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// file : libbutl/move-only-function.hxx -*- C++ -*-
// license : MIT; see accompanying LICENSE file
#pragma once
#include <utility>
#include <functional>
#include <type_traits>
namespace butl
{
// This is a move-only std::function version which is implemented in terms
// of std::function. It is similar to C++23 std::move_only_function but
// still provides target() (but not target_type()).
//
template <typename>
class move_only_function_ex;
// Alias butl::move_only_function to std::move_only_function if available
// and to move_only_function_ex otherwise.
//
#ifdef __cpp_lib_move_only_function
using std::move_only_function;
#else
template <typename F>
using move_only_function = move_only_function_ex<F>;
#endif
template <typename R, typename... A>
class move_only_function_ex<R (A...)>
{
public:
using result_type = R;
move_only_function_ex () = default;
move_only_function_ex (std::nullptr_t) noexcept {}
// Note: according to the spec we should also disable these if F is not
// callable, but that is not easy to do in C++14. Maybe we should do
// something for C++17 and later (without this the diagnostics is quite
// hairy).
//
template <typename F>
move_only_function_ex (F&& f, typename std::enable_if<!std::is_same<typename std::remove_reference<F>::type, move_only_function_ex>::value>::type* = 0)
{
using FV = typename std::decay<F>::type;
if (!null (f))
f_ = wrapper<FV> (std::forward<F> (f));
}
template <typename F>
typename std::enable_if<!std::is_same<typename std::remove_reference<F>::type, move_only_function_ex>::value, move_only_function_ex>::type&
operator= (F&& f)
{
move_only_function_ex (std::forward<F> (f)).swap (*this);
return *this;
}
move_only_function_ex&
operator= (std::nullptr_t) noexcept
{
f_ = nullptr;
return *this;
}
void swap (move_only_function_ex& f) noexcept
{
f_.swap (f.f_);
}
R operator() (A... args) const
{
return f_ (std::forward<A> (args)...);
}
explicit operator bool () const noexcept
{
return static_cast<bool> (f_);
}
template <typename T>
T* target() noexcept
{
wrapper<T>* r (f_.template target<wrapper<T>> ());
return r != nullptr ? &r->f : nullptr;
}
template <typename T>
const T* target() const noexcept
{
const wrapper<T>* r (f_.template target<wrapper<T>> ());
return r != nullptr ? &r->f : nullptr;
}
move_only_function_ex (move_only_function_ex&&) = default;
move_only_function_ex& operator= (move_only_function_ex&&) = default;
move_only_function_ex (const move_only_function_ex&) = delete;
move_only_function_ex& operator= (const move_only_function_ex&) = delete;
private:
template <typename F>
struct wrapper
{
struct empty {};
union
{
F f;
empty e;
};
explicit wrapper (F&& f_): f (std::move (f_)) {}
explicit wrapper (const F& f_): f (f_) {}
R operator() (A... args)
{
return f (std::forward<A> (args)...);
}
R operator() (A... args) const
{
return f (std::forward<A> (args)...);
}
wrapper (wrapper&& w)
noexcept (std::is_nothrow_move_constructible<F>::value)
: f (std::move (w.f)) {}
wrapper& operator= (wrapper&&) = delete; // Shouldn't be needed.
~wrapper () {f.~F ();}
// These shouldn't be called.
//
wrapper (const wrapper&) {}
wrapper& operator= (const wrapper&) {return *this;}
};
template <typename F> static bool null (const F&) {return false;}
template <typename R1, typename... A1> static bool null (R1 (*p) (A1...)) {return p == nullptr;}
template <typename R1, typename... A1> static bool null (const move_only_function_ex<R1 (A1...)>& f) {return !f;}
template <typename R1, typename C, typename... A1> static bool null (R1 (C::*p) (A1...)) {return p == nullptr;}
template <typename R1, typename C, typename... A1> static bool null (R1 (C::*p) (A1...) const) {return p == nullptr;}
std::function<R (A...)> f_;
};
template <typename R, typename... A>
inline bool
operator== (const move_only_function_ex<R (A...)>& f, std::nullptr_t) noexcept
{
return !f;
}
template <typename R, typename... A>
inline bool
operator== (std::nullptr_t, const move_only_function_ex<R (A...)>& f) noexcept
{
return !f;
}
template <typename R, typename... A>
inline bool
operator!= (const move_only_function_ex<R (A...)>& f, std::nullptr_t) noexcept
{
return static_cast<bool> (f);
}
template <typename R, typename... A>
inline bool
operator!= (std::nullptr_t, const move_only_function_ex<R (A...)>& f) noexcept
{
return static_cast<bool> (f);
}
}
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