我在下面提供了我想完成的完整示例。本质上,我希望树结构保存在内存映射中。下面的代码无法编译。我不理解(扩展)错误;这是某种类型转换失败。如何调整此代码以使其正常工作?
#include "boost/interprocess/allocators/allocator.hpp"
#include "boost/interprocess/managed_mapped_file.hpp"
#include <map>
#include <memory>
template <typename TKey, typename TData, template<class> class TAllocator = std::allocator>
class Node : std::enable_shared_from_this<Node<TKey, TData, TAllocator>> {
using TNode = Node<TKey, TData, TAllocator>;
std::map<TKey, TNode, std::less<TKey>, TAllocator<std::pair<const TKey, TData>>> _children;
TData _data;
public:
Node() = default;
explicit Node(const TAllocator<std::pair<const TKey, TData>>& allocator, const TData& data) : _children(allocator), _data(data) {}
TData& at() { return _data; }
const std::map<TKey, std::shared_ptr<TNode>>& children() { return _children; };
void add(const TKey& key, const TData& data) {
_children.emplace(key, TNode(_children.get_allocator(), data));
}
};
template <typename T>
using TAlloc = boost::interprocess::allocator<T, boost::interprocess::managed_mapped_file::segment_manager>;
using TMapTrie = Node<std::string, std::shared_ptr<std::size_t>, TAlloc>;
int main() {
boost::interprocess::managed_mapped_file file_vec(boost::interprocess::open_or_create, "/tmp/pfx_mmap.dat", 1 << 20);
TAlloc<std::pair<const std::string, std::shared_ptr<std::size_t>>> allocator(file_vec.get_segment_manager());
TMapTrie root(allocator, nullptr);
root.add("abc", std::make_shared<std::size_t>(42));
return 0;
}
您可以这样编译它:gcc demo.cpp -lboost_system -std=c++11 -lstdc++
。编译错误:
cannot convert ‘std::allocator_traits<boost::interprocess::allocator<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> > >, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> > >::pointer {aka boost::interprocess::offset_ptr<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> > >, long int, long unsigned int, 0>}’ to ‘std::_Rb_tree<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> > >, std::less<std::__cxx11::basic_string<char> >, boost::interprocess::allocator<std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> >, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> > >::_Link_type {aka std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Node<std::__cxx11::basic_string<char>, std::shared_ptr<long unsigned int>, TAlloc> > >*}’ in return
{ return _Alloc_traits::allocate(_M_get_Node_allocator(), 1); }
答案 0 :(得分:1)
这应该是在map
中实施libstdc++
的错误。其映射实现假定分配器的pointer
类型(即typename std::allocator_traits<some_allocator>::pointer
)实际上是指针。
这不是标准要求的,boost::interprocess
提供的分配器确实使用offset_ptr
作为pointer
类型,它是一个对象而不是实际的指针。
bits/stl_tree.h
中的罪魁祸首代码:
_Link_type
_M_get_node()
{ return _Alloc_traits::allocate(_M_get_Node_allocator(), 1); }
其中_Link_type
被直接声明为指针:
typedef _Rb_tree_node<_Val>* _Link_type;
正确的实现应使用allocator_traits<...>::pointer
来检索指针类型。
使用libc++
,GCC和Clang都正确接受代码(您的原始代码有一些不正确的错字...),并且其实现确实符合上述方法。
再现的最小示例:
#include "boost/interprocess/allocators/allocator.hpp"
#include "boost/interprocess/managed_mapped_file.hpp"
#include <map>
template <typename T>
using TAlloc = boost::interprocess::allocator<T, boost::interprocess::managed_mapped_file::segment_manager>;
int main() {
boost::interprocess::managed_mapped_file file_vec(boost::interprocess::open_or_create, "/tmp/pfx_mmap.dat", 1 << 20);
TAlloc<std::pair<const int, int>> allocator(file_vec.get_segment_manager());
std::map<int, int, std::less<int>, TAlloc<std::pair<const int, int>>> mp{allocator};
mp.insert({1, 2});
}