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244 lines
6.9 KiB
244 lines
6.9 KiB
/*============================================================================= |
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Copyright (c) 2012 Paul Fultz II |
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first_of.h |
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Distributed under the Boost Software License, Version 1.0. (See accompanying |
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
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==============================================================================*/ |
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#ifndef BOOST_HOF_GUARD_FUNCTION_CONDITIONAL_H |
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#define BOOST_HOF_GUARD_FUNCTION_CONDITIONAL_H |
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/// first_of |
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/// ======== |
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/// |
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/// Description |
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/// ----------- |
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/// |
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/// The `first_of` function adaptor combines several functions together. If |
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/// the first function can not be called, then it will try to call the next |
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/// function. This can be very useful when overloading functions using |
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/// template constraints(such as with `enable_if`). |
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/// |
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/// Note: This is different than the [`match`](match.md) function adaptor, which |
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/// can lead to ambiguities. Instead, `first_of` will call the first function |
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/// that is callable, regardless if there is another function that could be |
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/// called as well. |
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/// |
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/// Synopsis |
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/// -------- |
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/// |
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/// template<class... Fs> |
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/// constexpr first_of_adaptor<Fs...> first_of(Fs... fs); |
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/// |
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/// Requirements |
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/// ------------ |
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/// |
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/// Fs must be: |
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/// |
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/// * [ConstInvocable](ConstInvocable) |
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/// * MoveConstructible |
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/// |
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/// Example |
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/// ------- |
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/// |
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/// #include <boost/hof.hpp> |
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/// #include <iostream> |
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/// using namespace boost::hof; |
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/// |
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/// struct for_ints |
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/// { |
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/// void operator()(int) const |
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/// { |
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/// printf("Int\n"); |
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/// } |
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/// }; |
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/// |
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/// struct for_floats |
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/// { |
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/// void operator()(float) const |
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/// { |
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/// printf("Float\n"); |
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/// } |
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/// }; |
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/// |
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/// int main() { |
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/// first_of(for_ints(), for_floats())(3.0); |
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/// } |
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/// |
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/// This will print `Int` because the `for_floats` function object won't ever be |
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/// called. Due to the conversion rules in C++, the `for_ints` function can be |
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/// called on floats, so it is chosen by `first_of` first, even though |
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/// `for_floats` is a better match. |
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/// |
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/// So, the order of the functions in the `first_of_adaptor` are very important |
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/// to how the function is chosen. |
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/// |
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/// References |
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/// ---------- |
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/// |
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/// * [POO51](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0051r2.pdf) - Proposal for C++ |
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/// Proposal for C++ generic overload function |
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/// * [Conditional overloading](<Conditional overloading>) |
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/// |
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#include <boost/hof/reveal.hpp> |
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#include <boost/hof/detail/compressed_pair.hpp> |
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#include <boost/hof/detail/callable_base.hpp> |
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#include <boost/hof/detail/delegate.hpp> |
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#include <boost/hof/detail/join.hpp> |
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#include <boost/hof/detail/seq.hpp> |
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#include <boost/hof/detail/make.hpp> |
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#include <boost/hof/detail/static_const_var.hpp> |
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namespace boost { namespace hof { |
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namespace detail { |
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template<class F1, class F2> |
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struct basic_first_of_adaptor : F1, F2 |
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{ |
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BOOST_HOF_INHERIT_DEFAULT(basic_first_of_adaptor, F1, F2) |
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template<class A, class B, |
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BOOST_HOF_ENABLE_IF_CONVERTIBLE(A, F1), |
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BOOST_HOF_ENABLE_IF_CONVERTIBLE(B, F2)> |
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constexpr basic_first_of_adaptor(A&& f1, B&& f2) |
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noexcept(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(F1, A&&) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(F2, B&&)) |
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: F1(BOOST_HOF_FORWARD(A)(f1)), F2(BOOST_HOF_FORWARD(B)(f2)) |
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{} |
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template<class X, |
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class=typename std::enable_if< |
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BOOST_HOF_IS_CONVERTIBLE(X, F1) && |
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BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(F2) |
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>::type> |
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constexpr basic_first_of_adaptor(X&& x) |
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BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(F1, X&&) |
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: F1(BOOST_HOF_FORWARD(X)(x)) |
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{} |
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template<class... Ts> |
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struct select |
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: std::conditional |
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< |
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is_invocable<F1, Ts...>::value, |
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F1, |
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F2 |
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> |
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{}; |
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BOOST_HOF_RETURNS_CLASS(basic_first_of_adaptor); |
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template<class... Ts, class F=typename select<Ts...>::type> |
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constexpr BOOST_HOF_SFINAE_RESULT(typename select<Ts...>::type, id_<Ts>...) |
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operator()(Ts && ... xs) const |
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BOOST_HOF_SFINAE_RETURNS |
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( |
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BOOST_HOF_RETURNS_STATIC_CAST(const F&)(*BOOST_HOF_CONST_THIS)(BOOST_HOF_FORWARD(Ts)(xs)...) |
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); |
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}; |
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template <class F1, class F2> |
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constexpr const F1& which(std::true_type, const F1& f1, const F2&) noexcept |
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{ |
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return f1; |
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} |
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template <class F1, class F2> |
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constexpr const F2& which(std::false_type, const F1&, const F2& f2) noexcept |
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{ |
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return f2; |
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} |
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template<class F1, class F2> |
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struct conditional_kernel : compressed_pair<F1, F2> |
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{ |
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typedef compressed_pair<F1, F2> base; |
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BOOST_HOF_INHERIT_CONSTRUCTOR(conditional_kernel, base) |
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template<class... Ts> |
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struct select |
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: std::conditional |
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< |
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is_invocable<F1, Ts...>::value, |
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F1, |
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F2 |
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> |
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{}; |
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BOOST_HOF_RETURNS_CLASS(conditional_kernel); |
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template<class... Ts, class PickFirst=is_invocable<F1, Ts...>> |
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constexpr BOOST_HOF_SFINAE_RESULT(typename select<Ts...>::type, id_<Ts>...) |
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operator()(Ts && ... xs) const |
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BOOST_HOF_SFINAE_RETURNS |
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( |
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boost::hof::detail::which( |
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BOOST_HOF_RETURNS_CONSTRUCT(PickFirst)(), |
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BOOST_HOF_MANGLE_CAST(const F1&)(BOOST_HOF_CONST_THIS->first(xs...)), |
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BOOST_HOF_MANGLE_CAST(const F2&)(BOOST_HOF_CONST_THIS->second(xs...)) |
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) |
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(BOOST_HOF_FORWARD(Ts)(xs)...) |
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); |
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}; |
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} |
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template<class F, class... Fs> |
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struct first_of_adaptor |
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: detail::conditional_kernel<F, BOOST_HOF_JOIN(first_of_adaptor, Fs...) > |
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{ |
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typedef first_of_adaptor fit_rewritable_tag; |
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typedef BOOST_HOF_JOIN(first_of_adaptor, Fs...) kernel_base; |
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typedef detail::conditional_kernel<F, kernel_base > base; |
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BOOST_HOF_INHERIT_DEFAULT(first_of_adaptor, base) |
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template<class X, class... Xs, |
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BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(base, X, kernel_base), |
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BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(kernel_base, Xs...)> |
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constexpr first_of_adaptor(X&& f1, Xs&& ... fs) |
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noexcept(BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(base, X&&, kernel_base) && BOOST_HOF_IS_NOTHROW_CONSTRUCTIBLE(kernel_base, Xs&&...)) |
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: base(BOOST_HOF_FORWARD(X)(f1), kernel_base(BOOST_HOF_FORWARD(Xs)(fs)...)) |
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{} |
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template<class X, class... Xs, |
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BOOST_HOF_ENABLE_IF_CONSTRUCTIBLE(base, X)> |
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constexpr first_of_adaptor(X&& f1) |
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BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(base, X&&) |
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: base(BOOST_HOF_FORWARD(X)(f1)) |
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{} |
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struct failure |
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: failure_for<F, Fs...> |
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{}; |
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}; |
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template<class F> |
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struct first_of_adaptor<F> : F |
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{ |
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typedef first_of_adaptor fit_rewritable_tag; |
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BOOST_HOF_INHERIT_CONSTRUCTOR(first_of_adaptor, F); |
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struct failure |
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: failure_for<F> |
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{}; |
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}; |
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template<class F1, class F2> |
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struct first_of_adaptor<F1, F2> |
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: detail::conditional_kernel<F1, F2> |
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{ |
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typedef detail::conditional_kernel<F1, F2> base; |
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typedef first_of_adaptor fit_rewritable_tag; |
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BOOST_HOF_INHERIT_CONSTRUCTOR(first_of_adaptor, base); |
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struct failure |
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: failure_for<F1, F2> |
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{}; |
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}; |
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BOOST_HOF_DECLARE_STATIC_VAR(first_of, detail::make<first_of_adaptor>); |
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}} // namespace boost::hof |
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#endif
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