自定义 std::set 作为 unordered_map 的值类型答案

【问题标题】：Custom std::set as value type to unordered_map自定义 std::set 作为 unordered_map 的值类型
【发布时间】：2020-11-30 01:21:13
【问题描述】：

让代码说话：

auto SetCompare = [](const string& a, const string& b)
{
    size_t L = a.length();
    size_t R = b.length();
    if (L == R) return (a < b);
    return L < R;
};

using MySet = std::set<string, decltype(SetCompare)>;

unordered_map<string, MySet> Map;

插入或访问无效：

Map["abc"];
Map["xyz"].insert("mapped to xyz");

// Insert TO set works
MySet mySet(SetCompare); // HINT
mySet.insert("x");
mySet.insert("abc");

// But not to the map!
Map.insert({"pqr", mySet});

在提示的位置，我将SetCompare lambda（不仅仅是类型）传递给MySet 的构造函数。问题是如何将其传递给unordered_map的“Value”类型？

编辑（编译器错误）：

GCC C++17

In file included from /usr/include/c++/7/set:60:0,
                 from main.cpp:10:
/usr/include/c++/7/bits/stl_tree.h: In instantiation of ‘std::_Rb_tree_key_compare<_Key_compare>::_Rb_tree_key_compare() [with _Key_compare = main()::<lambda(const string&, const string&)>]’:
/usr/include/c++/7/bits/stl_tree.h:688:4:   required from ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {char&&}; long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const char; _T2 = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >]’
/usr/include/c++/7/tuple:1641:63:   required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {char&&}; _Args2 = {}; _T1 = const char; _T2 = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >]’
/usr/include/c++/7/ext/new_allocator.h:136:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _Tp = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >]’
/usr/include/c++/7/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _Tp = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >]’
/usr/include/c++/7/bits/hashtable_policy.h:2066:37:   required from ‘std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >, false> >; std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type = std::__detail::_Hash_node<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >, false>]’
/usr/include/c++/7/bits/hashtable_policy.h:750:8:   required from ‘std::__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 = char; _Pair = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Alloc = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >; _Equal = std::equal_to<char>; _H1 = std::hash<char>; _H2 = std::__detail::_Mod_range_hashing; _Hash = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<false, false, true>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type = char]’
/usr/include/c++/7/bits/unordered_map.h:977:20:   required from ‘std::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 = char; _Tp = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; _Hash = std::hash<char>; _Pred = std::equal_to<char>; _Alloc = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type = char]’
<span class="error_line" onclick="ide.gotoLine('main.cpp',31)">main.cpp:31:7</span>:   required from here
/usr/include/c++/7/bits/stl_tree.h:149:24: error: use of deleted function ‘main()::::()’
       : _M_key_compare()
                        ^
main.cpp:17:24: note: a lambda closure type has a deleted default constructor
     auto SetCompare = [](const string& a, const string& b)

GCC C++17

In file included from /usr/include/c++/6/set:60:0,
                 from main.cpp:10:
/usr/include/c++/6/bits/stl_tree.h: In instantiation of ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Rb_tree_impl<_Key_compare, <anonymous> >::_Rb_tree_impl() [with _Key_compare = main()::<lambda(const string&, const string&)>; bool <anonymous> = false; _Key = std::basic_string<char>; _Val = std::basic_string<char>; _KeyOfValue = std::_Identity<std::basic_string<char> >; _Compare = main()::<lambda(const string&, const string&)>; _Alloc = std::allocator<std::basic_string<char> >]’:
/usr/include/c++/6/bits/stl_tree.h:821:18:   required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Rb_tree() [with _Key = std::basic_string<char>; _Val = std::basic_string<char>; _KeyOfValue = std::_Identity<std::basic_string<char> >; _Compare = main()::<lambda(const string&, const string&)>; _Alloc = std::allocator<std::basic_string<char> >]’
/usr/include/c++/6/bits/stl_set.h:146:14:   required from ‘std::set<_Key, _Compare, _Alloc>::set() [with _Key = std::basic_string<char>; _Compare = main()::<lambda(const string&, const string&)>; _Alloc = std::allocator<std::basic_string<char> >]’
/usr/include/c++/6/tuple:1590:70:   required from ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {char&&}; long unsigned int ..._Indexes1 = {0ul}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const char; _T2 = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >]’
/usr/include/c++/6/tuple:1579:63:   required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {char&&}; _Args2 = {}; _T1 = const char; _T2 = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >]’
/usr/include/c++/6/ext/new_allocator.h:120:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _Tp = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >]’
/usr/include/c++/6/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _Tp = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >]’
/usr/include/c++/6/bits/hashtable_policy.h:1953:37:   required from ‘std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<char&&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >, false> >; std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type = std::__detail::_Hash_node<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >, false>]’
/usr/include/c++/6/bits/hashtable_policy.h:620:8:   required from ‘std::__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 = char; _Pair = std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > >; _Alloc = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >; _Equal = std::equal_to<char>; _H1 = std::hash<char>; _H2 = std::__detail::_Mod_range_hashing; _Hash = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<false, false, true>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type = char]’
/usr/include/c++/6/bits/unordered_map.h:908:20:   required from ‘std::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 = char; _Tp = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; _Hash = std::hash<char>; _Pred = std::equal_to<char>; _Alloc = std::allocator<std::pair<const char, std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> > > >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type = std::set<std::basic_string<char>, main()::<lambda(const string&, const string&)> >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type = char]’
<span class="error_line" onclick="ide.gotoLine('main.cpp',31)">main.cpp:31:7</span>:   required from here
/usr/include/c++/6/bits/stl_tree.h:602:21: error: use of deleted function ‘main()::::()’
      _M_node_count(0)
                     ^
main.cpp:17:24: note: a lambda closure type has a deleted default constructor
     auto SetCompare = [](const string& a, const string& b)
                        ^
In file included from /usr/include/c++/6/set:60:0,
                 from main.cpp:10:
/usr/include/c++/6/bits/stl_tree.h:628:4: warning: ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Rb_tree_impl<_Key_compare,  >::_M_initialize() [with _Key_compare = main()::; bool  = false; _Key = std::basic_string; _Val = std::basic_string; _KeyOfValue = std::_Identity >; _Compare = main()::; _Alloc = std::allocator >]’ used but never defined
    _M_initialize()
    ^~~~~~~~~~~~~

VC C++14/17

1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\set(84,1): error C2280: 'main::<lambda_b1395bcc88464fd844969fa0764f137c>::<lambda_b1395bcc88464fd844969fa0764f137c>(void)': attempting to reference a deleted function
1>xx.cpp(923): message : see declaration of 'main::<lambda_b1395bcc88464fd844969fa0764f137c>::<lambda_b1395bcc88464fd844969fa0764f137c>'
1>xxx.cpp(923,20): message : 'main::<lambda_b1395bcc88464fd844969fa0764f137c>::<lambda_b1395bcc88464fd844969fa0764f137c>(void)': function was explicitly deleted
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\set(84): message : while compiling class template member function 'std::set<std::string,main::<lambda_b1395bcc88464fd844969fa0764f137c>,std::allocator<std::string>>::set(void)'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.27.29110\include\tuple(975): message : see reference to function template instantiation 'std::set<std::string,main::<lambda_b1395bcc88464fd844969fa0764f137c>,std::allocator<std::string>>::set(void)' being compiled
1>xxx.cpp(937): message : see reference to class template instantiation 'std::set<std::string,main::<lambda_b1395bcc88464fd844969fa0764f137c>,std::allocator<std::string>>' being compiled

【问题讨论】：

“行不通”是什么意思？你有没有编译错误？它以前如何？您是否尝试了解编译错误的含义？诚然，没有人设法发明出一个以简洁明了的错误消息而闻名的 C++ 编译器，但任何以精通 C++ 为目标的人都应该至少尝试阅读和解释其编译器的错误消息。最终他们会成功并找出其中之一。然后另一个。然后更多。然后他们将始终能够找出编译器的错误，而无需寻求其他任何人的帮助。
不同的编译器给出不同的消息这一事实甚至有用。阅读所有这些，我可以向你保证，它们都有一些有用的信息可以提供。特别是如果一些编译器接受代码，而另一些不接受，这是非常有用的信息。
开始说“编译器错误消息并不重要”与“我总是需要其他人帮助我解决编译错误”是同义词。在某些时候，您将不得不承认，从长远来看，这将是不切实际的。确实，不同的编译器会给出不同的错误信息。但他们都报告了同样的问题。在一个人擅长破译自己编译器的错误之后，破译其他编译器的神秘错误就容易了几个数量级。我不使用VS，但我通常可以弄清楚它在吠叫什么。
@SamVarshavchik 添加了编译器错误。
它抱怨 lambda 类的默认构造函数被删除。 std::map::operator[] 默认构造映射值，如果它不存在，因此映射的值必须是可默认构造的。 Lambda 类只有 C++20 或更高版本才有默认构造函数。

标签： c++ c++14

【解决方案1】：

在 C++20 之前，Lambda 永远不能默认构造（参见 cppreference 的 ClosureType::ClosureType() 部分），因此在构造 MySet 时必须传递 SetCompare 变量。在 clang 或 GCC 上使用 -std=c++20/-std=c++2a 应该可以工作 (demo)，在 MSVC 上使用 /std:c++latest。

要解决早期 C++ 版本中的这一限制，请将 SetCompare 定义为函子类型：

struct SetCompare
{
    bool operator()(const string& a, const string& b)
    {
        size_t L = a.length();
        size_t R = b.length();
        if (L == R) return (a < b);
        return L < R;
    }
};

并在定义MySet时使用此类型

using MySet = std::set<std::string, SetCompare>;

【讨论】：