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