
  31     //! The value returned when `cond` is true-valued.
  32     //!
  33     //! @param else_
  34     //! The value returned when `cond` is false-valued.
  35     //!
  36     //!
  37     //! Example
  38     //! -------
  39     //! @include example/if.cpp
  40 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  41     constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) {
  42         return tag-dispatched;
  43     };
  44 #else
  45     template <typename L, typename = void>
  46     struct if_impl : if_impl<L, when<true>> { };
  47
  48     struct if_t {
  49         template <typename Cond, typename Then, typename Else>
  50         constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const;
  51     };
  52
  53     constexpr if_t if_{};
  54 #endif
  55 BOOST_HANA_NAMESPACE_END
  56
  57 #endif // !BOOST_HANA_FWD_IF_HPP