598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::while_`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_WHILE_HPP
#define BOOST_HANA_WHILE_HPP
#include <boost/hana/fwd/while.hpp>
#include <boost/hana/bool.hpp>
#include <boost/hana/concept/constant.hpp>
#include <boost/hana/concept/constant.hpp>
#include <boost/hana/concept/logical.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/to.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/detail/canonical_constant.hpp>
#include <type_traits>
namespace boost { namespace hana {
//! @cond
template <typename Pred, typename State, typename F>
constexpr decltype(auto) while_t::operator()(Pred&& pred, State&& state, F&& f) const {
using Cond = decltype(pred(state));
using Bool = typename hana::tag_of<Cond>::type;
using While = BOOST_HANA_DISPATCH_IF(while_impl<Bool>,
hana::Logical<Bool>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Logical<Bool>::value,
"hana::while_(pred, state, f) requires 'pred(state)' to be a Logical");
#endif
return While::apply(static_cast<Pred&&>(pred),
static_cast<State&&>(state),
static_cast<F&&>(f));
}
//! @endcond
template <typename L, bool condition>
struct while_impl<L, hana::when<condition>> : hana::default_ {
template <typename ...Args>
static constexpr auto apply(Args&& ...) = delete;
};
template <typename L>
struct while_impl<L, hana::when<std::is_arithmetic<L>::value>> {
template <typename Pred, typename State, typename F>
static auto apply(Pred&& pred, State&& state, F&& f)
-> decltype(
true ? f(static_cast<State&&>(state))
: static_cast<State&&>(state)
)
{
if (pred(state)) {
decltype(auto) r = f(static_cast<State&&>(state));
return hana::while_(static_cast<Pred&&>(pred),
static_cast<decltype(r)&&>(r),
static_cast<F&&>(f));
}
else {
return static_cast<State&&>(state);
}
}
};
template <typename C>
struct while_impl<C, hana::when<
hana::Constant<C>::value &&
hana::Logical<typename C::value_type>::value
>> {
template <typename Pred, typename State, typename F>
static constexpr State
while_helper(hana::false_, Pred&&, State&& state, F&&) {
return static_cast<State&&>(state);
}
template <typename Pred, typename State, typename F>
static constexpr decltype(auto)
while_helper(hana::true_, Pred&& pred, State&& state, F&& f) {
decltype(auto) r = f(static_cast<State&&>(state));
return hana::while_(static_cast<Pred&&>(pred),
static_cast<decltype(r)&&>(r),
static_cast<F&&>(f));
}
template <typename Pred, typename State, typename F>
static constexpr decltype(auto)
apply(Pred&& pred, State&& state, F&& f) {
// Since `pred(state)` returns a `Constant`, we do not actually
// need to call it; we only need its decltype. However, we still
// call it to run potential side effects. I'm not sure whether
// that is desirable, since we pretty much take for granted that
// functions are pure, but we'll do it like this for now. Also, I
// think there is something rather deep hidden behind this, and
// understanding what must be done here should give us a better
// understanding of something non-trivial.
auto cond_ = pred(state);
constexpr auto cond = hana::value(cond_);
constexpr bool truth_value = hana::if_(cond, true, false);
return while_helper(hana::bool_c<truth_value>,
static_cast<Pred&&>(pred),
static_cast<State&&>(state),
static_cast<F&&>(f));
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_WHILE_HPP
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