unordered_set 不支持 `unordered_set <vector<int> &gt;` 或 `unordered_set <pair<int,int> &gt;`? </pair<int,int></vector<int>

Question

unordered_set<pair<int,int>> vis;
unordered_set<vector<int>> vis;

他们都错了，但如果我把它们改成

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.

Answer 1

默认情况下，没有。 对于unordered_set ，它需要能够 hash 对象，并且std::pair和std::vector没有默认的 hash 实现。 有两种典型的方法可以做到这一点； 为您提供自己的Hash类型，或为这些类型实现std::hash函数。 让我们看看两者。

首先，这是std::unordered_set的定义方式：

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> 。 就目前而言，您的程序无法编译，因为std::hash<std::pair<int, int>>的特化不存在。 所以我们可以提供它：

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，我委托std::hash<int>来实现 hash，但是您可以随意实现它。

或者，您可以提供自己的结构而不是实现std::hash ，然后在定义unordered_set object 时必须声明它：

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.
}

有趣的是，在 C++20 中，您可以在未计算的上下文中使用 lambda，例如decltype ，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; 
}

Answer 2

std::unordered_set对其键类型进行散列，因此std::hash的特化需要可用于键类型，否则您需要在声明std::unordered_set时指定自己的散列 function 。 std::pair或std::vector没有std::hash的默认特化。

另一方面， std::set不是 hash 的关键。 它默认使用operator<比较键值， std::pair和std::vector都实现了。