#ifndef BOOST_FUNCTION_FUNCTION_TEMPLATE_HPP_INCLUDED
#define BOOST_FUNCTION_FUNCTION_TEMPLATE_HPP_INCLUDED

// Boost.Function library

//  Copyright Douglas Gregor 2001-2006
//  Copyright Emil Dotchevski 2007
//  Use, modification and distribution is subject to 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)

// For more information, see http://www.boost.org

#include <b/function/function_base.hpp>
#include <b/core/no_exceptions_support.hpp>
#include <b/mem_fn.hpp>
#include <b/throw_exception.hpp>
#include <b/config.hpp>
#include <algorithm>
#include <cassert>
#include <type_traits>

#if defined(BOOST_MSVC)
#   pragma warning( push )
#   pragma warning( disable : 4127 ) // "conditional expression is constant"
#endif

namespace boost {
  namespace detail {
    namespace function {
      template<
        typename FunctionPtr,
        typename R,
        typename... T
        >
      struct function_invoker
      {
        static R invoke(function_buffer& function_ptr,
                        T... a)
        {
          FunctionPtr f = reinterpret_cast<FunctionPtr>(function_ptr.members.func_ptr);
          return f(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionPtr,
        typename R,
        typename... T
        >
      struct void_function_invoker
      {
        static void
        invoke(function_buffer& function_ptr,
               T... a)

        {
          FunctionPtr f = reinterpret_cast<FunctionPtr>(function_ptr.members.func_ptr);
          f(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
      >
      struct function_obj_invoker
      {
        static R invoke(function_buffer& function_obj_ptr,
                        T... a)

        {
          FunctionObj* f;
          if (function_allows_small_object_optimization<FunctionObj>::value)
            f = reinterpret_cast<FunctionObj*>(function_obj_ptr.data);
          else
            f = reinterpret_cast<FunctionObj*>(function_obj_ptr.members.obj_ptr);
          return (*f)(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
      >
      struct void_function_obj_invoker
      {
        static void
        invoke(function_buffer& function_obj_ptr,
               T... a)

        {
          FunctionObj* f;
          if (function_allows_small_object_optimization<FunctionObj>::value)
            f = reinterpret_cast<FunctionObj*>(function_obj_ptr.data);
          else
            f = reinterpret_cast<FunctionObj*>(function_obj_ptr.members.obj_ptr);
          (*f)(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
      >
      struct function_ref_invoker
      {
        static R invoke(function_buffer& function_obj_ptr,
                        T... a)

        {
          FunctionObj* f =
            reinterpret_cast<FunctionObj*>(function_obj_ptr.members.obj_ptr);
          return (*f)(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
      >
      struct void_function_ref_invoker
      {
        static void
        invoke(function_buffer& function_obj_ptr,
               T... a)

        {
          FunctionObj* f =
            reinterpret_cast<FunctionObj*>(function_obj_ptr.members.obj_ptr);
          (*f)(static_cast<T&&>(a)...);
        }
      };

      /* Handle invocation of member pointers. */
      template<
        typename MemberPtr,
        typename R,
        typename... T
      >
      struct member_invoker
      {
        static R invoke(function_buffer& function_obj_ptr,
                        T... a)

        {
          MemberPtr* f =
            reinterpret_cast<MemberPtr*>(function_obj_ptr.data);
          return boost::mem_fn(*f)(static_cast<T&&>(a)...);
        }
      };

      template<
        typename MemberPtr,
        typename R,
        typename... T
      >
      struct void_member_invoker
      {
        static void
        invoke(function_buffer& function_obj_ptr,
               T... a)

        {
          MemberPtr* f =
            reinterpret_cast<MemberPtr*>(function_obj_ptr.data);
          boost::mem_fn(*f)(static_cast<T&&>(a)...);
        }
      };

      template<
        typename FunctionPtr,
        typename R,
        typename... T
      >
      struct get_function_invoker
      {
        typedef typename std::conditional<std::is_void<R>::value,
                            void_function_invoker<
                            FunctionPtr,
                            R,
                            T...
                          >,
                          function_invoker<
                            FunctionPtr,
                            R,
                            T...
                          >
                       >::type type;
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
       >
      struct get_function_obj_invoker
      {
        typedef typename std::conditional<std::is_void<R>::value,
                            void_function_obj_invoker<
                            FunctionObj,
                            R,
                            T...
                          >,
                          function_obj_invoker<
                            FunctionObj,
                            R,
                            T...
                          >
                       >::type type;
      };

      template<
        typename FunctionObj,
        typename R,
        typename... T
       >
      struct get_function_ref_invoker
      {
        typedef typename std::conditional<std::is_void<R>::value,
                            void_function_ref_invoker<
                            FunctionObj,
                            R,
                            T...
                          >,
                          function_ref_invoker<
                            FunctionObj,
                            R,
                            T...
                          >
                       >::type type;
      };

      /* Retrieve the appropriate invoker for a member pointer.  */
      template<
        typename MemberPtr,
        typename R,
        typename... T
       >
      struct get_member_invoker
      {
        typedef typename std::conditional<std::is_void<R>::value,
                            void_member_invoker<
                            MemberPtr,
                            R,
                            T...
                          >,
                          member_invoker<
                            MemberPtr,
                            R,
                            T...
                          >
                       >::type type;
      };

      /* Given the tag returned by get_function_tag, retrieve the
         actual invoker that will be used for the given function
         object.

         Each specialization contains an "apply_" nested class template
         that accepts the function object, return type, function
         argument types, and allocator. The resulting "apply_" class
         contains two typedefs, "invoker_type" and "manager_type",
         which correspond to the invoker and manager types. */
      template<typename Tag>
      struct get_invoker { };

      /* Retrieve the invoker for a function pointer. */
      template<>
      struct get_invoker<function_ptr_tag>
      {
        template<typename FunctionPtr,
                 typename R, typename... T>
        struct apply_
        {
          typedef typename get_function_invoker<
                             FunctionPtr,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager<FunctionPtr> manager_type;
        };

        template<typename FunctionPtr, typename Allocator,
                 typename R, typename... T>
        struct apply_a
        {
          typedef typename get_function_invoker<
                             FunctionPtr,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager<FunctionPtr> manager_type;
        };
      };

      /* Retrieve the invoker for a member pointer. */
      template<>
      struct get_invoker<member_ptr_tag>
      {
        template<typename MemberPtr,
                 typename R, typename... T>
        struct apply_
        {
          typedef typename get_member_invoker<
                             MemberPtr,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager<MemberPtr> manager_type;
        };

        template<typename MemberPtr, typename Allocator,
                 typename R, typename... T>
        struct apply_a
        {
          typedef typename get_member_invoker<
                             MemberPtr,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager<MemberPtr> manager_type;
        };
      };

      /* Retrieve the invoker for a function object. */
      template<>
      struct get_invoker<function_obj_tag>
      {
        template<typename FunctionObj,
                 typename R, typename... T>
        struct apply_
        {
          typedef typename get_function_obj_invoker<
                             FunctionObj,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager<FunctionObj> manager_type;
        };

        template<typename FunctionObj, typename Allocator,
                 typename R, typename... T>
        struct apply_a
        {
          typedef typename get_function_obj_invoker<
                             FunctionObj,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef functor_manager_a<FunctionObj, Allocator> manager_type;
        };
      };

      /* Retrieve the invoker for a reference to a function object. */
      template<>
      struct get_invoker<function_obj_ref_tag>
      {
        template<typename RefWrapper,
                 typename R, typename... T>
        struct apply_
        {
          typedef typename get_function_ref_invoker<
                             typename RefWrapper::type,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef reference_manager<typename RefWrapper::type> manager_type;
        };

        template<typename RefWrapper, typename Allocator,
                 typename R, typename... T>
        struct apply_a
        {
          typedef typename get_function_ref_invoker<
                             typename RefWrapper::type,
                             R,
                             T...
                           >::type
            invoker_type;

          typedef reference_manager<typename RefWrapper::type> manager_type;
        };
      };


      /**
       * vtable for a specific boost::function instance. This
       * structure must be an aggregate so that we can use static
       * initialization in boost::function's assign_to and assign_to_a
       * members. It therefore cannot have any constructors,
       * destructors, base classes, etc.
       */
      template<typename R, typename... T>
      struct basic_vtable
      {
        typedef R         result_type;

        typedef result_type (*invoker_type)(function_buffer&
                                           ,
                                            T...);

        template<typename F>
        bool assign_to(F f, function_buffer& functor) const
        {
          typedef typename get_function_tag<F>::type tag;
          return assign_to(std::move(f), functor, tag());
        }
        template<typename F,typename Allocator>
        bool assign_to_a(F f, function_buffer& functor, Allocator a) const
        {
          typedef typename get_function_tag<F>::type tag;
          return assign_to_a(std::move(f), functor, a, tag());
        }

        void clear(function_buffer& functor) const
        {
#if defined(BOOST_GCC) && (__GNUC__ >= 11)
# pragma GCC diagnostic push
// False positive in GCC 11/12 for empty function objects
# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
#endif
          if (base.manager)
            base.manager(functor, functor, destroy_functor_tag);
#if defined(BOOST_GCC) && (__GNUC__ >= 11)
# pragma GCC diagnostic pop
#endif
        }

      private:
        // Function pointers
        template<typename FunctionPtr>
        bool
        assign_to(FunctionPtr f, function_buffer& functor, function_ptr_tag) const
        {
          this->clear(functor);
          if (f) {
            functor.members.func_ptr = reinterpret_cast<void (*)()>(f);
            return true;
          } else {
            return false;
          }
        }
        template<typename FunctionPtr,typename Allocator>
        bool
        assign_to_a(FunctionPtr f, function_buffer& functor, Allocator, function_ptr_tag) const
        {
          return assign_to(std::move(f),functor,function_ptr_tag());
        }

        // Member pointers
        template<typename MemberPtr>
        bool assign_to(MemberPtr f, function_buffer& functor, member_ptr_tag) const
        {
          // DPG TBD: Add explicit support for member function
          // objects, so we invoke through mem_fn() but we retain the
          // right target_type() values.
          if (f) {
            this->assign_to(boost::mem_fn(f), functor);
            return true;
          } else {
            return false;
          }
        }
        template<typename MemberPtr,typename Allocator>
        bool assign_to_a(MemberPtr f, function_buffer& functor, Allocator a, member_ptr_tag) const
        {
          // DPG TBD: Add explicit support for member function
          // objects, so we invoke through mem_fn() but we retain the
          // right target_type() values.
          if (f) {
            this->assign_to_a(boost::mem_fn(f), functor, a);
            return true;
          } else {
            return false;
          }
        }

        // Function objects
        // Assign to a function object using the small object optimization
        template<typename FunctionObj>
        void
        assign_functor(FunctionObj f, function_buffer& functor, std::true_type) const
        {
          new (reinterpret_cast<void*>(functor.data)) FunctionObj(std::move(f));
        }
        template<typename FunctionObj,typename Allocator>
        void
        assign_functor_a(FunctionObj f, function_buffer& functor, Allocator, std::true_type) const
        {
          assign_functor(std::move(f),functor,std::true_type());
        }

        // Assign to a function object allocated on the heap.
        template<typename FunctionObj>
        void
        assign_functor(FunctionObj f, function_buffer& functor, std::false_type) const
        {
          functor.members.obj_ptr = new FunctionObj(std::move(f));
        }
        template<typename FunctionObj,typename Allocator>
        void
        assign_functor_a(FunctionObj f, function_buffer& functor, Allocator a, std::false_type) const
        {
          typedef functor_wrapper<FunctionObj,Allocator> functor_wrapper_type;

          using wrapper_allocator_type = typename std::allocator_traits<Allocator>::template rebind_alloc<functor_wrapper_type>;
          using wrapper_allocator_pointer_type = typename std::allocator_traits<wrapper_allocator_type>::pointer;

          wrapper_allocator_type wrapper_allocator(a);
          wrapper_allocator_pointer_type copy = wrapper_allocator.allocate(1);
          std::allocator_traits<wrapper_allocator_type>::construct(wrapper_allocator, copy, functor_wrapper_type(f,a));

          functor_wrapper_type* new_f = static_cast<functor_wrapper_type*>(copy);
          functor.members.obj_ptr = new_f;
        }

        template<typename FunctionObj>
        bool
        assign_to(FunctionObj f, function_buffer& functor, function_obj_tag) const
        {
          if (!boost::detail::function::has_empty_target(boost::addressof(f))) {
            assign_functor(std::move(f), functor,
                           std::integral_constant<bool, (function_allows_small_object_optimization<FunctionObj>::value)>());
            return true;
          } else {
            return false;
          }
        }
        template<typename FunctionObj,typename Allocator>
        bool
        assign_to_a(FunctionObj f, function_buffer& functor, Allocator a, function_obj_tag) const
        {
          if (!boost::detail::function::has_empty_target(boost::addressof(f))) {
            assign_functor_a(std::move(f), functor, a,
                           std::integral_constant<bool, (function_allows_small_object_optimization<FunctionObj>::value)>());
            return true;
          } else {
            return false;
          }
        }

        // Reference to a function object
        template<typename FunctionObj>
        bool
        assign_to(const reference_wrapper<FunctionObj>& f,
                  function_buffer& functor, function_obj_ref_tag) const
        {
          functor.members.obj_ref.obj_ptr = (void *)(f.get_pointer());
          functor.members.obj_ref.is_const_qualified = std::is_const<FunctionObj>::value;
          functor.members.obj_ref.is_volatile_qualified = std::is_volatile<FunctionObj>::value;
          return true;
        }
        template<typename FunctionObj,typename Allocator>
        bool
        assign_to_a(const reference_wrapper<FunctionObj>& f,
                  function_buffer& functor, Allocator, function_obj_ref_tag) const
        {
          return assign_to(f,functor,function_obj_ref_tag());
        }

      public:
        vtable_base base;
        invoker_type invoker;
      };

      template <typename... T>
      struct variadic_function_base
      {};

      template <typename T1>
      struct variadic_function_base<T1>
      {
        typedef T1 argument_type;
        typedef T1 arg1_type;
      };

      template <typename T1, typename T2>
      struct variadic_function_base<T1, T2>
      {
        typedef T1 first_argument_type;
        typedef T2 second_argument_type;
        typedef T1 arg1_type;
        typedef T2 arg2_type;
      };

      template <typename T1, typename T2, typename T3>
      struct variadic_function_base<T1, T2, T3>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
      };

      template <typename T1, typename T2, typename T3, typename T4>
      struct variadic_function_base<T1, T2, T3, T4>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5>
      struct variadic_function_base<T1, T2, T3, T4, T5>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5, typename T6>
      struct variadic_function_base<T1, T2, T3, T4, T5, T6>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
        typedef T6 arg6_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7>
      struct variadic_function_base<T1, T2, T3, T4, T5, T6, T7>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
        typedef T6 arg6_type;
        typedef T7 arg7_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7, typename T8>
      struct variadic_function_base<T1, T2, T3, T4, T5, T6, T7, T8>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
        typedef T6 arg6_type;
        typedef T7 arg7_type;
        typedef T8 arg8_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7, typename T8, typename T9>
      struct variadic_function_base<T1, T2, T3, T4, T5, T6, T7, T8, T9>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
        typedef T6 arg6_type;
        typedef T7 arg7_type;
        typedef T8 arg8_type;
        typedef T9 arg9_type;
      };

      template <typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7, typename T8, typename T9, typename T10>
      struct variadic_function_base<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>
      {
        typedef T1 arg1_type;
        typedef T2 arg2_type;
        typedef T3 arg3_type;
        typedef T4 arg4_type;
        typedef T5 arg5_type;
        typedef T6 arg6_type;
        typedef T7 arg7_type;
        typedef T8 arg8_type;
        typedef T9 arg9_type;
        typedef T10 arg10_type;
      };

#if defined( BOOST_LIBSTDCXX_VERSION ) && BOOST_LIBSTDCXX_VERSION < 50000

      template<class T> struct is_trivially_copyable: std::integral_constant<bool,
        __has_trivial_copy(T) && __has_trivial_assign(T) && __has_trivial_destructor(T)> {};

#else

      using std::is_trivially_copyable;

#endif

    } // end namespace function
  } // end namespace detail

  template<
    typename R,
    typename... T
  >
  class function_n : public function_base
                                , public detail::function::variadic_function_base<T...>
  {
  public:
    typedef R         result_type;

  private:
    typedef boost::detail::function::basic_vtable<
              R, T...>
      vtable_type;

    vtable_type* get_vtable() const {
      return reinterpret_cast<vtable_type*>(
               reinterpret_cast<std::size_t>(vtable) & ~static_cast<std::size_t>(0x01));
    }

    struct clear_type {};

  public:
    // add signature for boost::lambda
    template<typename Args>
    struct sig
    {
      typedef result_type type;
    };

    BOOST_STATIC_CONSTANT(int, arity = sizeof...(T));

    typedef function_n self_type;

    BOOST_DEFAULTED_FUNCTION(function_n(), : function_base() {})

    // MSVC chokes if the following two constructors are collapsed into
    // one with a default parameter.
    template<typename Functor>
    function_n(Functor f
                            ,typename std::enable_if<
                             !std::is_integral<Functor>::value,
                                        int>::type = 0
                            ) :
      function_base()
    {
      this->assign_to(std::move(f));
    }
    template<typename Functor,typename Allocator>
    function_n(Functor f, Allocator a
                            ,typename std::enable_if<
                              !std::is_integral<Functor>::value,
                                        int>::type = 0
                            ) :
      function_base()
    {
      this->assign_to_a(std::move(f),a);
    }

    function_n(clear_type*) : function_base() { }

    function_n(const function_n& f) : function_base()
    {
      this->assign_to_own(f);
    }

    function_n(function_n&& f) : function_base()
    {
      this->move_assign(f);
    }

    ~function_n() { clear(); }

    result_type operator()(T... a) const
    {
      if (this->empty())
        boost::throw_exception(bad_function_call());

      return get_vtable()->invoker
               (this->functor, static_cast<T&&>(a)...);
    }

    // The distinction between when to use function_n and
    // when to use self_type is obnoxious. MSVC cannot handle self_type as
    // the return type of these assignment operators, but Borland C++ cannot
    // handle function_n as the type of the temporary to
    // construct.
    template<typename Functor>
    typename std::enable_if<
                  !std::is_integral<Functor>::value,
               function_n&>::type
    operator=(Functor f)
    {
      this->clear();
      BOOST_TRY  {
        this->assign_to(f);
      } BOOST_CATCH (...) {
        vtable = 0;
        BOOST_RETHROW;
      }
      BOOST_CATCH_END
      return *this;
    }
    template<typename Functor,typename Allocator>
    void assign(Functor f, Allocator a)
    {
      this->clear();
      BOOST_TRY{
        this->assign_to_a(f,a);
      } BOOST_CATCH (...) {
        vtable = 0;
        BOOST_RETHROW;
      }
      BOOST_CATCH_END
    }

    function_n& operator=(clear_type*)
    {
      this->clear();
      return *this;
    }

    // Assignment from another function_n
    function_n& operator=(const function_n& f)
    {
      if (&f == this)
        return *this;

      this->clear();
      BOOST_TRY {
        this->assign_to_own(f);
      } BOOST_CATCH (...) {
        vtable = 0;
        BOOST_RETHROW;
      }
      BOOST_CATCH_END
      return *this;
    }

    // Move assignment from another function_n
    function_n& operator=(function_n&& f)
    {
      if (&f == this)
        return *this;

      this->clear();
      BOOST_TRY {
        this->move_assign(f);
      } BOOST_CATCH (...) {
        vtable = 0;
        BOOST_RETHROW;
      }
      BOOST_CATCH_END
      return *this;
    }

    void swap(function_n& other)
    {
      if (&other == this)
        return;

      function_n tmp;
      tmp.move_assign(*this);
      this->move_assign(other);
      other.move_assign(tmp);
    }

    // Clear out a target, if there is one
    void clear()
    {
      if (vtable) {
        if (!this->has_trivial_copy_and_destroy())
          get_vtable()->clear(this->functor);
        vtable = 0;
      }
    }

    explicit operator bool () const { return !this->empty(); }

  private:
    void assign_to_own(const function_n& f)
    {
      if (!f.empty()) {
        this->vtable = f.vtable;
        if (this->has_trivial_copy_and_destroy()) {
          // Don't operate on storage directly since union type doesn't relax
          // strict aliasing rules, despite of having member char type.
#         if defined(BOOST_GCC) && (BOOST_GCC >= 40700)
#           pragma GCC diagnostic push
            // This warning is technically correct, but we don't want to pay the price for initializing
            // just to silence a warning: https://github.com/boostorg/function/issues/27
#           pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
#           if (BOOST_GCC >= 110000)
              // GCC 11.3, 12 emit a different warning: https://github.com/boostorg/function/issues/42
#             pragma GCC diagnostic ignored "-Wuninitialized"
#           endif
#         endif
          std::memcpy(this->functor.data, f.functor.data, sizeof(boost::detail::function::function_buffer));
#         if defined(BOOST_GCC) && (BOOST_GCC >= 40700)
#           pragma GCC diagnostic pop
#         endif
        } else
          get_vtable()->base.manager(f.functor, this->functor,
                                     boost::detail::function::clone_functor_tag);
      }
    }

    template<typename Functor>
    void assign_to(Functor f)
    {
      using boost::detail::function::vtable_base;

      typedef typename boost::detail::function::get_function_tag<Functor>::type tag;
      typedef boost::detail::function::get_invoker<tag> get_invoker;
      typedef typename get_invoker::
                         template apply_<Functor, R,
                        T...>
        handler_type;

      typedef typename handler_type::invoker_type invoker_type;
      typedef typename handler_type::manager_type manager_type;

      // Note: it is extremely important that this initialization use
      // static initialization. Otherwise, we will have a race
      // condition here in multi-threaded code. See
      // http://thread.gmane.org/gmane.comp.lib.boost.devel/164902/.
      static const vtable_type stored_vtable =
        { { &manager_type::manage }, &invoker_type::invoke };

      if (stored_vtable.assign_to(std::move(f), functor)) {
        std::size_t value = reinterpret_cast<std::size_t>(&stored_vtable.base);
        // coverity[pointless_expression]: suppress coverity warnings on apparant if(const).
        if (boost::detail::function::is_trivially_copyable<Functor>::value &&
            boost::detail::function::function_allows_small_object_optimization<Functor>::value)
          value |= static_cast<std::size_t>(0x01);
        vtable = reinterpret_cast<boost::detail::function::vtable_base *>(value);
      } else
        vtable = 0;
    }

    template<typename Functor,typename Allocator>
    void assign_to_a(Functor f,Allocator a)
    {
      using boost::detail::function::vtable_base;

      typedef typename boost::detail::function::get_function_tag<Functor>::type tag;
      typedef boost::detail::function::get_invoker<tag> get_invoker;
      typedef typename get_invoker::
                         template apply_a<Functor, Allocator, R,
                         T...>
        handler_type;

      typedef typename handler_type::invoker_type invoker_type;
      typedef typename handler_type::manager_type manager_type;

      // Note: it is extremely important that this initialization use
      // static initialization. Otherwise, we will have a race
      // condition here in multi-threaded code. See
      // http://thread.gmane.org/gmane.comp.lib.boost.devel/164902/.
      static const vtable_type stored_vtable =
        { { &manager_type::manage }, &invoker_type::invoke };

      if (stored_vtable.assign_to_a(std::move(f), functor, a)) {
        std::size_t value = reinterpret_cast<std::size_t>(&stored_vtable.base);
        // coverity[pointless_expression]: suppress coverity warnings on apparant if(const).
        if (boost::detail::function::is_trivially_copyable<Functor>::value &&
            boost::detail::function::function_allows_small_object_optimization<Functor>::value)
          value |= static_cast<std::size_t>(0x01);
        vtable = reinterpret_cast<boost::detail::function::vtable_base *>(value);
      } else
        vtable = 0;
    }

    // Moves the value from the specified argument to *this. If the argument
    // has its function object allocated on the heap, move_assign will pass
    // its buffer to *this, and set the argument's buffer pointer to NULL.
    void move_assign(function_n& f)
    {
      if (&f == this)
        return;

      BOOST_TRY {
        if (!f.empty()) {
          this->vtable = f.vtable;
          if (this->has_trivial_copy_and_destroy()) {
            // Don't operate on storage directly since union type doesn't relax
            // strict aliasing rules, despite of having member char type.
#           if defined(BOOST_GCC) && (BOOST_GCC >= 40700)
#             pragma GCC diagnostic push
              // This warning is technically correct, but we don't want to pay the price for initializing
              // just to silence a warning: https://github.com/boostorg/function/issues/27
#             pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
#             if (BOOST_GCC >= 120000)
                // GCC 12 emits a different warning: https://github.com/boostorg/function/issues/42
#               pragma GCC diagnostic ignored "-Wuninitialized"
#             endif
#           endif
            std::memcpy(this->functor.data, f.functor.data, sizeof(this->functor.data));
#           if defined(BOOST_GCC) && (BOOST_GCC >= 40700)
#             pragma GCC diagnostic pop
#           endif
          } else
#if defined(BOOST_GCC) && (__GNUC__ >= 11)
# pragma GCC diagnostic push
// False positive in GCC 11/12 for empty function objects (function_n_test.cpp:673)
# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
#endif
            get_vtable()->base.manager(f.functor, this->functor,
                                     boost::detail::function::move_functor_tag);
#if defined(BOOST_GCC) && (__GNUC__ >= 11)
# pragma GCC diagnostic pop
#endif
          f.vtable = 0;
        } else {
          clear();
        }
      } BOOST_CATCH (...) {
        vtable = 0;
        BOOST_RETHROW;
      }
      BOOST_CATCH_END
    }
  };

  template<typename R, typename... T>
  inline void swap(function_n<
                     R,
                     T...
                   >& f1,
                   function_n<
                     R,
                     T...
                   >& f2)
  {
    f1.swap(f2);
  }

// Poison comparisons between boost::function objects of the same type.
template<typename R, typename... T>
  void operator==(const function_n<
                          R,
                          T...>&,
                  const function_n<
                          R,
                          T...>&);
template<typename R, typename... T>
  void operator!=(const function_n<
                          R,
                          T...>&,
                  const function_n<
                          R,
                          T...>& );

template<typename R,
         typename... T>
class function<R (T...)>
  : public function_n<R, T...>
{
  typedef function_n<R, T...> base_type;
  typedef function self_type;

  struct clear_type {};

public:

  BOOST_DEFAULTED_FUNCTION(function(), : base_type() {})

  template<typename Functor>
  function(Functor f
           ,typename std::enable_if<
                          !std::is_integral<Functor>::value,
                       int>::type = 0
           ) :
    base_type(std::move(f))
  {
  }
  template<typename Functor,typename Allocator>
  function(Functor f, Allocator a
           ,typename std::enable_if<
                           !std::is_integral<Functor>::value,
                       int>::type = 0
           ) :
    base_type(std::move(f),a)
  {
  }

  function(clear_type*) : base_type() {}

  function(const self_type& f) : base_type(static_cast<const base_type&>(f)){}

  function(const base_type& f) : base_type(static_cast<const base_type&>(f)){}

  // Move constructors
  function(self_type&& f): base_type(static_cast<base_type&&>(f)){}
  function(base_type&& f): base_type(static_cast<base_type&&>(f)){}

  self_type& operator=(const self_type& f)
  {
    self_type(f).swap(*this);
    return *this;
  }

  self_type& operator=(self_type&& f)
  {
    self_type(static_cast<self_type&&>(f)).swap(*this);
    return *this;
  }

  template<typename Functor>
  typename std::enable_if<
                         !std::is_integral<Functor>::value,
                      self_type&>::type
  operator=(Functor f)
  {
    self_type(f).swap(*this);
    return *this;
  }

  self_type& operator=(clear_type*)
  {
    this->clear();
    return *this;
  }

  self_type& operator=(const base_type& f)
  {
    self_type(f).swap(*this);
    return *this;
  }

  self_type& operator=(base_type&& f)
  {
    self_type(static_cast<base_type&&>(f)).swap(*this);
    return *this;
  }
};

} // end namespace boost

#if defined(BOOST_MSVC)
#   pragma warning( pop )
#endif

// Resolve C++20 issue with fn == bind(...)
// https://github.com/boostorg/function/issues/45

namespace boost
{

namespace _bi
{

template<class R, class F, class L> class bind_t;

} // namespace _bi

template<class S, class R, class F, class L> bool operator==( function<S> const& f, _bi::bind_t<R, F, L> const& b )
{
    return f.contains( b );
}

template<class S, class R, class F, class L> bool operator!=( function<S> const& f, _bi::bind_t<R, F, L> const& b )
{
    return !f.contains( b );
}

} // namespace boost

#endif // #ifndef BOOST_FUNCTION_FUNCTION_TEMPLATE_HPP_INCLUDED
