ceph/src/boost/boost/hana/fwd/eval.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::eval`.
   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_EVAL_HPP
  11 #define BOOST_HANA_FWD_EVAL_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Evaluate a lazy value and return it.
  19     //! @relates hana::lazy
  20     //!
  21     //! Given a lazy expression `expr`, `eval` evaluates `expr` and returns
  22     //! the result as a normal value. However, for convenience, `eval` can
  23     //! also be used with nullary and unary function objects. Specifically,
  24     //! if `expr` is not a `hana::lazy`, it is called with no arguments at
  25     //! all and the result of that call (`expr()`) is returned. Otherwise,
  26     //! if `expr()` is ill-formed, then `expr(hana::id)` is returned instead.
  27     //! If that expression is ill-formed, then a compile-time error is
  28     //! triggered.
  29     //!
  30     //! The reason for allowing nullary callables in `eval` is because this
  31     //! allows using nullary lambdas as lazy branches to `eval_if`, which
  32     //! is convenient. The reason for allowing unary callables and calling
  33     //! them with `hana::id` is because this allows deferring the
  34     //! compile-time evaluation of selected expressions inside the callable.
  35     //! How this can be achieved is documented by `hana::eval_if`.
  36     //!
  37     //!
  38     //! Example
  39     //! -------
  40     //! @include example/eval.cpp
  41 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  42     constexpr auto eval = [](auto&& see_documentation) -> decltype(auto) {
  43         return tag-dispatched;
  44     };
  45 #else
  46     template <typename T, typename = void>
  47     struct eval_impl : eval_impl<T, when<true>> { };
  48
  49     struct eval_t {
  50         template <typename Expr>
  51         constexpr decltype(auto) operator()(Expr&& expr) const;
  52     };
  53
  54     constexpr eval_t eval{};
  55 #endif
  56 BOOST_HANA_NAMESPACE_END
  57
  58 #endif // !BOOST_HANA_FWD_EVAL_HPP