1/*!
2@file
3Defines `boost::hana::eval`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_EVAL_HPP
11#define BOOST_HANA_EVAL_HPP
12
13#include <boost/hana/fwd/eval.hpp>
14
15#include <boost/hana/config.hpp>
16#include <boost/hana/core/dispatch.hpp>
17#include <boost/hana/detail/wrong.hpp>
18#include <boost/hana/functional/id.hpp>
19
20
21namespace boost { namespace hana {
22 //! @cond
23 template <typename Expr>
24 constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
25 return eval_impl<typename hana::tag_of<Expr>::type>::apply(
26 static_cast<Expr&&>(expr)
27 );
28 }
29 //! @endcond
30
31 template <typename T, bool condition>
32 struct eval_impl<T, when<condition>> : default_ {
33 template <typename Expr>
34 static constexpr auto eval_helper(Expr&& expr, int)
35 -> decltype(static_cast<Expr&&>(expr)())
36 { return static_cast<Expr&&>(expr)(); }
37
38 template <typename Expr>
39 static constexpr auto eval_helper(Expr&& expr, long)
40 -> decltype(static_cast<Expr&&>(expr)(hana::id))
41 { return static_cast<Expr&&>(expr)(hana::id); }
42
43 template <typename Expr>
44 static constexpr auto eval_helper(Expr&&, ...) {
45 static_assert(detail::wrong<Expr>{},
46 "hana::eval(expr) requires the expression to be a hana::lazy, "
47 "a nullary Callable or a unary Callable that may be "
48 "called with hana::id");
49 }
50
51 template <typename Expr>
52 static constexpr decltype(auto) apply(Expr&& expr)
53 { return eval_helper(static_cast<Expr&&>(expr), int{}); }
54 };
55}} // end namespace boost::hana
56
57#endif // !BOOST_HANA_EVAL_HPP
58

source code of boost/libs/hana/include/boost/hana/eval.hpp