std：：在boost::interprocess::managed_shared_memory中列出

Question

最近，我在受教育中学习了在unordered_map段中使用unordered_map的正确方法。到目前为止还不错，但是我需要再添加一些STL容器。

理想情况下，我希望能够对任何STL容器执行相同的操作。现在我需要一个std::list。我不能让它起作用。不过，我可以让std::vector发挥作用。

std：：向量

以下代码起作用：

#include <vector>
#include <boost/interprocess/managed_shared_memory.hpp>

namespace ipc = boost::interprocess;
using Segment = ipc::managed_shared_memory;
using Manager = Segment::segment_manager;
template <typename T> using Alloc = ipc::allocator<T, Manager>;
template <typename K> using Vector = std::vector<K, Alloc<K>>;

int main() {
  boost::interprocess::shared_memory_object::remove("test");
  Segment _segment{ipc::create_only, "test", 1ul<<40};
  Manager *mgr = _segment.get_segment_manager();
  Vector<int> *v = _segment.construct<Vector<int>>("v")(mgr);
  v->emplace_back(1);
}

std:清单

列表等效代码将导致错误。

#include <list>
#include <boost/interprocess/managed_shared_memory.hpp>

namespace ipc = boost::interprocess;
using Segment = ipc::managed_shared_memory;
using Manager = Segment::segment_manager;
template <typename T> using Alloc = ipc::allocator<T, Manager>;
template <typename K> using List = std::list<K, Alloc<K>>;

int main() {
  boost::interprocess::shared_memory_object::remove("test");
  Segment _segment{ipc::create_only, "test", 1ul<<40};
  Manager *mgr = _segment.get_segment_manager();
  List<int> *v = _segment.construct<List<int>>("v")(mgr);
  v->emplace_back(1);
}

编译标志(使用g++-7)和错误如下：

$ g++ -std=gnu++17 lol_list.cpp -lrt -pthread -rdynamic -Wfatal-errors && ./a.out && rm ./a.out
In file included from /usr/include/c++/7/list:63:0,
                 from lol_list.cpp:1:
/usr/include/c++/7/bits/stl_list.h: In instantiation of ‘std::__cxx11::list<_Tp, _Alloc>::_Node* std::__cxx11::list<_Tp, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {int}; _Tp = int; _Alloc = boost::interprocess::allocator<int, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> >; std::__cxx11::list<_Tp, _Alloc>::_Node = std::_List_node<int>]’:
/usr/include/c++/7/bits/stl_list.h:1801:32:   required from ‘void std::__cxx11::list<_Tp, _Alloc>::_M_insert(std::__cxx11::list<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {int}; _Tp = int; _Alloc = boost::interprocess::allocator<int, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> >; std::__cxx11::list<_Tp, _Alloc>::iterator = std::_List_iterator<int>]’
/usr/include/c++/7/bits/stl_list.h:1133:4:   required from ‘std::__cxx11::list<_Tp, _Alloc>::reference std::__cxx11::list<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {int}; _Tp = int; _Alloc = boost::interprocess::allocator<int, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> >; std::__cxx11::list<_Tp, _Alloc>::reference = int&]’
lol_list.cpp:16:20:   required from here
/usr/include/c++/7/bits/stl_list.h:578:11: error: cannot convert ‘boost::interprocess::offset_ptr<std::_List_node<int>, long int, long unsigned int, 0>’ to ‘std::__cxx11::list<int, boost::interprocess::allocator<int, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> > >::_Node* {aka std::_List_node<int>*}’ in return
    return __p;
           ^~~
compilation terminated due to -Wfatal-errors.

错误持续存在于g++-6或g++-8，甚至clang-6.0和clang-5.0。

Answer 1

并不是所有的标准库实现都完全支持有状态分配器(还没有)。

在这种情况下，您的std::list<>似乎没有。只需选择Boost容器，它也可以方便地通过Boost Interprocess报头获得：

#include <boost/interprocess/containers/list.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>

namespace ipc = boost::interprocess;
using Segment = ipc::managed_shared_memory;
using Manager = Segment::segment_manager;
template <typename T> using Alloc = ipc::allocator<T, Manager>;
template <typename K> using List = ipc::list<K, Alloc<K>>;

int main() {
    boost::interprocess::shared_memory_object::remove("test");
    Segment _segment{ipc::create_only, "test", 1ul<<40};
    Manager *mgr = _segment.get_segment_manager();
    List<int> *v = _segment.construct<List<int>>("v")(mgr);
    v->emplace_back(1);
}

Answer 2

[allocator.requirements]第9段： 分配器可以约束可以实例化它的类型和可以调用其构造成员的参数。如果某个类型不能与特定的分配器一起使用，则分配程序类或构造调用可能无法实例化。您的分配器可以拒绝为任何东西分配内存，除非T.会阻止它在基于节点的容器(如std::list )中使用，这些容器需要分配自己的内部节点类型(不仅仅是容器的value_type)，但是对于std::vector来说，它可以正常工作。

由@Jonathan在https://stackoverflow.com/a/28995278/4288486上发布