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 Hu Chunming
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/*!
@file
Defines `boost::hana::capture`.
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_FUNCTIONAL_CAPTURE_HPP
#define BOOST_HANA_FUNCTIONAL_CAPTURE_HPP
#include <boost/hana/basic_tuple.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/detail/decay.hpp>
#include <boost/hana/functional/partial.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @ingroup group-functional
//! Create a function capturing the given variables.
//!
//! Given 0 or more variables, `capture` creates a closure that can be
//! used to partially apply a function. This is very similar to `partial`,
//! except that `capture` allows the partially applied function to be
//! specified later. Specifically, `capture(vars...)` is a function object
//! taking a function `f` and returning `f` partially applied to `vars...`.
//! In other words,
//! @code
//! capture(vars...)(f)(args...) == f(vars..., args...)
//! @endcode
//!
//! @note
//! The arity of `f` must match the total number of arguments passed to
//! it, i.e. `sizeof...(vars) + sizeof...(args)`.
//!
//!
//! Example
//! -------
//! @include example/functional/capture.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto capture = [](auto&& ...variables) {
return [perfect-capture](auto&& f) {
return [perfect-capture](auto&& ...args) -> decltype(auto) {
return forwarded(f)(forwarded(variables)..., forwarded(args)...);
};
};
};
#else
namespace detail {
template <typename F, typename Closure, std::size_t ...i>
constexpr auto apply_capture(F&& f, Closure&& closure, std::index_sequence<i...>) {
return hana::partial(static_cast<F&&>(f),
hana::at_c<i>(static_cast<Closure&&>(closure).storage_)...
);
}
}
template <typename ...X>
struct capture_t;
struct make_capture_t {
struct secret { };
template <typename ...X>
constexpr capture_t<typename detail::decay<X>::type...>
operator()(X&& ...x) const {
return {secret{}, static_cast<X&&>(x)...};
}
};
template <typename ...X>
struct capture_t {
template <typename ...Y>
constexpr capture_t(make_capture_t::secret, Y&& ...y)
: storage_{static_cast<Y&&>(y)...}
{ }
basic_tuple<X...> storage_;
template <typename F>
constexpr auto operator()(F&& f) const& {
return detail::apply_capture(
static_cast<F&&>(f), *this,
std::make_index_sequence<sizeof...(X)>{}
);
}
template <typename F>
constexpr auto operator()(F&& f) & {
return detail::apply_capture(
static_cast<F&&>(f), *this,
std::make_index_sequence<sizeof...(X)>{}
);
}
template <typename F>
constexpr auto operator()(F&& f) && {
return detail::apply_capture(
static_cast<F&&>(f), static_cast<capture_t&&>(*this),
std::make_index_sequence<sizeof...(X)>{}
);
}
};
BOOST_HANA_INLINE_VARIABLE constexpr make_capture_t capture{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_CAPTURE_HPP
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