/*=============================================================================
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Copyright (c) 2012 Paul Fultz II
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implicit.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_IMPLICIT_H
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#define BOOST_HOF_GUARD_FUNCTION_IMPLICIT_H
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/// implicit
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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 `implicit` adaptor is a static function adaptor that uses the type
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/// that the return value can be converted to, in order to determine the type
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/// of the template parameter. In essence, it will deduce the type for the
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/// template parameter using the type of variable the result is assigned to.
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/// Since it is a static function adaptor, the function must be default
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/// constructible.
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///
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/// Synopsis
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/// --------
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///
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/// template<template <class...> class F>
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/// class implicit<F>;
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///
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/// Semantics
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/// ---------
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///
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/// assert(T(implicit<F>()(xs...)) == F<T>()(xs...));
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///
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/// Requirements
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/// ------------
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///
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/// F must be a template class, that is a:
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///
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/// * [ConstFunctionObject](ConstFunctionObject)
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/// * DefaultConstructible
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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 <cassert>
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/// using namespace boost::hof;
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///
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/// template<class T>
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/// struct auto_caster
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/// {
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/// template<class U>
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/// T operator()(U x)
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/// {
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/// return T(x);
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/// }
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/// };
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///
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/// static constexpr implicit<auto_caster> auto_cast = {};
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///
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/// struct auto_caster_foo
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/// {
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/// int i;
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/// explicit auto_caster_foo(int i_) : i(i_) {}
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///
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/// };
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///
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/// int main() {
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/// float f = 1.5;
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/// int i = auto_cast(f);
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/// auto_caster_foo x = auto_cast(1);
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/// assert(1 == i);
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/// assert(1 == x.i);
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/// }
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///
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#include <boost/hof/pack.hpp>
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#include <boost/hof/detail/result_of.hpp>
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namespace boost { namespace hof { namespace detail {
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template<class F, class Pack, class X, class=void>
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struct is_implicit_callable
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: std::false_type
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{};
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#if BOOST_HOF_NO_EXPRESSION_SFINAE
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template<class F, class Pack, class X>
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struct is_implicit_callable<F, Pack, X, typename std::enable_if<
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std::is_convertible<typename result_of<Pack, id_<F>>::type, X>::value
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>::type>
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: std::true_type
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{};
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#else
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template<class F, class Pack, class X>
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struct is_implicit_callable<F, Pack, X, typename std::enable_if<
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std::is_convertible<decltype(std::declval<Pack>()(std::declval<F>())), X>::value
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>::type>
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: std::true_type
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{};
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#endif
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}
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template<template <class...> class F>
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struct implicit
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{
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template<class Pack>
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struct invoker
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{
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Pack p;
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constexpr invoker(Pack pp) BOOST_HOF_NOEXCEPT_CONSTRUCTIBLE(Pack, Pack&&)
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: p(boost::hof::move(pp))
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{}
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template<class X, class=typename std::enable_if<detail::is_implicit_callable<F<X>, Pack, X>::value>::type>
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constexpr operator X() const BOOST_HOF_NOEXCEPT(noexcept(p(F<X>())))
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{
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return p(F<X>());
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}
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#if !(defined(__GNUC__) && !defined (__clang__) && __GNUC__ == 4 && __GNUC_MINOR__ < 7)
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invoker (const invoker&) = delete;
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invoker& operator= (const invoker&) = delete;
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private:
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friend struct implicit;
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invoker (invoker&&) = default;
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#endif
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};
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struct make_invoker
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{
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template<class Pack>
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constexpr invoker<Pack> operator()(Pack p) const BOOST_HOF_NOEXCEPT(noexcept(invoker<Pack>(boost::hof::move(p))))
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{
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return invoker<Pack>(boost::hof::move(p));
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}
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};
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template<class... Ts>
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constexpr auto operator()(Ts&&... xs) const
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BOOST_HOF_RETURNS
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(
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BOOST_HOF_RETURNS_CONSTRUCT(make_invoker)()(boost::hof::pack_basic(BOOST_HOF_FORWARD(Ts)(xs)...))
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);
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};
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}} // namespace boost::hof
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#endif
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