// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___FUNCTIONAL_FUNCTION_H
#define _LIBCPP___FUNCTIONAL_FUNCTION_H

#include <__assert>
#include <__config>
#include <__exception/exception.h>
#include <__functional/binary_function.h>
#include <__functional/invoke.h>
#include <__functional/unary_function.h>
#include <__iterator/iterator_traits.h>
#include <__memory/addressof.h>
#include <__memory/allocator.h>
#include <__memory/allocator_destructor.h>
#include <__memory/allocator_traits.h>
#include <__memory/builtin_new_allocator.h>
#include <__memory/compressed_pair.h>
#include <__memory/unique_ptr.h>
#include <__type_traits/aligned_storage.h>
#include <__type_traits/is_trivially_copy_constructible.h>
#include <__type_traits/is_trivially_destructible.h>
#include <__type_traits/strip_signature.h>
#include <__utility/forward.h>
#include <__utility/move.h>
#include <__utility/piecewise_construct.h>
#include <__utility/swap.h>
#include <__verbose_abort>
#include <new>
#include <tuple>
#include <typeinfo>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

#ifndef _LIBCPP_CXX03_LANG

_LIBCPP_BEGIN_NAMESPACE_STD

// bad_function_call

_LIBCPP_DIAGNOSTIC_PUSH
_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
class _LIBCPP_EXCEPTION_ABI bad_function_call
    : public exception
{
public:
// Note that when a key function is not used, every translation unit that uses
// bad_function_call will end up containing a weak definition of the vtable and
// typeinfo.
#ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
    ~bad_function_call() _NOEXCEPT override;
#else
    ~bad_function_call() _NOEXCEPT override {}
#endif

#ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
    const char* what() const _NOEXCEPT override;
#endif
};
_LIBCPP_DIAGNOSTIC_POP

_LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY
void __throw_bad_function_call()
{
#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
    throw bad_function_call();
#else
    _LIBCPP_VERBOSE_ABORT("bad_function_call was thrown in -fno-exceptions mode");
#endif
}

template<class _Fp> class _LIBCPP_TEMPLATE_VIS function; // undefined

namespace __function
{

template<class _Rp>
struct __maybe_derive_from_unary_function
{
};

template<class _Rp, class _A1>
struct __maybe_derive_from_unary_function<_Rp(_A1)>
    : public __unary_function<_A1, _Rp>
{
};

template<class _Rp>
struct __maybe_derive_from_binary_function
{
};

template<class _Rp, class _A1, class _A2>
struct __maybe_derive_from_binary_function<_Rp(_A1, _A2)>
    : public __binary_function<_A1, _A2, _Rp>
{
};

template <class _Fp>
_LIBCPP_INLINE_VISIBILITY
bool __not_null(_Fp const&) { return true; }

template <class _Fp>
_LIBCPP_INLINE_VISIBILITY
bool __not_null(_Fp* __ptr) { return __ptr; }

template <class _Ret, class _Class>
_LIBCPP_INLINE_VISIBILITY
bool __not_null(_Ret _Class::*__ptr) { return __ptr; }

template <class _Fp>
_LIBCPP_INLINE_VISIBILITY
bool __not_null(function<_Fp> const& __f) { return !!__f; }

#ifdef _LIBCPP_HAS_EXTENSION_BLOCKS
template <class _Rp, class ..._Args>
_LIBCPP_INLINE_VISIBILITY
bool __not_null(_Rp (^__p)(_Args...)) { return __p; }
#endif

} // namespace __function

namespace __function {

// __alloc_func holds a functor and an allocator.

template <class _Fp, class _Ap, class _FB> class __alloc_func;
template <class _Fp, class _FB>
class __default_alloc_func;

template <class _Fp, class _Ap, class _Rp, class... _ArgTypes>
class __alloc_func<_Fp, _Ap, _Rp(_ArgTypes...)>
{
    __compressed_pair<_Fp, _Ap> __f_;

  public:
    typedef _LIBCPP_NODEBUG _Fp _Target;
    typedef _LIBCPP_NODEBUG _Ap _Alloc;

    _LIBCPP_INLINE_VISIBILITY
    const _Target& __target() const { return __f_.first(); }

    // WIN32 APIs may define __allocator, so use __get_allocator instead.
    _LIBCPP_INLINE_VISIBILITY
    const _Alloc& __get_allocator() const { return __f_.second(); }

    _LIBCPP_INLINE_VISIBILITY
    explicit __alloc_func(_Target&& __f)
        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
               _VSTD::forward_as_tuple())
    {
    }

    _LIBCPP_INLINE_VISIBILITY
    explicit __alloc_func(const _Target& __f, const _Alloc& __a)
        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
               _VSTD::forward_as_tuple(__a))
    {
    }

    _LIBCPP_INLINE_VISIBILITY
    explicit __alloc_func(const _Target& __f, _Alloc&& __a)
        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
               _VSTD::forward_as_tuple(_VSTD::move(__a)))
    {
    }

    _LIBCPP_INLINE_VISIBILITY
    explicit __alloc_func(_Target&& __f, _Alloc&& __a)
        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
               _VSTD::forward_as_tuple(_VSTD::move(__a)))
    {
    }

    _LIBCPP_INLINE_VISIBILITY
    _Rp operator()(_ArgTypes&&... __arg)
    {
        typedef __invoke_void_return_wrapper<_Rp> _Invoker;
        return _Invoker::__call(__f_.first(),
                                _VSTD::forward<_ArgTypes>(__arg)...);
    }

    _LIBCPP_INLINE_VISIBILITY
    __alloc_func* __clone() const
    {
        typedef allocator_traits<_Alloc> __alloc_traits;
        typedef __rebind_alloc<__alloc_traits, __alloc_func> _AA;
        _AA __a(__f_.second());
        typedef __allocator_destructor<_AA> _Dp;
        unique_ptr<__alloc_func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
        ::new ((void*)__hold.get()) __alloc_func(__f_.first(), _Alloc(__a));
        return __hold.release();
    }

    _LIBCPP_INLINE_VISIBILITY
    void destroy() _NOEXCEPT { __f_.~__compressed_pair<_Target, _Alloc>(); }

    static void __destroy_and_delete(__alloc_func* __f) {
      typedef allocator_traits<_Alloc> __alloc_traits;
      typedef __rebind_alloc<__alloc_traits, __alloc_func> _FunAlloc;
      _FunAlloc __a(__f->__get_allocator());
      __f->destroy();
      __a.deallocate(__f, 1);
    }
};

template <class _Fp, class _Rp, class... _ArgTypes>
class __default_alloc_func<_Fp, _Rp(_ArgTypes...)> {
  _Fp __f_;

public:
  typedef _LIBCPP_NODEBUG _Fp _Target;

  _LIBCPP_INLINE_VISIBILITY
  const _Target& __target() const { return __f_; }

  _LIBCPP_INLINE_VISIBILITY
  explicit __default_alloc_func(_Target&& __f) : __f_(_VSTD::move(__f)) {}

  _LIBCPP_INLINE_VISIBILITY
  explicit __default_alloc_func(const _Target& __f) : __f_(__f) {}

  _LIBCPP_INLINE_VISIBILITY
  _Rp operator()(_ArgTypes&&... __arg) {
    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
    return _Invoker::__call(__f_, _VSTD::forward<_ArgTypes>(__arg)...);
  }

  _LIBCPP_INLINE_VISIBILITY
  __default_alloc_func* __clone() const {
      __builtin_new_allocator::__holder_t __hold =
        __builtin_new_allocator::__allocate_type<__default_alloc_func>(1);
    __default_alloc_func* __res =
        ::new ((void*)__hold.get()) __default_alloc_func(__f_);
    (void)__hold.release();
    return __res;
  }

  _LIBCPP_INLINE_VISIBILITY
  void destroy() _NOEXCEPT { __f_.~_Target(); }

  static void __destroy_and_delete(__default_alloc_func* __f) {
    __f->destroy();
      __builtin_new_allocator::__deallocate_type<__default_alloc_func>(__f, 1);
  }
};

// __base provides an abstract interface for copyable functors.

template<class _Fp> class _LIBCPP_TEMPLATE_VIS __base;

template<class _Rp, class ..._ArgTypes>
class __base<_Rp(_ArgTypes...)>
{
    __base(const __base&);
    __base& operator=(const __base&);
public:
    _LIBCPP_INLINE_VISIBILITY __base() {}
    _LIBCPP_HIDE_FROM_ABI_VIRTUAL virtual ~__base() {}
    virtual __base* __clone() const = 0;
    virtual void __clone(__base*) const = 0;
    virtual void destroy() _NOEXCEPT = 0;
    virtual void destroy_deallocate() _NOEXCEPT = 0;
    virtual _Rp operator()(_ArgTypes&& ...) = 0;
#ifndef _LIBCPP_HAS_NO_RTTI
    virtual const void* target(const type_info&) const _NOEXCEPT = 0;
    virtual const std::type_info& target_type() const _NOEXCEPT = 0;
#endif // _LIBCPP_HAS_NO_RTTI
};

// __func implements __base for a given functor type.

template<class _FD, class _Alloc, class _FB> class __func;

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
class __func<_Fp, _Alloc, _Rp(_ArgTypes...)>
    : public  __base<_Rp(_ArgTypes...)>
{
    __alloc_func<_Fp, _Alloc, _Rp(_ArgTypes...)> __f_;
public:
    _LIBCPP_INLINE_VISIBILITY
    explicit __func(_Fp&& __f)
        : __f_(_VSTD::move(__f)) {}

    _LIBCPP_INLINE_VISIBILITY
    explicit __func(const _Fp& __f, const _Alloc& __a)
        : __f_(__f, __a) {}

    _LIBCPP_INLINE_VISIBILITY
    explicit __func(const _Fp& __f, _Alloc&& __a)
        : __f_(__f, _VSTD::move(__a)) {}

    _LIBCPP_INLINE_VISIBILITY
    explicit __func(_Fp&& __f, _Alloc&& __a)
        : __f_(_VSTD::move(__f), _VSTD::move(__a)) {}

    virtual __base<_Rp(_ArgTypes...)>* __clone() const;
    virtual void __clone(__base<_Rp(_ArgTypes...)>*) const;
    virtual void destroy() _NOEXCEPT;
    virtual void destroy_deallocate() _NOEXCEPT;
    virtual _Rp operator()(_ArgTypes&&... __arg);
#ifndef _LIBCPP_HAS_NO_RTTI
    virtual const void* target(const type_info&) const _NOEXCEPT;
    virtual const std::type_info& target_type() const _NOEXCEPT;
#endif // _LIBCPP_HAS_NO_RTTI
};

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
__base<_Rp(_ArgTypes...)>*
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone() const
{
    typedef allocator_traits<_Alloc> __alloc_traits;
    typedef __rebind_alloc<__alloc_traits, __func> _Ap;
    _Ap __a(__f_.__get_allocator());
    typedef __allocator_destructor<_Ap> _Dp;
    unique_ptr<__func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
    ::new ((void*)__hold.get()) __func(__f_.__target(), _Alloc(__a));
    return __hold.release();
}

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
void
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone(__base<_Rp(_ArgTypes...)>* __p) const
{
    ::new ((void*)__p) __func(__f_.__target(), __f_.__get_allocator());
}

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
void
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy() _NOEXCEPT
{
    __f_.destroy();
}

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
void
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy_deallocate() _NOEXCEPT
{
    typedef allocator_traits<_Alloc> __alloc_traits;
    typedef __rebind_alloc<__alloc_traits, __func> _Ap;
    _Ap __a(__f_.__get_allocator());
    __f_.destroy();
    __a.deallocate(this, 1);
}

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
_Rp
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::operator()(_ArgTypes&& ... __arg)
{
    return __f_(_VSTD::forward<_ArgTypes>(__arg)...);
}

#ifndef _LIBCPP_HAS_NO_RTTI

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
const void*
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::target(const type_info& __ti) const _NOEXCEPT
{
    if (__ti == typeid(_Fp))
        return _VSTD::addressof(__f_.__target());
    return nullptr;
}

template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
const std::type_info&
__func<_Fp, _Alloc, _Rp(_ArgTypes...)>::target_type() const _NOEXCEPT
{
    return typeid(_Fp);
}

#endif // _LIBCPP_HAS_NO_RTTI

// __value_func creates a value-type from a __func.

template <class _Fp> class __value_func;

template <class _Rp, class... _ArgTypes> class __value_func<_Rp(_ArgTypes...)>
{
    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
    typename aligned_storage<3 * sizeof(void*)>::type __buf_;
    _LIBCPP_SUPPRESS_DEPRECATED_POP

    typedef __base<_Rp(_ArgTypes...)> __func;
    __func* __f_;

    _LIBCPP_NO_CFI static __func* __as_base(void* __p)
    {
        return reinterpret_cast<__func*>(__p);
    }

  public:
    _LIBCPP_INLINE_VISIBILITY
    __value_func() _NOEXCEPT : __f_(nullptr) {}

    template <class _Fp, class _Alloc>
    _LIBCPP_INLINE_VISIBILITY __value_func(_Fp&& __f, const _Alloc& __a)
        : __f_(nullptr)
    {
        typedef allocator_traits<_Alloc> __alloc_traits;
        typedef __function::__func<_Fp, _Alloc, _Rp(_ArgTypes...)> _Fun;
        typedef __rebind_alloc<__alloc_traits, _Fun> _FunAlloc;

        if (__function::__not_null(__f))
        {
            _FunAlloc __af(__a);
            if (sizeof(_Fun) <= sizeof(__buf_) &&
                is_nothrow_copy_constructible<_Fp>::value &&
                is_nothrow_copy_constructible<_FunAlloc>::value)
            {
                __f_ =
                    ::new ((void*)&__buf_) _Fun(_VSTD::move(__f), _Alloc(__af));
            }
            else
            {
                typedef __allocator_destructor<_FunAlloc> _Dp;
                unique_ptr<__func, _Dp> __hold(__af.allocate(1), _Dp(__af, 1));
                ::new ((void*)__hold.get()) _Fun(_VSTD::move(__f), _Alloc(__a));
                __f_ = __hold.release();
            }
        }
    }

    template <class _Fp,
        class = typename enable_if<!is_same<__decay_t<_Fp>, __value_func>::value>::type>
    _LIBCPP_INLINE_VISIBILITY explicit __value_func(_Fp&& __f)
        : __value_func(_VSTD::forward<_Fp>(__f), allocator<_Fp>()) {}

    _LIBCPP_INLINE_VISIBILITY
    __value_func(const __value_func& __f)
    {
        if (__f.__f_ == nullptr)
            __f_ = nullptr;
        else if ((void*)__f.__f_ == &__f.__buf_)
        {
            __f_ = __as_base(&__buf_);
            __f.__f_->__clone(__f_);
        }
        else
            __f_ = __f.__f_->__clone();
    }

    _LIBCPP_INLINE_VISIBILITY
    __value_func(__value_func&& __f) _NOEXCEPT
    {
        if (__f.__f_ == nullptr)
            __f_ = nullptr;
        else if ((void*)__f.__f_ == &__f.__buf_)
        {
            __f_ = __as_base(&__buf_);
            __f.__f_->__clone(__f_);
        }
        else
        {
            __f_ = __f.__f_;
            __f.__f_ = nullptr;
        }
    }

    _LIBCPP_INLINE_VISIBILITY
    ~__value_func()
    {
        if ((void*)__f_ == &__buf_)
            __f_->destroy();
        else if (__f_)
            __f_->destroy_deallocate();
    }

    _LIBCPP_INLINE_VISIBILITY
    __value_func& operator=(__value_func&& __f)
    {
        *this = nullptr;
        if (__f.__f_ == nullptr)
            __f_ = nullptr;
        else if ((void*)__f.__f_ == &__f.__buf_)
        {
            __f_ = __as_base(&__buf_);
            __f.__f_->__clone(__f_);
        }
        else
        {
            __f_ = __f.__f_;
            __f.__f_ = nullptr;
        }
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    __value_func& operator=(nullptr_t)
    {
        __func* __f = __f_;
        __f_ = nullptr;
        if ((void*)__f == &__buf_)
            __f->destroy();
        else if (__f)
            __f->destroy_deallocate();
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    _Rp operator()(_ArgTypes&&... __args) const
    {
        if (__f_ == nullptr)
            __throw_bad_function_call();
        return (*__f_)(_VSTD::forward<_ArgTypes>(__args)...);
    }

    _LIBCPP_INLINE_VISIBILITY
    void swap(__value_func& __f) _NOEXCEPT
    {
        if (&__f == this)
            return;
        if ((void*)__f_ == &__buf_ && (void*)__f.__f_ == &__f.__buf_)
        {
            _LIBCPP_SUPPRESS_DEPRECATED_PUSH
            typename aligned_storage<sizeof(__buf_)>::type __tempbuf;
            _LIBCPP_SUPPRESS_DEPRECATED_POP
            __func* __t = __as_base(&__tempbuf);
            __f_->__clone(__t);
            __f_->destroy();
            __f_ = nullptr;
            __f.__f_->__clone(__as_base(&__buf_));
            __f.__f_->destroy();
            __f.__f_ = nullptr;
            __f_ = __as_base(&__buf_);
            __t->__clone(__as_base(&__f.__buf_));
            __t->destroy();
            __f.__f_ = __as_base(&__f.__buf_);
        }
        else if ((void*)__f_ == &__buf_)
        {
            __f_->__clone(__as_base(&__f.__buf_));
            __f_->destroy();
            __f_ = __f.__f_;
            __f.__f_ = __as_base(&__f.__buf_);
        }
        else if ((void*)__f.__f_ == &__f.__buf_)
        {
            __f.__f_->__clone(__as_base(&__buf_));
            __f.__f_->destroy();
            __f.__f_ = __f_;
            __f_ = __as_base(&__buf_);
        }
        else
            _VSTD::swap(__f_, __f.__f_);
    }

    _LIBCPP_INLINE_VISIBILITY
    explicit operator bool() const _NOEXCEPT { return __f_ != nullptr; }

#ifndef _LIBCPP_HAS_NO_RTTI
    _LIBCPP_INLINE_VISIBILITY
    const std::type_info& target_type() const _NOEXCEPT
    {
        if (__f_ == nullptr)
            return typeid(void);
        return __f_->target_type();
    }

    template <typename _Tp>
    _LIBCPP_INLINE_VISIBILITY const _Tp* target() const _NOEXCEPT
    {
        if (__f_ == nullptr)
            return nullptr;
        return (const _Tp*)__f_->target(typeid(_Tp));
    }
#endif // _LIBCPP_HAS_NO_RTTI
};

// Storage for a functor object, to be used with __policy to manage copy and
// destruction.
union __policy_storage
{
    mutable char __small[sizeof(void*) * 2];
    void* __large;
};

// True if _Fun can safely be held in __policy_storage.__small.
template <typename _Fun>
struct __use_small_storage
    : public integral_constant<
          bool, sizeof(_Fun) <= sizeof(__policy_storage) &&
                    _LIBCPP_ALIGNOF(_Fun) <= _LIBCPP_ALIGNOF(__policy_storage) &&
                    is_trivially_copy_constructible<_Fun>::value &&
                    is_trivially_destructible<_Fun>::value> {};

// Policy contains information about how to copy, destroy, and move the
// underlying functor. You can think of it as a vtable of sorts.
struct __policy
{
    // Used to copy or destroy __large values. null for trivial objects.
    void* (*const __clone)(const void*);
    void (*const __destroy)(void*);

    // True if this is the null policy (no value).
    const bool __is_null;

    // The target type. May be null if RTTI is disabled.
    const std::type_info* const __type_info;

    // Returns a pointer to a static policy object suitable for the functor
    // type.
    template <typename _Fun>
    _LIBCPP_INLINE_VISIBILITY static const __policy* __create()
    {
        return __choose_policy<_Fun>(__use_small_storage<_Fun>());
    }

    _LIBCPP_INLINE_VISIBILITY
    static const __policy* __create_empty()
    {
        static const _LIBCPP_CONSTEXPR __policy __policy = {nullptr, nullptr,
                                                            true,
#ifndef _LIBCPP_HAS_NO_RTTI
                                                            &typeid(void)
#else
                                                            nullptr
#endif
        };
        return &__policy;
    }

  private:
    template <typename _Fun> static void* __large_clone(const void* __s)
    {
        const _Fun* __f = static_cast<const _Fun*>(__s);
        return __f->__clone();
    }

    template <typename _Fun>
    static void __large_destroy(void* __s) {
      _Fun::__destroy_and_delete(static_cast<_Fun*>(__s));
    }

    template <typename _Fun>
    _LIBCPP_INLINE_VISIBILITY static const __policy*
    __choose_policy(/* is_small = */ false_type) {
      static const _LIBCPP_CONSTEXPR __policy __policy = {
          &__large_clone<_Fun>, &__large_destroy<_Fun>, false,
#ifndef _LIBCPP_HAS_NO_RTTI
          &typeid(typename _Fun::_Target)
#else
          nullptr
#endif
      };
        return &__policy;
    }

    template <typename _Fun>
    _LIBCPP_INLINE_VISIBILITY static const __policy*
        __choose_policy(/* is_small = */ true_type)
    {
        static const _LIBCPP_CONSTEXPR __policy __policy = {
            nullptr, nullptr, false,
#ifndef _LIBCPP_HAS_NO_RTTI
            &typeid(typename _Fun::_Target)
#else
            nullptr
#endif
        };
        return &__policy;
    }
};

// Used to choose between perfect forwarding or pass-by-value. Pass-by-value is
// faster for types that can be passed in registers.
template <typename _Tp>
using __fast_forward = __conditional_t<is_scalar<_Tp>::value, _Tp, _Tp&&>;

// __policy_invoker calls an instance of __alloc_func held in __policy_storage.

template <class _Fp> struct __policy_invoker;

template <class _Rp, class... _ArgTypes>
struct __policy_invoker<_Rp(_ArgTypes...)>
{
    typedef _Rp (*__Call)(const __policy_storage*,
                          __fast_forward<_ArgTypes>...);

    __Call __call_;

    // Creates an invoker that throws bad_function_call.
    _LIBCPP_INLINE_VISIBILITY
    __policy_invoker() : __call_(&__call_empty) {}

    // Creates an invoker that calls the given instance of __func.
    template <typename _Fun>
    _LIBCPP_INLINE_VISIBILITY static __policy_invoker __create()
    {
        return __policy_invoker(&__call_impl<_Fun>);
    }

  private:
    _LIBCPP_INLINE_VISIBILITY
    explicit __policy_invoker(__Call __c) : __call_(__c) {}

    static _Rp __call_empty(const __policy_storage*,
                            __fast_forward<_ArgTypes>...)
    {
        __throw_bad_function_call();
    }

    template <typename _Fun>
    static _Rp __call_impl(const __policy_storage* __buf,
                           __fast_forward<_ArgTypes>... __args)
    {
        _Fun* __f = reinterpret_cast<_Fun*>(__use_small_storage<_Fun>::value
                                                ? &__buf->__small
                                                : __buf->__large);
        return (*__f)(_VSTD::forward<_ArgTypes>(__args)...);
    }
};

// __policy_func uses a __policy and __policy_invoker to create a type-erased,
// copyable functor.

template <class _Fp> class __policy_func;

template <class _Rp, class... _ArgTypes> class __policy_func<_Rp(_ArgTypes...)>
{
    // Inline storage for small objects.
    __policy_storage __buf_;

    // Calls the value stored in __buf_. This could technically be part of
    // policy, but storing it here eliminates a level of indirection inside
    // operator().
    typedef __function::__policy_invoker<_Rp(_ArgTypes...)> __invoker;
    __invoker __invoker_;

    // The policy that describes how to move / copy / destroy __buf_. Never
    // null, even if the function is empty.
    const __policy* __policy_;

  public:
    _LIBCPP_INLINE_VISIBILITY
    __policy_func() : __policy_(__policy::__create_empty()) {}

    template <class _Fp, class _Alloc>
    _LIBCPP_INLINE_VISIBILITY __policy_func(_Fp&& __f, const _Alloc& __a)
        : __policy_(__policy::__create_empty())
    {
        typedef __alloc_func<_Fp, _Alloc, _Rp(_ArgTypes...)> _Fun;
        typedef allocator_traits<_Alloc> __alloc_traits;
        typedef __rebind_alloc<__alloc_traits, _Fun> _FunAlloc;

        if (__function::__not_null(__f))
        {
            __invoker_ = __invoker::template __create<_Fun>();
            __policy_ = __policy::__create<_Fun>();

            _FunAlloc __af(__a);
            if (__use_small_storage<_Fun>())
            {
                ::new ((void*)&__buf_.__small)
                    _Fun(_VSTD::move(__f), _Alloc(__af));
            }
            else
            {
                typedef __allocator_destructor<_FunAlloc> _Dp;
                unique_ptr<_Fun, _Dp> __hold(__af.allocate(1), _Dp(__af, 1));
                ::new ((void*)__hold.get())
                    _Fun(_VSTD::move(__f), _Alloc(__af));
                __buf_.__large = __hold.release();
            }
        }
    }

    template <class _Fp, class = typename enable_if<!is_same<__decay_t<_Fp>, __policy_func>::value>::type>
    _LIBCPP_INLINE_VISIBILITY explicit __policy_func(_Fp&& __f)
        : __policy_(__policy::__create_empty()) {
      typedef __default_alloc_func<_Fp, _Rp(_ArgTypes...)> _Fun;

      if (__function::__not_null(__f)) {
        __invoker_ = __invoker::template __create<_Fun>();
        __policy_ = __policy::__create<_Fun>();
        if (__use_small_storage<_Fun>()) {
          ::new ((void*)&__buf_.__small) _Fun(_VSTD::move(__f));
        } else {
          __builtin_new_allocator::__holder_t __hold =
              __builtin_new_allocator::__allocate_type<_Fun>(1);
          __buf_.__large = ::new ((void*)__hold.get()) _Fun(_VSTD::move(__f));
          (void)__hold.release();
        }
      }
    }

    _LIBCPP_INLINE_VISIBILITY
    __policy_func(const __policy_func& __f)
        : __buf_(__f.__buf_), __invoker_(__f.__invoker_),
          __policy_(__f.__policy_)
    {
        if (__policy_->__clone)
            __buf_.__large = __policy_->__clone(__f.__buf_.__large);
    }

    _LIBCPP_INLINE_VISIBILITY
    __policy_func(__policy_func&& __f)
        : __buf_(__f.__buf_), __invoker_(__f.__invoker_),
          __policy_(__f.__policy_)
    {
        if (__policy_->__destroy)
        {
            __f.__policy_ = __policy::__create_empty();
            __f.__invoker_ = __invoker();
        }
    }

    _LIBCPP_INLINE_VISIBILITY
    ~__policy_func()
    {
        if (__policy_->__destroy)
            __policy_->__destroy(__buf_.__large);
    }

    _LIBCPP_INLINE_VISIBILITY
    __policy_func& operator=(__policy_func&& __f)
    {
        *this = nullptr;
        __buf_ = __f.__buf_;
        __invoker_ = __f.__invoker_;
        __policy_ = __f.__policy_;
        __f.__policy_ = __policy::__create_empty();
        __f.__invoker_ = __invoker();
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    __policy_func& operator=(nullptr_t)
    {
        const __policy* __p = __policy_;
        __policy_ = __policy::__create_empty();
        __invoker_ = __invoker();
        if (__p->__destroy)
            __p->__destroy(__buf_.__large);
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    _Rp operator()(_ArgTypes&&... __args) const
    {
        return __invoker_.__call_(_VSTD::addressof(__buf_),
                                  _VSTD::forward<_ArgTypes>(__args)...);
    }

    _LIBCPP_INLINE_VISIBILITY
    void swap(__policy_func& __f)
    {
        _VSTD::swap(__invoker_, __f.__invoker_);
        _VSTD::swap(__policy_, __f.__policy_);
        _VSTD::swap(__buf_, __f.__buf_);
    }

    _LIBCPP_INLINE_VISIBILITY
    explicit operator bool() const _NOEXCEPT
    {
        return !__policy_->__is_null;
    }

#ifndef _LIBCPP_HAS_NO_RTTI
    _LIBCPP_INLINE_VISIBILITY
    const std::type_info& target_type() const _NOEXCEPT
    {
        return *__policy_->__type_info;
    }

    template <typename _Tp>
    _LIBCPP_INLINE_VISIBILITY const _Tp* target() const _NOEXCEPT
    {
        if (__policy_->__is_null || typeid(_Tp) != *__policy_->__type_info)
            return nullptr;
        if (__policy_->__clone) // Out of line storage.
            return reinterpret_cast<const _Tp*>(__buf_.__large);
        else
            return reinterpret_cast<const _Tp*>(&__buf_.__small);
    }
#endif // _LIBCPP_HAS_NO_RTTI
};

#if defined(_LIBCPP_HAS_BLOCKS_RUNTIME)

extern "C" void *_Block_copy(const void *);
extern "C" void _Block_release(const void *);

template<class _Rp1, class ..._ArgTypes1, class _Alloc, class _Rp, class ..._ArgTypes>
class __func<_Rp1(^)(_ArgTypes1...), _Alloc, _Rp(_ArgTypes...)>
    : public  __base<_Rp(_ArgTypes...)>
{
    typedef _Rp1(^__block_type)(_ArgTypes1...);
    __block_type __f_;

public:
    _LIBCPP_INLINE_VISIBILITY
    explicit __func(__block_type const& __f)
#ifdef _LIBCPP_HAS_OBJC_ARC
        : __f_(__f)
#else
        : __f_(reinterpret_cast<__block_type>(__f ? _Block_copy(__f) : nullptr))
#endif
    { }

    // [TODO] add && to save on a retain

    _LIBCPP_INLINE_VISIBILITY
    explicit __func(__block_type __f, const _Alloc& /* unused */)
#ifdef _LIBCPP_HAS_OBJC_ARC
        : __f_(__f)
#else
        : __f_(reinterpret_cast<__block_type>(__f ? _Block_copy(__f) : nullptr))
#endif
    { }

    virtual __base<_Rp(_ArgTypes...)>* __clone() const {
        _LIBCPP_ASSERT(false,
            "Block pointers are just pointers, so they should always fit into "
            "std::function's small buffer optimization. This function should "
            "never be invoked.");
        return nullptr;
    }

    virtual void __clone(__base<_Rp(_ArgTypes...)>* __p) const {
        ::new ((void*)__p) __func(__f_);
    }

    virtual void destroy() _NOEXCEPT {
#ifndef _LIBCPP_HAS_OBJC_ARC
        if (__f_)
            _Block_release(__f_);
#endif
        __f_ = 0;
    }

    virtual void destroy_deallocate() _NOEXCEPT {
        _LIBCPP_ASSERT(false,
            "Block pointers are just pointers, so they should always fit into "
            "std::function's small buffer optimization. This function should "
            "never be invoked.");
    }

    virtual _Rp operator()(_ArgTypes&& ... __arg) {
        return _VSTD::__invoke(__f_, _VSTD::forward<_ArgTypes>(__arg)...);
    }

#ifndef _LIBCPP_HAS_NO_RTTI
    virtual const void* target(type_info const& __ti) const _NOEXCEPT {
        if (__ti == typeid(__func::__block_type))
            return &__f_;
        return (const void*)nullptr;
    }

    virtual const std::type_info& target_type() const _NOEXCEPT {
        return typeid(__func::__block_type);
    }
#endif // _LIBCPP_HAS_NO_RTTI
};

#endif // _LIBCPP_HAS_EXTENSION_BLOCKS

} // namespace __function

template<class _Rp, class ..._ArgTypes>
class _LIBCPP_TEMPLATE_VIS function<_Rp(_ArgTypes...)>
    : public __function::__maybe_derive_from_unary_function<_Rp(_ArgTypes...)>,
      public __function::__maybe_derive_from_binary_function<_Rp(_ArgTypes...)>
{
#ifndef _LIBCPP_ABI_OPTIMIZED_FUNCTION
    typedef __function::__value_func<_Rp(_ArgTypes...)> __func;
#else
    typedef __function::__policy_func<_Rp(_ArgTypes...)> __func;
#endif

    __func __f_;

    template <class _Fp, bool = _And<
        _IsNotSame<__remove_cvref_t<_Fp>, function>,
        __invokable<_Fp, _ArgTypes...>
    >::value>
    struct __callable;
    template <class _Fp>
        struct __callable<_Fp, true>
        {
            static const bool value = is_void<_Rp>::value ||
                __is_core_convertible<typename __invoke_of<_Fp, _ArgTypes...>::type,
                                      _Rp>::value;
        };
    template <class _Fp>
        struct __callable<_Fp, false>
        {
            static const bool value = false;
        };

  template <class _Fp>
  using _EnableIfLValueCallable = typename enable_if<__callable<_Fp&>::value>::type;
public:
    typedef _Rp result_type;

    // construct/copy/destroy:
    _LIBCPP_INLINE_VISIBILITY
    function() _NOEXCEPT { }
    _LIBCPP_INLINE_VISIBILITY
    function(nullptr_t) _NOEXCEPT {}
    function(const function&);
    function(function&&) _NOEXCEPT;
    template<class _Fp, class = _EnableIfLValueCallable<_Fp>>
    function(_Fp);

#if _LIBCPP_STD_VER <= 14
    template<class _Alloc>
      _LIBCPP_INLINE_VISIBILITY
      function(allocator_arg_t, const _Alloc&) _NOEXCEPT {}
    template<class _Alloc>
      _LIBCPP_INLINE_VISIBILITY
      function(allocator_arg_t, const _Alloc&, nullptr_t) _NOEXCEPT {}
    template<class _Alloc>
      function(allocator_arg_t, const _Alloc&, const function&);
    template<class _Alloc>
      function(allocator_arg_t, const _Alloc&, function&&);
    template<class _Fp, class _Alloc, class = _EnableIfLValueCallable<_Fp>>
      function(allocator_arg_t, const _Alloc& __a, _Fp __f);
#endif

    function& operator=(const function&);
    function& operator=(function&&) _NOEXCEPT;
    function& operator=(nullptr_t) _NOEXCEPT;
    template<class _Fp, class = _EnableIfLValueCallable<__decay_t<_Fp>>>
    function& operator=(_Fp&&);

    ~function();

    // function modifiers:
    void swap(function&) _NOEXCEPT;

#if _LIBCPP_STD_VER <= 14
    template<class _Fp, class _Alloc>
      _LIBCPP_INLINE_VISIBILITY
      void assign(_Fp&& __f, const _Alloc& __a)
        {function(allocator_arg, __a, _VSTD::forward<_Fp>(__f)).swap(*this);}
#endif

    // function capacity:
    _LIBCPP_INLINE_VISIBILITY
    explicit operator bool() const _NOEXCEPT {
      return static_cast<bool>(__f_);
    }

    // deleted overloads close possible hole in the type system
    template<class _R2, class... _ArgTypes2>
      bool operator==(const function<_R2(_ArgTypes2...)>&) const = delete;
    template<class _R2, class... _ArgTypes2>
      bool operator!=(const function<_R2(_ArgTypes2...)>&) const = delete;
public:
    // function invocation:
    _Rp operator()(_ArgTypes...) const;

#ifndef _LIBCPP_HAS_NO_RTTI
    // function target access:
    const std::type_info& target_type() const _NOEXCEPT;
    template <typename _Tp> _Tp* target() _NOEXCEPT;
    template <typename _Tp> const _Tp* target() const _NOEXCEPT;
#endif // _LIBCPP_HAS_NO_RTTI
};

#if _LIBCPP_STD_VER >= 17
template<class _Rp, class ..._Ap>
function(_Rp(*)(_Ap...)) -> function<_Rp(_Ap...)>;

template<class _Fp, class _Stripped = typename __strip_signature<decltype(&_Fp::operator())>::type>
function(_Fp) -> function<_Stripped>;
#endif // _LIBCPP_STD_VER >= 17

template<class _Rp, class ..._ArgTypes>
function<_Rp(_ArgTypes...)>::function(const function& __f) : __f_(__f.__f_) {}

#if _LIBCPP_STD_VER <= 14
template<class _Rp, class ..._ArgTypes>
template <class _Alloc>
function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc&,
                                     const function& __f) : __f_(__f.__f_) {}
#endif

template <class _Rp, class... _ArgTypes>
function<_Rp(_ArgTypes...)>::function(function&& __f) _NOEXCEPT
    : __f_(_VSTD::move(__f.__f_)) {}

#if _LIBCPP_STD_VER <= 14
template<class _Rp, class ..._ArgTypes>
template <class _Alloc>
function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc&,
                                      function&& __f)
    : __f_(_VSTD::move(__f.__f_)) {}
#endif

template <class _Rp, class... _ArgTypes>
template <class _Fp, class>
function<_Rp(_ArgTypes...)>::function(_Fp __f) : __f_(_VSTD::move(__f)) {}

#if _LIBCPP_STD_VER <= 14
template <class _Rp, class... _ArgTypes>
template <class _Fp, class _Alloc, class>
function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc& __a,
                                      _Fp __f)
    : __f_(_VSTD::move(__f), __a) {}
#endif

template<class _Rp, class ..._ArgTypes>
function<_Rp(_ArgTypes...)>&
function<_Rp(_ArgTypes...)>::operator=(const function& __f)
{
    function(__f).swap(*this);
    return *this;
}

template<class _Rp, class ..._ArgTypes>
function<_Rp(_ArgTypes...)>&
function<_Rp(_ArgTypes...)>::operator=(function&& __f) _NOEXCEPT
{
    __f_ = _VSTD::move(__f.__f_);
    return *this;
}

template<class _Rp, class ..._ArgTypes>
function<_Rp(_ArgTypes...)>&
function<_Rp(_ArgTypes...)>::operator=(nullptr_t) _NOEXCEPT
{
    __f_ = nullptr;
    return *this;
}

template<class _Rp, class ..._ArgTypes>
template <class _Fp, class>
function<_Rp(_ArgTypes...)>&
function<_Rp(_ArgTypes...)>::operator=(_Fp&& __f)
{
    function(_VSTD::forward<_Fp>(__f)).swap(*this);
    return *this;
}

template<class _Rp, class ..._ArgTypes>
function<_Rp(_ArgTypes...)>::~function() {}

template<class _Rp, class ..._ArgTypes>
void
function<_Rp(_ArgTypes...)>::swap(function& __f) _NOEXCEPT
{
    __f_.swap(__f.__f_);
}

template<class _Rp, class ..._ArgTypes>
_Rp
function<_Rp(_ArgTypes...)>::operator()(_ArgTypes... __arg) const
{
    return __f_(_VSTD::forward<_ArgTypes>(__arg)...);
}

#ifndef _LIBCPP_HAS_NO_RTTI

template<class _Rp, class ..._ArgTypes>
const std::type_info&
function<_Rp(_ArgTypes...)>::target_type() const _NOEXCEPT
{
    return __f_.target_type();
}

template<class _Rp, class ..._ArgTypes>
template <typename _Tp>
_Tp*
function<_Rp(_ArgTypes...)>::target() _NOEXCEPT
{
    return (_Tp*)(__f_.template target<_Tp>());
}

template<class _Rp, class ..._ArgTypes>
template <typename _Tp>
const _Tp*
function<_Rp(_ArgTypes...)>::target() const _NOEXCEPT
{
    return __f_.template target<_Tp>();
}

#endif // _LIBCPP_HAS_NO_RTTI

template <class _Rp, class... _ArgTypes>
inline _LIBCPP_INLINE_VISIBILITY
bool
operator==(const function<_Rp(_ArgTypes...)>& __f, nullptr_t) _NOEXCEPT {return !__f;}

template <class _Rp, class... _ArgTypes>
inline _LIBCPP_INLINE_VISIBILITY
bool
operator==(nullptr_t, const function<_Rp(_ArgTypes...)>& __f) _NOEXCEPT {return !__f;}

template <class _Rp, class... _ArgTypes>
inline _LIBCPP_INLINE_VISIBILITY
bool
operator!=(const function<_Rp(_ArgTypes...)>& __f, nullptr_t) _NOEXCEPT {return (bool)__f;}

template <class _Rp, class... _ArgTypes>
inline _LIBCPP_INLINE_VISIBILITY
bool
operator!=(nullptr_t, const function<_Rp(_ArgTypes...)>& __f) _NOEXCEPT {return (bool)__f;}

template <class _Rp, class... _ArgTypes>
inline _LIBCPP_INLINE_VISIBILITY
void
swap(function<_Rp(_ArgTypes...)>& __x, function<_Rp(_ArgTypes...)>& __y) _NOEXCEPT
{return __x.swap(__y);}

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_CXX03_LANG

#endif // _LIBCPP___FUNCTIONAL_FUNCTION_H