通过[]运算符访问unordered_map时出现错误

Question

我偶然发现了 unordered_map 的一个奇怪问题。

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首先，我生成了一条unordered_map<string, Person>记录（“Bob”，Person（1，“Bob”））并将其插入表中。然后我尝试使用带有键“Bob”的 [] 运算符来访问记录，但发生了错误。

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这是代码：

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#include<iostream>\n#include<unordered_map>\nusing namespace std;\n\nclass Person\n{\n    public:\n        int play;\n        string name;\n        Person(int p, string n):play(p), name(n) {}\n};\n\nint main()\n{\n    unordered_map<string,Person> test;\n    test.insert(std::make_pair("haha",Person(1,"haha")));\n    cout<<test["haha"].name<<endl;\n    return 0;\n}\n

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当我使用“g++ -S hash.cpp”编译代码时出现错误\n输出：

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In file included from /usr/include/c++/7/unordered_map:41:0,\n                 from hash.cpp:2:\n/usr/include/c++/7/tuple: In instantiation of \xe2\x80\x98std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Person]\xe2\x80\x99:\n/usr/include/c++/7/tuple:1641:63:   required from \xe2\x80\x98std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; _Args2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Person]\xe2\x80\x99\n/usr/include/c++/7/ext/new_allocator.h:136:4:   required from \xe2\x80\x98void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Person>; _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Tp = std::pair<const std::__cxx11::basic_string<char>, Person>]\xe2\x80\x99\n/usr/include/c++/7/bits/alloc_traits.h:475:4:   required from \xe2\x80\x98static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Person>; _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Tp = std::pair<const std::__cxx11::basic_string<char>, Person>; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Person> >]\xe2\x80\x99\n/usr/include/c++/7/bits/hashtable_policy.h:2066:37:   required from \xe2\x80\x98std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Person>, true> >; std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type = std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Person>, true>]\xe2\x80\x99\n/usr/include/c++/7/bits/hashtable_policy.h:750:8:   required from \xe2\x80\x98std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type& std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::operator[](std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Pair = std::pair<const std::__cxx11::basic_string<char>, Person>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Person> >; _Equal = std::equal_to<std::__cxx11::basic_string<char> >; _H1 = std::hash<std::__cxx11::basic_string<char> >; _H2 = std::__detail::_Mod_range_hashing; _Hash = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<true, false, true>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type = Person; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type = std::__cxx11::basic_string<char>]\xe2\x80\x99\n/usr/include/c++/7/bits/unordered_map.h:980:20:   required from \xe2\x80\x98std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type& std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::operator[](std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = Person; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Person> >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type = Person; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type = std::__cxx11::basic_string<char>]\xe2\x80\x99\nhash.cpp:17:19:   required from here\n/usr/include/c++/7/tuple:1652:70: error: no matching function for call to \xe2\x80\x98Person::Person()\xe2\x80\x99\n         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)\n                                                                      ^\nhash.cpp:10:3: note: candidate: Person::Person(int, std::__cxx11::string)\n   Person(int p, string n):play(p), name(n) {}\n   ^~~~~~\nhash.cpp:10:3: note:   candidate expects 2 arguments, 0 provided\nhash.cpp:5:7: note: candidate: Person::Person(const Person&)\n class Person\n       ^~~~~~\nhash.cpp:5:7: note:   candidate expects 1 argument, 0 provided\nhash.cpp:5:7: note: candidate: Person::Person(Person&&)\nhash.cpp:5:7: note:   candidate expects 1 argument, 0 provided\n

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我已经插入了记录，但地图似乎没有意识到这一点并尝试再次插入记录。

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那么g++编译器是如何处理[]运算符的呢？

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谢谢。

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Answer 1

unordered_map 的映射类型必须是 DefaultConstructible 下的operator[]。即，如果您希望能够使用 .Person() ，则必须有一个默认构造函数operator[]。

作为替代方案，使用 at()：

int main() 
{ 
 std::unordered_map<string,Person> test;
 test.insert(std::make_pair("haha",Person(1,"haha")));
 std::cout<<test.at("haha").name<<'\n';
 return 0; 
}

映射类型必须是 DefaultConstructible，因为operator[]应该能够创建缺失的条目。新条目中的数据是默认构造的。