std::list in a boost::interprocess::managed_shared_memory

问题

Recently I got schooled and learnt the proper way of having a unordered_map inside a boost::interprocess::managed_shared_memory segment. So far so good, but I will need to add a few more STL containers.

Ideally, I would want to be able to follow the same for any STL container. Right now I'm in need of a std::list. I can't make it work. I can make a std::vector work, though.

std::vector

The following code works:

#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::list

The list-equivalent code results in an error.

#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);
}

The compilation flags (using g++-7) and error are the following:

$ 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.

The error persists with g++-6 or g++-8 and even with clang-6.0 and clang-5.0.

回答1:

Not all standard library implementations fully support stateful allocators (yet?).

In this case it appears that your std::list<> doesn't. Simply opt for the one for Boost Container, which is conveniently also available through Boost Interprocess headers:

#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);
}

回答2:

[allocator.requirements] paragraph 9:

An allocator may constrain the types on which it can be instantiated and the arguments for which its construct member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to construct may fail to instantiate.

It's OK for your allocator to refuse to allocate memory for anything except a T. That will prevent it being used in node-based containers such as std::list which need to allocate their own internal node types (not just the container's value_type) but it will work fine for std::vector

As posted in https://stackoverflow.com/a/28995278/4288486 by @Jonathan-Wakely

来源：https://stackoverflow.com/questions/51883848/stdlist-in-a-boostinterprocessmanaged-shared-memory