unordered_set不支持`unordered_set<vector<int>>`还是`unordered_set<pair<int，int>>`？
unordered_set不支持`unordered_set<vector<int>>`还是`unordered_set<pair<int，int>>`？
提问于 2022-09-27 19:29:51
unordered_set<pair<int,int>> vis;
unordered_set<vector<int>> vis;他们都错了，但如果我把他们改成
set<vector<int>> vis;
set<pair<int,int>> vis;那么他们是正确的。为什么？
int test()
{
unordered_set<pair<int,int>> vis;
return 0;
}编译错误：
error: call to implicitly-deleted default constructor of 'unordered_set<pair<int, int>>'
unordered_set<pair<int,int>> vis;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unordered_set.h:135:7: note: explicitly defaulted function was implicitly deleted here
unordered_set() = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unordered_set.h:100:18: note: default constructor of 'unordered_set<std::pair<int, int>, std::hash<std::pair<int, int>>, std::equal_to<std::pair<int, int>>, std::allocator<std::pair<int, int>>>' is implicitly deleted because field '_M_h' has a deleted default constructor
_Hashtable _M_h;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable.h:414:7: note: explicitly defaulted function was implicitly deleted here
_Hashtable() = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable.h:174:7: note: default constructor of '_Hashtable<std::pair<int, int>, std::pair<int, int>, std::allocator<std::pair<int, int>>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true>>' is implicitly deleted because base class '__detail::_Hashtable_base<pair<int, int>, pair<int, int>, _Identity, equal_to<pair<int, int>>, hash<pair<int, int>>, _Mod_range_hashing, _Default_ranged_hash, _Hashtable_traits<true, true, true>>' has a deleted default constructor
: public __detail::_Hashtable_base<_Key, _Value, _ExtractKey, _Equal,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1822:5: note: explicitly defaulted function was implicitly deleted here
_Hashtable_base() = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1771:5: note: default constructor of '_Hashtable_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::equal_to<std::pair<int, int>>, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Hashtable_traits<true, true, true>>' is implicitly deleted because base class '_Hash_code_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, _Hashtable_traits<true, true, true>::__hash_cached::value>' has a deleted default constructor
: public _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2, _Hash,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1373:7: note: explicitly defaulted function was implicitly deleted here
_Hash_code_base() = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1349:7: note: default constructor of '_Hash_code_base<std::pair<int, int>, std::pair<int, int>, std::__detail::_Identity, std::hash<std::pair<int, int>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>' is implicitly deleted because base class '_Hashtable_ebo_helper<1, std::hash<std::pair<int, int>>>' has a deleted default constructor
private _Hashtable_ebo_helper<1, _H1>,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1096:7: note: explicitly defaulted function was implicitly deleted here
_Hashtable_ebo_helper() = default;
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1094:7: note: default constructor of '_Hashtable_ebo_helper<1, std::hash<std::pair<int, int>>, true>' is implicitly deleted because base class 'std::hash<std::pair<int, int>>' has a deleted default constructor
: private _Tp
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/functional_hash.h:101:19: note: default constructor of 'hash<std::pair<int, int>>' is implicitly deleted because base class '__hash_enum<std::pair<int, int>>' has no default constructor
struct hash : __hash_enum<_Tp>
^
1 error generated.回答 2
Stack Overflow用户
回答已采纳
发布于 2022-09-27 19:37:59
默认情况下不。对于unordered_set，它需要能够散列对象，而std::pair和std::vector没有默认的哈希实现。有两种典型的方法可以做到这一点:提供您自己的Hash类型，或者为这些类型实现std::hash函式。让我们看看这两个。
首先，std::unordered_set是这样定义的：
template<
class Key,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator<Key>
> class unordered_set;我们感兴趣的是第二个模板参数Hash = std::hash<Key>。目前，由于不存在std::hash<std::pair<int, int>>的专门化，您的程序无法编译。所以我们可以提供：
template <>
struct std::hash<std::pair<int, int>>
{
std::size_t operator()(std::pair<int, int> p) {
return std::hash<int>{}(p.first) ^ std::hash<int>{}(p.second);
}
};在这里，我将委托给std::hash<int>来实现哈希，但是您可以任意实现这个哈希。
或者，您可以提供自己的结构，而不是实现std::hash，您只需在定义unordered_set对象时声明它：
struct MyHash
{
std::size_t operator()(std::pair<int, int> p) {
return std::hash<int>{}(p.first) ^ std::hash<int>{}(p.second);
}
};
int main() {
std::unordered_set<std::pair<int, int>, MyHash> mySet; // Using my own hashing class.
}为了好玩，在C++20中，您可以在未评估的上下文(如decltype )中使用lambda，因此以下内容也是有效的：
int main() {
const auto hash = [](std::pair<int, int> p) {
return std::hash<int>{}(p.first) ^ std::hash<int>{}(p.second);
}
std::unordered_set<std::pair<int, int>, decltype(hash)> mySet;
}Stack Overflow用户
发布于 2022-09-27 19:37:36
std::unordered_set散列其键类型，因此需要为密钥类型提供std::hash的专门化，或者在声明std::unordered_set时需要指定自己的哈希函数。std::hash对于std::pair或std::vector没有默认的专门化。
另一方面，std::set没有散列它的密钥。它比较了默认情况下使用operator<的键值，std::pair和std::vector都实现了这一点。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/73872919
复制相关文章
相似问题
腾讯云开发者
