与unordered_map相关的类型 [英] Codependent types with unordered_map
问题描述
假设我要在unordered_map< int，int>的条目
之间保持一定的顺序。一种高效的内存存储方式似乎是在映射条目之间保留一个链表。即，我将使用unordered_map
Suppose that I want to keep a certain ordering between the entries of an unordered_map<int, int>. A memory efficient way to do that seems to be keeping a linked list between the entries of the map. Namely instead of having an unordered_map<int, int>, I will use an unordered_map<int, Node> where Node is defined as
struct Node {
int val;
typename std::unordered_map<int, Node>::iterator up;
};
这是有效的C ++吗？ Clang和gcc不允许这样说Node是不完整的类型。有关完整的错误消息，请参见下文。两者都接受以下内容：
Is this valid C++? Clang and gcc do not permit this saying Node is an incomplete type. See below for the full error message. The following is accepted by both:
template<typename Key, typename Value>
struct Map {
struct MapEntry {
Key key;
Value val;
MapEntry *prev, *next;
};
using Iterator = MapEntry*;
};
struct Node {
int val;
Map<int, Node>::Iterator up;
};
这里的规则到底是什么？为什么第一个不被接受但是第二个很好呢？在一个相关的问题中，出现了类似的问题，需要同样的解释在这里，必须保证unordered_map包含一个Value对象而没有任何间接调用。即，unordered_map的定义应如下所示：
What exactly is the rule here? Why is it that the first is not accepted but the second is fine? In a related question, a similar issue arose, however, for the same explanation to apply here it must be that unordered_map contains a Value object without any indirection. Namely the definition of unordered_map should look like this:
template<typename Key, typename Value>
class unordered_map {
Value val;
using value_type = std::pair<const Key, Value>;
using entry = std::tuple<value_type, entry*, entry*>;
using iterator = entry*;
};
我不明白为什么unordered_map应该直接存储值。否则，
就像我在上面给出的玩具示例struct Map中一样，没有依赖周期。
I don't see why unordered_map should store a Value directly. Otherwise, as in the toy example struct Map I gave above, there is no dependency cycle.
In file included from /usr/include/c++/8/unordered_map:43,
from test.cpp:1:
/usr/include/c++/8/bits/stl_pair.h: In instantiation of ‘struct std::pair<const int, Node>’:
/usr/include/c++/8/ext/aligned_buffer.h:91:28: required from ‘struct __gnu_cxx::__aligned_buffer<std::pair<const int, Node> >’
/usr/include/c++/8/bits/hashtable_policy.h:234:43: required from ‘struct std::__detail::_Hash_node_value_base<std::pair<const int, Node> >’
/usr/include/c++/8/bits/hashtable_policy.h:280:12: required from ‘struct std::__detail::_Hash_node<std::pair<const int, Node>, false>’
/usr/include/c++/8/bits/hashtable_policy.h:2027:49: required from ‘struct std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const int, Node>, false> > >’
/usr/include/c++/8/bits/hashtable.h:173:11: required from ‘class std::_Hashtable<int, std::pair<const int, Node>, std::allocator<std::pair<const int, Node> >, std::__detail::_Select1st, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >’
/usr/include/c++/8/bits/unordered_map.h:105:18: required from ‘class std::unordered_map<int, Node>’
test.cpp:5:32: required from here
/usr/include/c++/8/bits/stl_pair.h:215:11: error: ‘std::pair<_T1, _T2>::second’ has incomplete type
_T2 second; /// @c second is a copy of the second object
^~~~~~
test.cpp:3:8: note: forward declaration of ‘struct Node’
struct Node {
推荐答案
A，
struct Node {
int val;
typename std::unordered_map<int, Node>::iterator up;
};
不是有效的C ++。向 std :: vector ， std :: list 提供不完整的类型 Node code>和 std :: forward_list 根据最近接受的不完整类型建议到C ++ 17，但是unordered_map仍将提供完整类型。
is not valid C++ as of C++17 standard. Providing the incomplete type Node to a std::vector, std::list and std::forward_list is valid as per the recent acceptance of the incomplete types proposal into C++17, but unordered_map is still to be provided with complete types.
MSVC和libc ++可以很好地使用不完整的值，但这是由于它们超出了标准的要求。
MSVC and libc++ work fine with incomplete Value, but this is due to them going beyond the requirements of the standard.
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