89 lines
2.7 KiB
C++
89 lines
2.7 KiB
C++
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/*!
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@file
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Defines `boost::hana::overload`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP
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#define BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/detail/decay.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @ingroup group-functional
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//! Pick one of several functions to call based on overload resolution.
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//!
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//! Specifically, `overload(f1, f2, ..., fn)` is a function object such
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//! that
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//! @code
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//! overload(f1, f2, ..., fn)(x...) == fk(x...)
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//! @endcode
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//!
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//! where `fk` is the function of `f1, ..., fn` that would be called if
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//! overload resolution was performed amongst that set of functions only.
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//! If more than one function `fk` would be picked by overload resolution,
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//! then the call is ambiguous.
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//!
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//! ### Example
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//! @include example/functional/overload.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto overload = [](auto&& f1, auto&& f2, ..., auto&& fn) {
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return [perfect-capture](auto&& ...x) -> decltype(auto) {
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return forwarded(fk)(forwarded(x)...);
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};
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};
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#else
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template <typename F, typename ...G>
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struct overload_t
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: overload_t<F>::type
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, overload_t<G...>::type
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{
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using type = overload_t;
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using overload_t<F>::type::operator();
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using overload_t<G...>::type::operator();
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template <typename F_, typename ...G_>
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constexpr explicit overload_t(F_&& f, G_&& ...g)
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: overload_t<F>::type(static_cast<F_&&>(f))
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, overload_t<G...>::type(static_cast<G_&&>(g)...)
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{ }
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};
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template <typename F>
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struct overload_t<F> { using type = F; };
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template <typename R, typename ...Args>
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struct overload_t<R(*)(Args...)> {
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using type = overload_t;
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R (*fptr_)(Args...);
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explicit constexpr overload_t(R (*fp)(Args...))
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: fptr_(fp)
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{ }
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constexpr R operator()(Args ...args) const
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{ return fptr_(static_cast<Args&&>(args)...); }
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};
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struct make_overload_t {
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template <typename ...F,
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typename Overload = typename overload_t<
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typename detail::decay<F>::type...
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>::type
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>
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constexpr Overload operator()(F&& ...f) const {
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return Overload(static_cast<F&&>(f)...);
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}
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};
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constexpr make_overload_t overload{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FUNCTIONAL_OVERLOAD_HPP
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