ceph/src/boost/boost/hana/fwd/core/make.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::make`.
   4
   5 @copyright Louis Dionne 2013-2017
   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_CORE_MAKE_HPP
  11 #define BOOST_HANA_FWD_CORE_MAKE_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15
  16 BOOST_HANA_NAMESPACE_BEGIN
  17     //! @ingroup group-core
  18     //! Create an object of the given tag with the given arguments.
  19     //!
  20     //! This function serves the same purpose as constructors in usual C++.
  21     //! However, instead of creating an object of a specific C++ type, it
  22     //! creates an object of a specific tag, regardless of the C++ type
  23     //! of that object.
  24     //!
  25     //! This function is actually a variable template, so `make<T>` can be
  26     //! passed around as a function object creating an object of tag `T`.
  27     //! Also, it uses tag-dispatching so this is how it should be customized
  28     //! for user-defined tags.
  29     //!
  30     //! Finally, the default implementation of `make` is equivalent to calling
  31     //! the constructor of the given tag with the corresponding arguments.
  32     //! In other words, by default,
  33     //! @code
  34     //!     make<T>(args...) == T(args...)
  35     //! @endcode
  36     //!
  37     //! Note that the arguments are perfectly forwarded and the form of
  38     //! construction which is used is exactly as documented, i.e. `T(args...)`.
  39     //! However, if `T(args...)` is not a valid expression, a compilation
  40     //! error is triggered. This default behavior is useful because it makes
  41     //! foreign C++ types that have no notion of tag constructible with `make`
  42     //! out-of-the-box, since their tag is exactly themselves.
  43     //!
  44     //!
  45     //! Example
  46     //! -------
  47     //! @include example/core/make.cpp
  48 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  49     template <typename Tag>
  50     constexpr auto make = [](auto&& ...x) -> decltype(auto) {
  51         return tag-dispatched;
  52     };
  53 #else
  54     template <typename Tag, typename = void>
  55     struct make_impl;
  56
  57     template <typename Tag>
  58     struct make_t {
  59         template <typename ...X>
  60         constexpr decltype(auto) operator()(X&& ...x) const {
  61             return make_impl<Tag>::apply(static_cast<X&&>(x)...);
  62         }
  63     };
  64
  65     template <typename Tag>
  66     constexpr make_t<Tag> make{};
  67 #endif
  68 BOOST_HANA_NAMESPACE_END
  69
  70 #endif // !BOOST_HANA_FWD_CORE_MAKE_HPP