245 lines
6.9 KiB
C++
245 lines
6.9 KiB
C++
/*=============================================================================
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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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