c++ - 定义一个 unordered_set 对

Question

为什么这适用于有序集 -

set<pair<int, int>> us;

但不适用于无序 -

unordered_set<pair<int, int>> us;

我不需要订购我的套装。

错误日志：

Char 39: fatal error: call to implicitly-deleted default constructor of 'unordered_set<pair<int, int> >'
        unordered_set<pair<int, int>> us;
                                      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/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/8/../../../../include/c++/8/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/8/../../../../include/c++/8/bits/hashtable.h:408:7: note: explicitly defaulted function was implicitly deleted here
      _Hashtable() = default;
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/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/8/../../../../include/c++/8/bits/hashtable_policy.h:1824:5: note: explicitly defaulted function was implicitly deleted here
    _Hashtable_base() = default;
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/hashtable_policy.h:1773:5: 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/8/../../../../include/c++/8/bits/hashtable_policy.h:1379:7: note: explicitly defaulted function was implicitly deleted here
      _Hash_code_base() = default;
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/hashtable_policy.h:1355:7: 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/8/../../../../include/c++/8/bits/hashtable_policy.h:1106:7: note: explicitly defaulted function was implicitly deleted here
      _Hashtable_ebo_helper() = default;
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/hashtable_policy.h:1104:7: 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/8/../../../../include/c++/8/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.

标签： c++setstd-pairunordered-set