Poly.h

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/*
 * Copyright 2017-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// TODO: [x] "cast" from Poly<C&> to Poly<C&&>
// TODO: [ ] copy/move from Poly<C&>/Poly<C&&> to Poly<C>
// TODO: [ ] copy-on-write?
// TODO: [ ] down- and cross-casting? (Possible?)
// TODO: [ ] shared ownership? (Dubious.)
// TODO: [ ] can games be played with making the VTable a member of a struct
//           with strange alignment such that the address of the VTable can
//           be used to tell whether the object is stored in-situ or not?

#pragma once

#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
#error Folly.Poly requires gcc-5 or greater
#endif

#include <cassert>
#include <new>
#include <type_traits>
#include <typeinfo>
#include <utility>

#include <folly/CPortability.h>
#include <folly/CppAttributes.h>
#include <folly/Traits.h>
#include <folly/detail/TypeList.h>
#include <folly/lang/Assume.h>

#if !defined(__cpp_inline_variables)
#define FOLLY_INLINE_CONSTEXPR constexpr
#else
#define FOLLY_INLINE_CONSTEXPR inline constexpr
#endif

#include <folly/PolyException.h>
#include <folly/detail/PolyDetail.h>

namespace folly {
template <class I>
struct Poly;

template <
    class Node,
    class Tfx = detail::MetaIdentity,
    class Access = detail::PolyAccess>
using PolySelf = decltype(Access::template self_<Node, Tfx>());

using PolyDecay = detail::MetaQuote<std::decay_t>;

#if !defined(__cpp_template_auto)

#define FOLLY_POLY_MEMBERS(...)                     \
  typename decltype(::folly::detail::deduceMembers( \
      __VA_ARGS__))::template Members<__VA_ARGS__>

#define FOLLY_POLY_MEMBER(SIG, MEM) \
  ::folly::detail::MemberDef<       \
      ::folly::detail::Member<decltype(::folly::sig<SIG>(MEM)), MEM>>::value

template <class... Ts>
using PolyMembers = detail::TypeList<Ts...>;

#else
#define FOLLY_POLY_MEMBER(SIG, MEM) ::folly::sig<SIG>(MEM)
#define FOLLY_POLY_MEMBERS(...) ::folly::PolyMembers<__VA_ARGS__>

template <auto... Ps>
struct PolyMembers {};

#endif

template <class... I>
struct PolyExtends : virtual I... {
  using Subsumptions = detail::TypeList<I...>;

  template <class Base>
  struct Interface : Base {
    Interface() = default;
    using Base::Base;
  };

  template <class...>
  using Members = PolyMembers<>;
};


template <std::size_t N, typename This, typename... As>
auto poly_call(This&& _this, As&&... as)
    -> decltype(detail::PolyAccess::call<N>(
        static_cast<This&&>(_this),
        static_cast<As&&>(as)...)) {
  return detail::PolyAccess::call<N>(
      static_cast<This&&>(_this), static_cast<As&&>(as)...);
}

template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail>&& _this, As&&... as) {
  using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
  return detail::PolyAccess::call<N>(
      static_cast<This&&>(_this), static_cast<As&&>(as)...);
}

template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail>& _this, As&&... as) {
  using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
  return detail::PolyAccess::call<N>(
      static_cast<This&>(_this), static_cast<As&&>(as)...);
}

template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail> const& _this, As&&... as) {
  using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
  return detail::PolyAccess::call<N>(
      static_cast<This const&>(_this), static_cast<As&&>(as)...);
}

template <
    std::size_t N,
    class I,
    class Poly,
    typename... As,
    std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
auto poly_call(Poly&& _this, As&&... as) -> decltype(poly_call<N, I>(
    static_cast<Poly&&>(_this).get(),
    static_cast<As&&>(as)...)) {
  return poly_call<N, I>(
      static_cast<Poly&&>(_this).get(), static_cast<As&&>(as)...);
}

template <std::size_t N, class I, typename... As>
[[noreturn]] detail::Bottom poly_call(detail::ArchetypeBase const&, As&&...) {
  assume_unreachable();
}


template <class T, class I>
detail::AddCvrefOf<T, I>&& poly_cast(detail::PolyRoot<I>&& that) {
  return detail::PolyAccess::cast<T>(std::move(that));
}

template <class T, class I>
detail::AddCvrefOf<T, I>& poly_cast(detail::PolyRoot<I>& that) {
  return detail::PolyAccess::cast<T>(that);
}

template <class T, class I>
detail::AddCvrefOf<T, I> const& poly_cast(detail::PolyRoot<I> const& that) {
  return detail::PolyAccess::cast<T>(that);
}

template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I>&& poly_cast(detail::ArchetypeRoot<I>&&) {
  assume_unreachable();
}

template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I>& poly_cast(detail::ArchetypeRoot<I>&) {
  assume_unreachable();
}

template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I> const& poly_cast(
    detail::ArchetypeRoot<I> const&) {
  assume_unreachable();
}

template <
    class T,
    class Poly,
    std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_cast(Poly&& that)
    -> decltype(poly_cast<T>(std::declval<Poly>().get())) {
  return poly_cast<T>(static_cast<Poly&&>(that).get());
}


template <class I>
std::type_info const& poly_type(detail::PolyRoot<I> const& that) noexcept {
  return detail::PolyAccess::type(that);
}

[[noreturn]] inline std::type_info const& poly_type(
    detail::ArchetypeBase const&) noexcept {
  assume_unreachable();
}

template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_type(Poly const& that) noexcept
    -> decltype(poly_type(that.get())) {
  return poly_type(that.get());
}


template <class I>
bool poly_empty(detail::PolyRoot<I> const& that) noexcept {
  return detail::State::eEmpty == detail::PolyAccess::vtable(that)->state_;
}

template <class I>
constexpr bool poly_empty(detail::PolyRoot<I&&> const&) noexcept {
  return false;
}

template <class I>
constexpr bool poly_empty(detail::PolyRoot<I&> const&) noexcept {
  return false;
}

template <class I>
constexpr bool poly_empty(Poly<I&&> const&) noexcept {
  return false;
}

template <class I>
constexpr bool poly_empty(Poly<I&> const&) noexcept {
  return false;
}

[[noreturn]] inline bool poly_empty(detail::ArchetypeBase const&) noexcept {
  assume_unreachable();
}


template <
    class I,
    std::enable_if_t<detail::Not<std::is_reference<I>>::value, int> = 0>
constexpr Poly<I>&& poly_move(detail::PolyRoot<I>& that) noexcept {
  return static_cast<Poly<I>&&>(static_cast<Poly<I>&>(that));
}

template <
    class I,
    std::enable_if_t<detail::Not<std::is_const<I>>::value, int> = 0>
Poly<I&&> poly_move(detail::PolyRoot<I&> const& that) noexcept {
  return detail::PolyAccess::move(that);
}

template <class I>
Poly<I const&> poly_move(detail::PolyRoot<I const&> const& that) noexcept {
  return detail::PolyAccess::move(that);
}

[[noreturn]] inline detail::ArchetypeBase poly_move(
    detail::ArchetypeBase const&) noexcept {
  assume_unreachable();
}

template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_move(Poly& that) noexcept
    -> decltype(poly_move(that.get())) {
  return poly_move(that.get());
}

namespace detail {
template <class I>
struct PolyVal : PolyImpl<I> {
 private:
  friend PolyAccess;

  struct NoneSuch {};
  using Copyable = std::is_copy_constructible<PolyImpl<I>>;
  using PolyOrNonesuch = If<Copyable::value, PolyVal, NoneSuch>;

  using PolyRoot<I>::vptr_;

  PolyRoot<I>& _polyRoot_() noexcept {
    return *this;
  }
  PolyRoot<I> const& _polyRoot_() const noexcept {
    return *this;
  }

  Data* _data_() noexcept {
    return PolyAccess::data(*this);
  }
  Data const* _data_() const noexcept {
    return PolyAccess::data(*this);
  }

 public:
  PolyVal() = default;
  PolyVal(PolyVal&& that) noexcept;
  /* implicit */ PolyVal(PolyOrNonesuch const& that);

  ~PolyVal();

  using PolyImpl<I>::PolyImpl;

  Poly<I>& operator=(PolyVal that) noexcept;

  template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
  /* implicit */ PolyVal(T&& t);

  template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0>
  /* implicit */ PolyVal(Poly<I2> that);

  template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
  Poly<I>& operator=(T&& t);

  template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0>
  Poly<I>& operator=(Poly<I2> that);

  void swap(Poly<I>& that) noexcept;
};


template <class I>
struct PolyRef : private PolyImpl<I> {
 private:
  friend PolyAccess;

  AddCvrefOf<PolyRoot<I>, I>& _polyRoot_() const noexcept;

  Data* _data_() noexcept {
    return PolyAccess::data(*this);
  }
  Data const* _data_() const noexcept {
    return PolyAccess::data(*this);
  }

  static constexpr RefType refType() noexcept;

 protected:
  template <class That, class I2>
  PolyRef(That&& that, Type<I2>);

 public:
  PolyRef(PolyRef const& that) noexcept;

  Poly<I>& operator=(PolyRef const& that) noexcept;

  template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
  /* implicit */ PolyRef(T&& t) noexcept;

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0>
  /* implicit */ PolyRef(Poly<I2>&& that) noexcept(
      std::is_reference<I2>::value);

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0>
  /* implicit */ PolyRef(Poly<I2>& that) noexcept(std::is_reference<I2>::value)
      : PolyRef{that, Type<I2>{}} {}

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0>
  /* implicit */ PolyRef(Poly<I2> const& that) noexcept(
      std::is_reference<I2>::value)
      : PolyRef{that, Type<I2>{}} {}

  template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
  Poly<I>& operator=(T&& t) noexcept;

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0>
  Poly<I>& operator=(Poly<I2>&& that) noexcept(std::is_reference<I2>::value);

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0>
  Poly<I>& operator=(Poly<I2>& that) noexcept(std::is_reference<I2>::value);

  template <
      class I2,
      std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0>
  Poly<I>& operator=(Poly<I2> const& that) noexcept(
      std::is_reference<I2>::value);

  void swap(Poly<I>& that) noexcept;

  AddCvrefOf<PolyImpl<I>, I>& get() const noexcept;

  AddCvrefOf<PolyImpl<I>, I>& operator*() const noexcept {
    return get();
  }

  auto operator-> () const noexcept {
    return &get();
  }
};

template <class I>
using PolyValOrRef = If<std::is_reference<I>::value, PolyRef<I>, PolyVal<I>>;
} // namespace detail

template <class I>
struct Poly final : detail::PolyValOrRef<I> {
  friend detail::PolyAccess;
  Poly() = default;
  using detail::PolyValOrRef<I>::PolyValOrRef;
  using detail::PolyValOrRef<I>::operator=;
};

template <class I>
void swap(Poly<I>& left, Poly<I>& right) noexcept {
  left.swap(right);
}

template <class Sig>
FOLLY_INLINE_CONSTEXPR detail::Sig<Sig> const sig = {};

} // namespace folly

#include <folly/Poly-inl.h>

#undef FOLLY_INLINE_CONSTEXPR