poly_collection/detail/allocator_adaptor.hpp

/* Copyright 2018 Joaquin M Lopez Munoz.
 * Distributed under 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)
 *
 * See http://www.boost.org/libs/poly_collection for library home page.
 */

#ifndef BOOST_POLY_COLLECTION_DETAIL_ALLOCATOR_ADAPTOR_HPP
#define BOOST_POLY_COLLECTION_DETAIL_ALLOCATOR_ADAPTOR_HPP

#if defined(_MSC_VER)
#pragma once
#endif

#include <boost/mp11/function.hpp>
#include <boost/mp11/integer_sequence.hpp>
#include <boost/poly_collection/detail/is_constructible.hpp>
#include <new>
#include <memory>
#include <tuple>
#include <type_traits>
#include <utility>

namespace boost{

namespace poly_collection{

namespace detail{

/* [container.requirements.general]/3 state that containers must use the
 * allocator's construct/destroy member functions to construct/destroy
 * elements and *not at all* for any other type. Since poly_collection is
 * implemented as a multi-level structure of container and container-like
 * objects, we need to use an adaptor for the user-provided Allocator that
 * prevents intermediate entities from calling Allocator::[construct|destroy].
 * allocator_adaptor<Allocator> does this by taking advantage of the fact that
 * elements are ultimately held within a value_holder:
 *  - construct(value_holder<T>*,...) uses placement new construction and
 *    passes the wrapped Allocator object for value_holder<T> to use for
 *    its inner construction of T.
 *  - For the rest of types, construct uses placement new construction and
 *    passes down the adaptor object itself as an argument following an
 *    approach analogous to that of std::scoped_allocator_adaptor.
 *  - destroy(value_holder<T>) resorts to Allocator::destroy to destroy the
 *    contained T element.
 *  - For the rest of types, destroy(T) calls ~T directly.
 *
 * Code has been ripped and adapted from libc++'s implementation of
 * std::scoped_allocator_adaptor.
 */

template<typename T>
class value_holder_base;

template<typename T>
class value_holder;

template<typename T,typename Allocator,typename... Args>
struct uses_alloc_ctor_impl
{
    using RawAllocator=typename std::remove_cv<
      typename std::remove_reference<Allocator>::type
    >::type;

    static const bool ua=std::uses_allocator<T,RawAllocator>::value;
    static const int  ic=is_constructible<
      T,std::allocator_arg_t,Allocator,Args...>::value?1:0;
    static const int  value=ua?2-ic:0;
};

template<typename T,typename Allocator,typename... Args>
struct uses_alloc_ctor:
  std::integral_constant<int,uses_alloc_ctor_impl<T,Allocator,Args...>::value>
{};

template<typename Allocator,typename=void>
struct allocator_is_always_equal:std::is_empty<Allocator>{};

template<typename Allocator>
struct allocator_is_always_equal<
  Allocator,
  mp11::mp_void<
    typename std::allocator_traits<Allocator>::is_always_equal
  >
>:std::allocator_traits<Allocator>::is_always_equal{};

template<typename Allocator>
struct allocator_adaptor:Allocator
{
  using traits=std::allocator_traits<Allocator>;

  using value_type=typename traits::value_type;
  using size_type=typename traits::size_type;
  using difference_type=typename traits::difference_type;
  using pointer=typename traits::pointer;
  using const_pointer=typename traits::const_pointer;
  using void_pointer=typename traits::void_pointer;
  using const_void_pointer=typename traits::const_void_pointer;
  using propagate_on_container_copy_assignment=
    typename traits::propagate_on_container_copy_assignment;
  using propagate_on_container_move_assignment=
    typename traits::propagate_on_container_move_assignment;
  using propagate_on_container_swap=
    typename traits::propagate_on_container_swap;
  using is_always_equal=typename allocator_is_always_equal<Allocator>::type;

  template<typename U>
  struct rebind
  {
    using other=allocator_adaptor<typename traits::template rebind_alloc<U>>;
  };

  allocator_adaptor()=default;
  allocator_adaptor(const allocator_adaptor&)=default;

  template<
    typename Allocator2,
    typename std::enable_if<
      is_constructible<Allocator,const Allocator2&>::value
    >::type* =nullptr
  >
  allocator_adaptor(const Allocator2& x)noexcept:Allocator{x}{}

  template<
    typename Allocator2,
    typename std::enable_if<
      is_constructible<Allocator,const Allocator2&>::value
    >::type* =nullptr
  >
  allocator_adaptor(const allocator_adaptor<Allocator2>& x)noexcept:
    Allocator{x.allocator()}{}

  allocator_adaptor& operator=(const allocator_adaptor&)=default;

  Allocator&       allocator()noexcept{return *this;}
  const Allocator& allocator()const noexcept{return *this;}

  template<typename T,typename... Args>
  void construct(T* p,Args&&... args)
  {
    construct_(
      uses_alloc_ctor<T,allocator_adaptor&,Args...>{},
      p,std::forward<Args>(args)...);
  }

  template<typename T,typename... Args>
  void construct(value_holder<T>* p,Args&&... args)
  {
    ::new ((void*)p) value_holder<T>(allocator(),std::forward<Args>(args)...);
  }

  template<typename T1,typename T2,typename... Args1,typename... Args2>
  void construct(
    std::pair<T1,T2>* p,std::piecewise_construct_t,
    std::tuple<Args1...> x,std::tuple<Args2...> y)
  {
    ::new ((void*)p) std::pair<T1,T2>(
       std::piecewise_construct,
       transform_tuple(
         uses_alloc_ctor<T1,allocator_adaptor&,Args1...>{},
         std::move(x),
         mp11::make_index_sequence<sizeof...(Args1)>{}),
       transform_tuple(
         uses_alloc_ctor<T2,allocator_adaptor&,Args2...>{},
         std::move(y),
         mp11::make_index_sequence<sizeof...(Args2)>{})
    );
  }

  template<typename T1,typename T2>
  void construct(std::pair<T1,T2>* p)
  {
    construct(p,std::piecewise_construct,std::tuple<>{},std::tuple<>{});
  }

  template<typename T1,typename T2,typename U,typename V>
  void construct(std::pair<T1,T2>* p,U&& x,V&& y)
  {
    construct(
      p,std::piecewise_construct,
      std::forward_as_tuple(std::forward<U>(x)),
      std::forward_as_tuple(std::forward<V>(y)));
  }

  template<typename T1,typename T2,typename U,typename V>
  void construct(std::pair<T1,T2>* p,const std::pair<U,V>& x)
  {
    construct(
      p,std::piecewise_construct,
      std::forward_as_tuple(x.first),std::forward_as_tuple(x.second));
  }

  template<typename T1,typename T2,typename U,typename V>
  void construct(std::pair<T1,T2>* p,std::pair<U,V>&& x)
  {
    construct(
      p,std::piecewise_construct,
      std::forward_as_tuple(std::forward<U>(x.first)),
      std::forward_as_tuple(std::forward<V>(x.second)));
  }

  template<typename T>
  void destroy(T* p)
  {
    p->~T();
  }

  template<typename T>
  void destroy(value_holder<T>* p)
  {
    traits::destroy(
      allocator(),
      reinterpret_cast<T*>(static_cast<value_holder_base<T>*>(p)));
  }

  allocator_adaptor
  select_on_container_copy_construction()const noexcept
  {
    return traits::select_on_container_copy_construction(allocator());
  }

private:
  template<typename T,typename... Args>
  void construct_(
    std::integral_constant<int,0>,  /* doesn't use allocator */
    T* p,Args&&... args)
  {
    ::new ((void*)p) T(std::forward<Args>(args)...);
  }

  template<typename T,typename... Args>
  void construct_(
    std::integral_constant<int,1>, /* with std::allocator_arg */
    T* p,Args&&... args)
  {
    ::new ((void*)p) T(std::allocator_arg,*this,std::forward<Args>(args)...);
  }

  template<typename T,typename... Args>
  void construct_(
    std::integral_constant<int,2>, /* allocator at the end */
    T* p,Args&&... args)
  {
    ::new ((void*)p) T(std::forward<Args>(args)...,*this);
  }

  template<typename... Args,std::size_t... I>
  std::tuple<Args&&...> transform_tuple(
    std::integral_constant<int,0>,  /* doesn't use allocator */
    std::tuple<Args...>&& t,mp11::index_sequence<I...>)
  {
    return std::tuple<Args&&...>(std::get<I>(std::move(t))...);
  }

  template<typename... Args,std::size_t... I>
  std::tuple<std::allocator_arg_t,allocator_adaptor&,Args&&...>
  transform_tuple(
    std::integral_constant<int,1>, /* with std::allocator_arg */
    std::tuple<Args...>&& t,mp11::index_sequence<I...>)
  {
    return std::tuple<
      std::allocator_arg_t,allocator_adaptor&,Args&&...>(
      std::allocator_arg,*this,std::get<I>(std::move(t))...);
  }

  template<typename... Args,std::size_t... I>
  std::tuple<Args&&...,allocator_adaptor&>
  transform_tuple(
    std::integral_constant<int,2>, /* allocator at the end */
    std::tuple<Args...>&& t,mp11::index_sequence<I...>)
  {
    return std::tuple<Args&&...,allocator_adaptor&>(
      std::get<I>(std::move(t))...,*this);
  }
};

template<typename Allocator1,typename Allocator2>
bool operator==(
  const allocator_adaptor<Allocator1>& x,
  const allocator_adaptor<Allocator2>& y)noexcept
{
  return x.allocator()==y.allocator();
}

template<typename Allocator1,typename Allocator2>
bool operator!=(
  const allocator_adaptor<Allocator1>& x,
  const allocator_adaptor<Allocator2>& y)noexcept
{
  return !(x==y);
}

} /* namespace poly_collection::detail */

} /* namespace poly_collection */

} /* namespace boost */

#endif