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 Hu Chunming
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/*!
@file
Defines `boost::hana::overload`.
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_OVERLOAD_HPP
#define BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/detail/decay.hpp>
namespace boost { namespace hana {
//! @ingroup group-functional
//! Pick one of several functions to call based on overload resolution.
//!
//! Specifically, `overload(f1, f2, ..., fn)` is a function object such
//! that
//! @code
//! overload(f1, f2, ..., fn)(x...) == fk(x...)
//! @endcode
//!
//! where `fk` is the function of `f1, ..., fn` that would be called if
//! overload resolution was performed amongst that set of functions only.
//! If more than one function `fk` would be picked by overload resolution,
//! then the call is ambiguous.
//!
//! ### Example
//! @include example/functional/overload.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto overload = [](auto&& f1, auto&& f2, ..., auto&& fn) {
return [perfect-capture](auto&& ...x) -> decltype(auto) {
return forwarded(fk)(forwarded(x)...);
};
};
#else
template <typename F, typename ...G>
struct overload_t
: overload_t<F>::type
, overload_t<G...>::type
{
using type = overload_t;
using overload_t<F>::type::operator();
using overload_t<G...>::type::operator();
template <typename F_, typename ...G_>
constexpr explicit overload_t(F_&& f, G_&& ...g)
: overload_t<F>::type(static_cast<F_&&>(f))
, overload_t<G...>::type(static_cast<G_&&>(g)...)
{ }
};
template <typename F>
struct overload_t<F> { using type = F; };
template <typename R, typename ...Args>
struct overload_t<R(*)(Args...)> {
using type = overload_t;
R (*fptr_)(Args...);
explicit constexpr overload_t(R (*fp)(Args...))
: fptr_(fp)
{ }
constexpr R operator()(Args ...args) const
{ return fptr_(static_cast<Args&&>(args)...); }
};
struct make_overload_t {
template <typename ...F,
typename Overload = typename overload_t<
typename detail::decay<F>::type...
>::type
>
constexpr Overload operator()(F&& ...f) const {
return Overload(static_cast<F&&>(f)...);
}
};
BOOST_HANA_INLINE_VARIABLE constexpr make_overload_t overload{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP
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