function.hpp

//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012-2017 OpenVPN Technologies, Inc.
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU General Public License Version 3
//    as published by the Free Software Foundation.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU General Public License for more details.
//
//    You should have received a copy of the GNU General Public License
//    along with this program in the COPYING file.
//    If not, see <http://www.gnu.org/licenses/>.

// High-performance functor with move-only semantics.

#ifndef OPENVPN_COMMON_FUNCTION_H
#define OPENVPN_COMMON_FUNCTION_H

#include <cstddef>  // for std::size_t
#include <utility>  // for std::move
#include <new>

namespace openvpn {
  // F -- function type (usually a lambda expression)
  // N (default=3) -- max size of functor in void* machine words before we overflow to dynamic allocation
  // INTERN_ONLY (default=false) -- if true, throw a static assertion if functor cannot be stored internally
  template <typename F, std::size_t N=3, bool INTERN_ONLY=false>
  class Function;

  template <typename R, typename ... A, std::size_t N, bool INTERN_ONLY>
  class Function<R(A...), N, INTERN_ONLY>
  {
  public:
    Function() noexcept
    {
      methods = nullptr;
    }

    template <typename T>
    Function(T&& functor) noexcept
    {
      construct(std::move(functor));
    }

    Function(Function&& other) noexcept
    {
      methods = other.methods;
      other.methods = nullptr;
      if (methods)
	methods->move(data, other.data);
    }

    Function& operator=(Function&& other) noexcept
    {
      if (methods)
	methods->destruct(data);
      methods = other.methods;
      other.methods = nullptr;
      if (methods)
	methods->move(data, other.data);
      return *this;
    }

    ~Function()
    {
      if (methods)
	methods->destruct(data);
    }

    template <typename T>
    void reset(T&& functor) noexcept
    {
      if (methods)
	methods->destruct(data);
      construct(std::move(functor));
    }

    void reset() noexcept
    {
      if (methods)
	{
	  methods->destruct(data);
	  methods = nullptr;
	}
    }

    R operator()(A... args)
    {
      return methods->invoke(data, args...);
    }

    explicit operator bool() const noexcept
    {
      return methods != nullptr;
    }

  private:
    template <typename T>
    void construct(T&& functor) noexcept
    {
      constexpr bool is_intern = (sizeof(Intern<T>) <= sizeof(data));
      static_assert(!INTERN_ONLY || is_intern, "Function: Intern<T> doesn't fit in data[] and INTERN_ONLY=true");
      static_assert(sizeof(Extern<T>) <= sizeof(data), "Function: Extern<T> doesn't fit in data[]");

      if (is_intern)
	{
	  // store functor internally (in data)
	  setup_methods_intern<T>();
	  new (data) Intern<T>(std::move(functor));
	}
      else
	{
	  // store functor externally (using new)
	  setup_methods_extern<T>();
	  new (data) Extern<T>(std::move(functor));
	}
    }

    struct Methods
    {
      R (*invoke)(void *, A...);
      void (*move)(void *, void *);
      void (*destruct)(void *);
    };

    template <typename T>
    void setup_methods_intern()
    {
      static const struct Methods m = {
	&Intern<T>::invoke,
	&Intern<T>::move,
	&Intern<T>::destruct,
      };
      methods = &m;
    }

    template <typename T>
    void setup_methods_extern()
    {
      static const struct Methods m = {
	&Extern<T>::invoke,
	&Extern<T>::move,
	&Extern<T>::destruct,
      };
      methods = &m;
    }

    // store functor internally (in data)
    template <typename T>
    class Intern
    {
    public:
      Intern(T&& functor) noexcept
        : functor_(std::move(functor))
      {
      }

      static R invoke(void *ptr, A... args)
      {
	Intern* self = reinterpret_cast<Intern<T>*>(ptr);
	return self->functor_(args...);
      }

      static void move(void *dest, void *src)
      {
	Intern* s = reinterpret_cast<Intern<T>*>(src);
	new (dest) Intern(std::move(*s));
      }

      static void destruct(void *ptr)
      {
	Intern* self = reinterpret_cast<Intern<T>*>(ptr);
	self->~Intern();
      }

    private:
      T functor_;
    };

    // store functor externally (using new)
    template <typename T>
    class Extern
    {
    public:
      Extern(T&& functor) noexcept
        : functor_(new T(std::move(functor)))
      {
      }

      static R invoke(void *ptr, A... args)
      {
	Extern* self = reinterpret_cast<Extern<T>*>(ptr);
	return (*self->functor_)(args...);
      }

      static void move(void *dest, void *src)
      {
	Extern* d = reinterpret_cast<Extern<T>*>(dest);
	Extern* s = reinterpret_cast<Extern<T>*>(src);
	d->functor_ = s->functor_;
	// no need to set s->functor_=nullptr because parent will not destruct src after move
      }

      static void destruct(void *ptr)
      {
	Extern* self = reinterpret_cast<Extern<T>*>(ptr);
	delete self->functor_;
      }

    private:
      T* functor_;
    };

    const Methods* methods;
    void* data[N];
  };
}

#endif